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This is the bulk of GPIO changes for the v4.17 kernel cycle:

Browse source 
New drivers:
 
 - Nintendo Wii GameCube GPIO, known as "Hollywood"
 
 - Raspberry Pi mailbox service GPIO expander
 
 - Spreadtrum main SC9860 SoC and IEC GPIO controllers.
 
 Improvements:
 
 - Implemented .get_multiple() callback for most of the
   high-performance industrial GPIO cards for the ISA bus.
 
 - ISA GPIO drivers now select the ISA_BUS_API instead of
   depending on it. This is merged with the same pattern
   for all the ISA drivers and some other Kconfig cleanups
   related to this.
 
 Cleanup:
 
 - Delete the TZ1090 GPIO drivers following the deletion of
   this SoC from the ARM tree.
 
 - Move the documentation over to driver-api to conform with
   the rest of the kernel documentation build.
 
 - Continue to make the GPIO drivers include only
   <linux/gpio/driver.h> and not the too broad <linux/gpio.h>
   that we want to get rid of.
 
 - Managed to remove VLA allocation from two drivers pending
   more fixes in this area for the next merge window.
 
 - Misc janitorial fixes.
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Merge tag 'gpio-v4.17-1' of git://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-gpio

Pull GPIO updates from Linus Walleij:
 "This is the bulk of GPIO changes for the v4.17 kernel cycle:

  New drivers:

   - Nintendo Wii GameCube GPIO, known as "Hollywood"

   - Raspberry Pi mailbox service GPIO expander

   - Spreadtrum main SC9860 SoC and IEC GPIO controllers.

  Improvements:

   - Implemented .get_multiple() callback for most of the
     high-performance industrial GPIO cards for the ISA bus.

   - ISA GPIO drivers now select the ISA_BUS_API instead of depending on
     it. This is merged with the same pattern for all the ISA drivers
     and some other Kconfig cleanups related to this.

  Cleanup:

   - Delete the TZ1090 GPIO drivers following the deletion of this SoC
     from the ARM tree.

   - Move the documentation over to driver-api to conform with the rest
     of the kernel documentation build.

   - Continue to make the GPIO drivers include only
     <linux/gpio/driver.h> and not the too broad <linux/gpio.h> that we
     want to get rid of.

   - Managed to remove VLA allocation from two drivers pending more
     fixes in this area for the next merge window.

   - Misc janitorial fixes"

* tag 'gpio-v4.17-1' of git://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-gpio: (77 commits)
  gpio: Add Spreadtrum PMIC EIC driver support
  gpio: Add Spreadtrum EIC driver support
  dt-bindings: gpio: Add Spreadtrum EIC controller documentation
  gpio: ath79: Fix potential NULL dereference in ath79_gpio_probe()
  pinctrl: qcom: Don't allow protected pins to be requested
  gpiolib: Support 'gpio-reserved-ranges' property
  gpiolib: Change bitmap allocation to kmalloc_array
  gpiolib: Extract mask allocation into subroutine
  dt-bindings: gpio: Add a gpio-reserved-ranges property
  gpio: mockup: fix a potential crash when creating debugfs entries
  gpio: pca953x: add compatibility for pcal6524 and pcal9555a
  gpio: dwapb: Add support for a bus clock
  gpio: Remove VLA from xra1403 driver
  gpio: Remove VLA from MAX3191X driver
  gpio: ws16c48: Implement get_multiple callback
  gpio: gpio-mm: Implement get_multiple callback
  gpio: 104-idi-48: Implement get_multiple callback
  gpio: 104-dio-48e: Implement get_multiple callback
  gpio: pcie-idio-24: Implement get_multiple/set_multiple callbacks
  gpio: pci-idio-16: Implement get_multiple callback
  ...
This commit is contained in:
Linus Torvalds 2018-04-05 09:51:41 -07:00
commit 1b2951dd99
77 changed files with 2771 additions and 1274 deletions
Download patch file Download diff file Expand all files Collapse all files

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=============
GPIO Mappings
=============
This document explains how GPIOs can be assigned to given devices and functions.
Note that it only applies to the new descriptor-based interface. For a
description of the deprecated integer-based GPIO interface please refer to
gpio-legacy.txt (actually, there is no real mapping possible with the old
interface; you just fetch an integer from somewhere and request the
corresponding GPIO).
All platforms can enable the GPIO library, but if the platform strictly
requires GPIO functionality to be present, it needs to select GPIOLIB from its
Kconfig. Then, how GPIOs are mapped depends on what the platform uses to
describe its hardware layout. Currently, mappings can be defined through device
tree, ACPI, and platform data.
Device Tree
-----------
GPIOs can easily be mapped to devices and functions in the device tree. The
exact way to do it depends on the GPIO controller providing the GPIOs, see the
device tree bindings for your controller.
GPIOs mappings are defined in the consumer device's node, in a property named
<function>-gpios, where <function> is the function the driver will request
through gpiod_get(). For example::
foo_device {
compatible = "acme,foo";
...
led-gpios = <&gpio 15 GPIO_ACTIVE_HIGH>, /* red */
<&gpio 16 GPIO_ACTIVE_HIGH>, /* green */
<&gpio 17 GPIO_ACTIVE_HIGH>; /* blue */
power-gpios = <&gpio 1 GPIO_ACTIVE_LOW>;
};
Properties named <function>-gpio are also considered valid and old bindings use
it but are only supported for compatibility reasons and should not be used for
newer bindings since it has been deprecated.
This property will make GPIOs 15, 16 and 17 available to the driver under the
"led" function, and GPIO 1 as the "power" GPIO::
struct gpio_desc *red, *green, *blue, *power;
red = gpiod_get_index(dev, "led", 0, GPIOD_OUT_HIGH);
green = gpiod_get_index(dev, "led", 1, GPIOD_OUT_HIGH);
blue = gpiod_get_index(dev, "led", 2, GPIOD_OUT_HIGH);
power = gpiod_get(dev, "power", GPIOD_OUT_HIGH);
The led GPIOs will be active high, while the power GPIO will be active low (i.e.
gpiod_is_active_low(power) will be true).
The second parameter of the gpiod_get() functions, the con_id string, has to be
the <function>-prefix of the GPIO suffixes ("gpios" or "gpio", automatically
looked up by the gpiod functions internally) used in the device tree. With above
"led-gpios" example, use the prefix without the "-" as con_id parameter: "led".
Internally, the GPIO subsystem prefixes the GPIO suffix ("gpios" or "gpio")
with the string passed in con_id to get the resulting string
(``snprintf(... "%s-%s", con_id, gpio_suffixes[]``).
ACPI
----
ACPI also supports function names for GPIOs in a similar fashion to DT.
The above DT example can be converted to an equivalent ACPI description
with the help of _DSD (Device Specific Data), introduced in ACPI 5.1::
Device (FOO) {
Name (_CRS, ResourceTemplate () {
GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
"\\_SB.GPI0") {15} // red
GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
"\\_SB.GPI0") {16} // green
GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
"\\_SB.GPI0") {17} // blue
GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
"\\_SB.GPI0") {1} // power
})
Name (_DSD, Package () {
ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
Package () {
Package () {
"led-gpios",
Package () {
^FOO, 0, 0, 1,
^FOO, 1, 0, 1,
^FOO, 2, 0, 1,
}
},
Package () {
"power-gpios",
Package () {^FOO, 3, 0, 0},
},
}
})
}
For more information about the ACPI GPIO bindings see
Documentation/acpi/gpio-properties.txt.
Platform Data
-------------
Finally, GPIOs can be bound to devices and functions using platform data. Board
files that desire to do so need to include the following header::
#include <linux/gpio/machine.h>
GPIOs are mapped by the means of tables of lookups, containing instances of the
gpiod_lookup structure. Two macros are defined to help declaring such mappings::
GPIO_LOOKUP(chip_label, chip_hwnum, con_id, flags)
GPIO_LOOKUP_IDX(chip_label, chip_hwnum, con_id, idx, flags)
where
- chip_label is the label of the gpiod_chip instance providing the GPIO
- chip_hwnum is the hardware number of the GPIO within the chip
- con_id is the name of the GPIO function from the device point of view. It
can be NULL, in which case it will match any function.
- idx is the index of the GPIO within the function.
- flags is defined to specify the following properties:
* GPIO_ACTIVE_HIGH - GPIO line is active high
* GPIO_ACTIVE_LOW - GPIO line is active low
* GPIO_OPEN_DRAIN - GPIO line is set up as open drain
* GPIO_OPEN_SOURCE - GPIO line is set up as open source
* GPIO_PERSISTENT - GPIO line is persistent during
suspend/resume and maintains its value
* GPIO_TRANSITORY - GPIO line is transitory and may loose its
electrical state during suspend/resume
In the future, these flags might be extended to support more properties.
Note that GPIO_LOOKUP() is just a shortcut to GPIO_LOOKUP_IDX() where idx = 0.
A lookup table can then be defined as follows, with an empty entry defining its
end. The 'dev_id' field of the table is the identifier of the device that will
make use of these GPIOs. It can be NULL, in which case it will be matched for
calls to gpiod_get() with a NULL device.
.. code-block:: c
struct gpiod_lookup_table gpios_table = {
.dev_id = "foo.0",
.table = {
GPIO_LOOKUP_IDX("gpio.0", 15, "led", 0, GPIO_ACTIVE_HIGH),
GPIO_LOOKUP_IDX("gpio.0", 16, "led", 1, GPIO_ACTIVE_HIGH),
GPIO_LOOKUP_IDX("gpio.0", 17, "led", 2, GPIO_ACTIVE_HIGH),
GPIO_LOOKUP("gpio.0", 1, "power", GPIO_ACTIVE_LOW),
{ },
},
};
And the table can be added by the board code as follows::
gpiod_add_lookup_table(&gpios_table);
The driver controlling "foo.0" will then be able to obtain its GPIOs as follows::
struct gpio_desc *red, *green, *blue, *power;
red = gpiod_get_index(dev, "led", 0, GPIOD_OUT_HIGH);
green = gpiod_get_index(dev, "led", 1, GPIOD_OUT_HIGH);
blue = gpiod_get_index(dev, "led", 2, GPIOD_OUT_HIGH);
power = gpiod_get(dev, "power", GPIOD_OUT_HIGH);
Since the "led" GPIOs are mapped as active-high, this example will switch their
signals to 1, i.e. enabling the LEDs. And for the "power" GPIO, which is mapped
as active-low, its actual signal will be 0 after this code. Contrary to the
legacy integer GPIO interface, the active-low property is handled during
mapping and is thus transparent to GPIO consumers.
A set of functions such as gpiod_set_value() is available to work with
the new descriptor-oriented interface.

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==================================
GPIO Descriptor Consumer Interface
==================================
This document describes the consumer interface of the GPIO framework. Note that
it describes the new descriptor-based interface. For a description of the
deprecated integer-based GPIO interface please refer to gpio-legacy.txt.
Guidelines for GPIOs consumers
==============================
Drivers that can't work without standard GPIO calls should have Kconfig entries
that depend on GPIOLIB or select GPIOLIB. The functions that allow a driver to
obtain and use GPIOs are available by including the following file:
#include <linux/gpio/consumer.h>
There are static inline stubs for all functions in the header file in the case
where GPIOLIB is disabled. When these stubs are called they will emit
warnings. These stubs are used for two use cases:
- Simple compile coverage with e.g. COMPILE_TEST - it does not matter that
the current platform does not enable or select GPIOLIB because we are not
going to execute the system anyway.
- Truly optional GPIOLIB support - where the driver does not really make use
of the GPIOs on certain compile-time configurations for certain systems, but
will use it under other compile-time configurations. In this case the
consumer must make sure not to call into these functions, or the user will
be met with console warnings that may be perceived as intimidating.
All the functions that work with the descriptor-based GPIO interface are
prefixed with ``gpiod_``. The ``gpio_`` prefix is used for the legacy
interface. No other function in the kernel should use these prefixes. The use
of the legacy functions is strongly discouraged, new code should use
<linux/gpio/consumer.h> and descriptors exclusively.
Obtaining and Disposing GPIOs
=============================
With the descriptor-based interface, GPIOs are identified with an opaque,
non-forgeable handler that must be obtained through a call to one of the
gpiod_get() functions. Like many other kernel subsystems, gpiod_get() takes the
device that will use the GPIO and the function the requested GPIO is supposed to
fulfill::
struct gpio_desc *gpiod_get(struct device *dev, const char *con_id,
enum gpiod_flags flags)
If a function is implemented by using several GPIOs together (e.g. a simple LED
device that displays digits), an additional index argument can be specified::
struct gpio_desc *gpiod_get_index(struct device *dev,
const char *con_id, unsigned int idx,
enum gpiod_flags flags)
For a more detailed description of the con_id parameter in the DeviceTree case
see Documentation/gpio/board.txt
The flags parameter is used to optionally specify a direction and initial value
for the GPIO. Values can be:
* GPIOD_ASIS or 0 to not initialize the GPIO at all. The direction must be set
later with one of the dedicated functions.
* GPIOD_IN to initialize the GPIO as input.
* GPIOD_OUT_LOW to initialize the GPIO as output with a value of 0.
* GPIOD_OUT_HIGH to initialize the GPIO as output with a value of 1.
* GPIOD_OUT_LOW_OPEN_DRAIN same as GPIOD_OUT_LOW but also enforce the line
to be electrically used with open drain.
* GPIOD_OUT_HIGH_OPEN_DRAIN same as GPIOD_OUT_HIGH but also enforce the line
to be electrically used with open drain.
The two last flags are used for use cases where open drain is mandatory, such
as I2C: if the line is not already configured as open drain in the mappings
(see board.txt), then open drain will be enforced anyway and a warning will be
printed that the board configuration needs to be updated to match the use case.
Both functions return either a valid GPIO descriptor, or an error code checkable
with IS_ERR() (they will never return a NULL pointer). -ENOENT will be returned
if and only if no GPIO has been assigned to the device/function/index triplet,
other error codes are used for cases where a GPIO has been assigned but an error
occurred while trying to acquire it. This is useful to discriminate between mere
errors and an absence of GPIO for optional GPIO parameters. For the common
pattern where a GPIO is optional, the gpiod_get_optional() and
gpiod_get_index_optional() functions can be used. These functions return NULL
instead of -ENOENT if no GPIO has been assigned to the requested function::
struct gpio_desc *gpiod_get_optional(struct device *dev,
const char *con_id,
enum gpiod_flags flags)
struct gpio_desc *gpiod_get_index_optional(struct device *dev,
const char *con_id,
unsigned int index,
enum gpiod_flags flags)
Note that gpio_get*_optional() functions (and their managed variants), unlike
the rest of gpiolib API, also return NULL when gpiolib support is disabled.
This is helpful to driver authors, since they do not need to special case
-ENOSYS return codes. System integrators should however be careful to enable
gpiolib on systems that need it.
For a function using multiple GPIOs all of those can be obtained with one call::
struct gpio_descs *gpiod_get_array(struct device *dev,
const char *con_id,
enum gpiod_flags flags)
This function returns a struct gpio_descs which contains an array of
descriptors::
struct gpio_descs {
unsigned int ndescs;
struct gpio_desc *desc[];
}
The following function returns NULL instead of -ENOENT if no GPIOs have been
assigned to the requested function::
struct gpio_descs *gpiod_get_array_optional(struct device *dev,
const char *con_id,
enum gpiod_flags flags)
Device-managed variants of these functions are also defined::
struct gpio_desc *devm_gpiod_get(struct device *dev, const char *con_id,
enum gpiod_flags flags)
struct gpio_desc *devm_gpiod_get_index(struct device *dev,
const char *con_id,
unsigned int idx,
enum gpiod_flags flags)
struct gpio_desc *devm_gpiod_get_optional(struct device *dev,
const char *con_id,
enum gpiod_flags flags)
struct gpio_desc *devm_gpiod_get_index_optional(struct device *dev,
const char *con_id,
unsigned int index,
enum gpiod_flags flags)
struct gpio_descs *devm_gpiod_get_array(struct device *dev,
const char *con_id,
enum gpiod_flags flags)
struct gpio_descs *devm_gpiod_get_array_optional(struct device *dev,
const char *con_id,
enum gpiod_flags flags)
A GPIO descriptor can be disposed of using the gpiod_put() function::
void gpiod_put(struct gpio_desc *desc)
For an array of GPIOs this function can be used::
void gpiod_put_array(struct gpio_descs *descs)
It is strictly forbidden to use a descriptor after calling these functions.
It is also not allowed to individually release descriptors (using gpiod_put())
from an array acquired with gpiod_get_array().
The device-managed variants are, unsurprisingly::
void devm_gpiod_put(struct device *dev, struct gpio_desc *desc)
void devm_gpiod_put_array(struct device *dev, struct gpio_descs *descs)
Using GPIOs
===========
Setting Direction
-----------------
The first thing a driver must do with a GPIO is setting its direction. If no
direction-setting flags have been given to gpiod_get*(), this is done by
invoking one of the gpiod_direction_*() functions::
int gpiod_direction_input(struct gpio_desc *desc)
int gpiod_direction_output(struct gpio_desc *desc, int value)
The return value is zero for success, else a negative errno. It should be
checked, since the get/set calls don't return errors and since misconfiguration
is possible. You should normally issue these calls from a task context. However,
for spinlock-safe GPIOs it is OK to use them before tasking is enabled, as part
of early board setup.
For output GPIOs, the value provided becomes the initial output value. This
helps avoid signal glitching during system startup.
A driver can also query the current direction of a GPIO::
int gpiod_get_direction(const struct gpio_desc *desc)
This function returns 0 for output, 1 for input, or an error code in case of error.
Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO
without setting its direction first is illegal and will result in undefined
behavior!**
Spinlock-Safe GPIO Access
-------------------------
Most GPIO controllers can be accessed with memory read/write instructions. Those
don't need to sleep, and can safely be done from inside hard (non-threaded) IRQ
handlers and similar contexts.
Use the following calls to access GPIOs from an atomic context::
int gpiod_get_value(const struct gpio_desc *desc);
void gpiod_set_value(struct gpio_desc *desc, int value);
The values are boolean, zero for low, nonzero for high. When reading the value
of an output pin, the value returned should be what's seen on the pin. That
won't always match the specified output value, because of issues including
open-drain signaling and output latencies.
The get/set calls do not return errors because "invalid GPIO" should have been
reported earlier from gpiod_direction_*(). However, note that not all platforms
can read the value of output pins; those that can't should always return zero.
Also, using these calls for GPIOs that can't safely be accessed without sleeping
(see below) is an error.
GPIO Access That May Sleep
--------------------------
Some GPIO controllers must be accessed using message based buses like I2C or
SPI. Commands to read or write those GPIO values require waiting to get to the
head of a queue to transmit a command and get its response. This requires
sleeping, which can't be done from inside IRQ handlers.
Platforms that support this type of GPIO distinguish them from other GPIOs by
returning nonzero from this call::
int gpiod_cansleep(const struct gpio_desc *desc)
To access such GPIOs, a different set of accessors is defined::
int gpiod_get_value_cansleep(const struct gpio_desc *desc)
void gpiod_set_value_cansleep(struct gpio_desc *desc, int value)
Accessing such GPIOs requires a context which may sleep, for example a threaded
IRQ handler, and those accessors must be used instead of spinlock-safe
accessors without the cansleep() name suffix.
Other than the fact that these accessors might sleep, and will work on GPIOs
that can't be accessed from hardIRQ handlers, these calls act the same as the
spinlock-safe calls.
The active low and open drain semantics
---------------------------------------
As a consumer should not have to care about the physical line level, all of the
gpiod_set_value_xxx() or gpiod_set_array_value_xxx() functions operate with
the *logical* value. With this they take the active low property into account.
This means that they check whether the GPIO is configured to be active low,
and if so, they manipulate the passed value before the physical line level is
driven.
The same is applicable for open drain or open source output lines: those do not
actively drive their output high (open drain) or low (open source), they just
switch their output to a high impedance value. The consumer should not need to
care. (For details read about open drain in driver.txt.)
With this, all the gpiod_set_(array)_value_xxx() functions interpret the
parameter "value" as "asserted" ("1") or "de-asserted" ("0"). The physical line
level will be driven accordingly.
As an example, if the active low property for a dedicated GPIO is set, and the
gpiod_set_(array)_value_xxx() passes "asserted" ("1"), the physical line level
will be driven low.
To summarize::
Function (example) line property physical line
gpiod_set_raw_value(desc, 0); don't care low
gpiod_set_raw_value(desc, 1); don't care high
gpiod_set_value(desc, 0); default (active high) low
gpiod_set_value(desc, 1); default (active high) high
gpiod_set_value(desc, 0); active low high
gpiod_set_value(desc, 1); active low low
gpiod_set_value(desc, 0); default (active high) low
gpiod_set_value(desc, 1); default (active high) high
gpiod_set_value(desc, 0); open drain low
gpiod_set_value(desc, 1); open drain high impedance
gpiod_set_value(desc, 0); open source high impedance
gpiod_set_value(desc, 1); open source high
It is possible to override these semantics using the set_raw/get_raw functions
but it should be avoided as much as possible, especially by system-agnostic drivers
which should not need to care about the actual physical line level and worry about
the logical value instead.
Accessing raw GPIO values
-------------------------
Consumers exist that need to manage the logical state of a GPIO line, i.e. the value
their device will actually receive, no matter what lies between it and the GPIO
line.
The following set of calls ignore the active-low or open drain property of a GPIO and
work on the raw line value::
int gpiod_get_raw_value(const struct gpio_desc *desc)
void gpiod_set_raw_value(struct gpio_desc *desc, int value)
int gpiod_get_raw_value_cansleep(const struct gpio_desc *desc)
void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value)
int gpiod_direction_output_raw(struct gpio_desc *desc, int value)
The active low state of a GPIO can also be queried using the following call::
int gpiod_is_active_low(const struct gpio_desc *desc)
Note that these functions should only be used with great moderation; a driver
should not have to care about the physical line level or open drain semantics.
Access multiple GPIOs with a single function call
-------------------------------------------------
The following functions get or set the values of an array of GPIOs::
int gpiod_get_array_value(unsigned int array_size,
struct gpio_desc **desc_array,
int *value_array);
int gpiod_get_raw_array_value(unsigned int array_size,
struct gpio_desc **desc_array,
int *value_array);
int gpiod_get_array_value_cansleep(unsigned int array_size,
struct gpio_desc **desc_array,
int *value_array);
int gpiod_get_raw_array_value_cansleep(unsigned int array_size,
struct gpio_desc **desc_array,
int *value_array);
void gpiod_set_array_value(unsigned int array_size,
struct gpio_desc **desc_array,
int *value_array)
void gpiod_set_raw_array_value(unsigned int array_size,
struct gpio_desc **desc_array,
int *value_array)
void gpiod_set_array_value_cansleep(unsigned int array_size,
struct gpio_desc **desc_array,
int *value_array)
void gpiod_set_raw_array_value_cansleep(unsigned int array_size,
struct gpio_desc **desc_array,
int *value_array)
The array can be an arbitrary set of GPIOs. The functions will try to access
GPIOs belonging to the same bank or chip simultaneously if supported by the
corresponding chip driver. In that case a significantly improved performance
can be expected. If simultaneous access is not possible the GPIOs will be
accessed sequentially.
The functions take three arguments:
* array_size - the number of array elements
* desc_array - an array of GPIO descriptors
* value_array - an array to store the GPIOs' values (get) or
an array of values to assign to the GPIOs (set)
The descriptor array can be obtained using the gpiod_get_array() function
or one of its variants. If the group of descriptors returned by that function
matches the desired group of GPIOs, those GPIOs can be accessed by simply using
the struct gpio_descs returned by gpiod_get_array()::
struct gpio_descs *my_gpio_descs = gpiod_get_array(...);
gpiod_set_array_value(my_gpio_descs->ndescs, my_gpio_descs->desc,
my_gpio_values);
It is also possible to access a completely arbitrary array of descriptors. The
descriptors may be obtained using any combination of gpiod_get() and
gpiod_get_array(). Afterwards the array of descriptors has to be setup
manually before it can be passed to one of the above functions.
Note that for optimal performance GPIOs belonging to the same chip should be
contiguous within the array of descriptors.
The return value of gpiod_get_array_value() and its variants is 0 on success
or negative on error. Note the difference to gpiod_get_value(), which returns
0 or 1 on success to convey the GPIO value. With the array functions, the GPIO
values are stored in value_array rather than passed back as return value.
GPIOs mapped to IRQs
--------------------
GPIO lines can quite often be used as IRQs. You can get the IRQ number
corresponding to a given GPIO using the following call::
int gpiod_to_irq(const struct gpio_desc *desc)
It will return an IRQ number, or a negative errno code if the mapping can't be
done (most likely because that particular GPIO cannot be used as IRQ). It is an
unchecked error to use a GPIO that wasn't set up as an input using
gpiod_direction_input(), or to use an IRQ number that didn't originally come
from gpiod_to_irq(). gpiod_to_irq() is not allowed to sleep.
Non-error values returned from gpiod_to_irq() can be passed to request_irq() or
free_irq(). They will often be stored into IRQ resources for platform devices,
by the board-specific initialization code. Note that IRQ trigger options are
part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are system wakeup
capabilities.
GPIOs and ACPI
==============
On ACPI systems, GPIOs are described by GpioIo()/GpioInt() resources listed by
the _CRS configuration objects of devices. Those resources do not provide
connection IDs (names) for GPIOs, so it is necessary to use an additional
mechanism for this purpose.
Systems compliant with ACPI 5.1 or newer may provide a _DSD configuration object
which, among other things, may be used to provide connection IDs for specific
GPIOs described by the GpioIo()/GpioInt() resources in _CRS. If that is the
case, it will be handled by the GPIO subsystem automatically. However, if the
_DSD is not present, the mappings between GpioIo()/GpioInt() resources and GPIO
connection IDs need to be provided by device drivers.
For details refer to Documentation/acpi/gpio-properties.txt
Interacting With the Legacy GPIO Subsystem
==========================================
Many kernel subsystems still handle GPIOs using the legacy integer-based
interface. Although it is strongly encouraged to upgrade them to the safer
descriptor-based API, the following two functions allow you to convert a GPIO
descriptor into the GPIO integer namespace and vice-versa::
int desc_to_gpio(const struct gpio_desc *desc)
struct gpio_desc *gpio_to_desc(unsigned gpio)
The GPIO number returned by desc_to_gpio() can be safely used as long as the
GPIO descriptor has not been freed. All the same, a GPIO number passed to
gpio_to_desc() must have been properly acquired, and usage of the returned GPIO
descriptor is only possible after the GPIO number has been released.
Freeing a GPIO obtained by one API with the other API is forbidden and an
unchecked error.

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================================
GPIO Descriptor Driver Interface
================================
This document serves as a guide for GPIO chip drivers writers. Note that it
describes the new descriptor-based interface. For a description of the
deprecated integer-based GPIO interface please refer to gpio-legacy.txt.
Each GPIO controller driver needs to include the following header, which defines
the structures used to define a GPIO driver:
#include <linux/gpio/driver.h>
Internal Representation of GPIOs
================================
Inside a GPIO driver, individual GPIOs are identified by their hardware number,
which is a unique number between 0 and n, n being the number of GPIOs managed by
the chip. This number is purely internal: the hardware number of a particular
GPIO descriptor is never made visible outside of the driver.
On top of this internal number, each GPIO also need to have a global number in
the integer GPIO namespace so that it can be used with the legacy GPIO
interface. Each chip must thus have a "base" number (which can be automatically
assigned), and for each GPIO the global number will be (base + hardware number).
Although the integer representation is considered deprecated, it still has many
users and thus needs to be maintained.
So for example one platform could use numbers 32-159 for GPIOs, with a
controller defining 128 GPIOs at a "base" of 32 ; while another platform uses
numbers 0..63 with one set of GPIO controllers, 64-79 with another type of GPIO
controller, and on one particular board 80-95 with an FPGA. The numbers need not
be contiguous; either of those platforms could also use numbers 2000-2063 to
identify GPIOs in a bank of I2C GPIO expanders.
Controller Drivers: gpio_chip
=============================
In the gpiolib framework each GPIO controller is packaged as a "struct
gpio_chip" (see linux/gpio/driver.h for its complete definition) with members
common to each controller of that type:
- methods to establish GPIO line direction
- methods used to access GPIO line values
- method to set electrical configuration to a a given GPIO line
- method to return the IRQ number associated to a given GPIO line
- flag saying whether calls to its methods may sleep
- optional line names array to identify lines
- optional debugfs dump method (showing extra state like pullup config)
- optional base number (will be automatically assigned if omitted)
- optional label for diagnostics and GPIO chip mapping using platform data
The code implementing a gpio_chip should support multiple instances of the
controller, possibly using the driver model. That code will configure each
gpio_chip and issue ``gpiochip_add[_data]()`` or ``devm_gpiochip_add_data()``.
Removing a GPIO controller should be rare; use ``[devm_]gpiochip_remove()``
when it is unavoidable.
Often a gpio_chip is part of an instance-specific structure with states not
exposed by the GPIO interfaces, such as addressing, power management, and more.
Chips such as audio codecs will have complex non-GPIO states.
Any debugfs dump method should normally ignore signals which haven't been
requested as GPIOs. They can use gpiochip_is_requested(), which returns either
NULL or the label associated with that GPIO when it was requested.
RT_FULL: the GPIO driver should not use spinlock_t or any sleepable APIs
(like PM runtime) in its gpio_chip implementation (.get/.set and direction
control callbacks) if it is expected to call GPIO APIs from atomic context
on -RT (inside hard IRQ handlers and similar contexts). Normally this should
not be required.
GPIO electrical configuration
-----------------------------
GPIOs can be configured for several electrical modes of operation by using the
.set_config() callback. Currently this API supports setting debouncing and
single-ended modes (open drain/open source). These settings are described
below.
The .set_config() callback uses the same enumerators and configuration
semantics as the generic pin control drivers. This is not a coincidence: it is
possible to assign the .set_config() to the function gpiochip_generic_config()
which will result in pinctrl_gpio_set_config() being called and eventually
ending up in the pin control back-end "behind" the GPIO controller, usually
closer to the actual pins. This way the pin controller can manage the below
listed GPIO configurations.
If a pin controller back-end is used, the GPIO controller or hardware
description needs to provide "GPIO ranges" mapping the GPIO line offsets to pin
numbers on the pin controller so they can properly cross-reference each other.
GPIOs with debounce support
---------------------------
Debouncing is a configuration set to a pin indicating that it is connected to
a mechanical switch or button, or similar that may bounce. Bouncing means the
line is pulled high/low quickly at very short intervals for mechanical
reasons. This can result in the value being unstable or irqs fireing repeatedly
unless the line is debounced.
Debouncing in practice involves setting up a timer when something happens on
the line, wait a little while and then sample the line again, so see if it
still has the same value (low or high). This could also be repeated by a clever
state machine, waiting for a line to become stable. In either case, it sets
a certain number of milliseconds for debouncing, or just "on/off" if that time
is not configurable.
GPIOs with open drain/source support
------------------------------------
Open drain (CMOS) or open collector (TTL) means the line is not actively driven
high: instead you provide the drain/collector as output, so when the transistor
is not open, it will present a high-impedance (tristate) to the external rail::
CMOS CONFIGURATION TTL CONFIGURATION
||--- out +--- out
in ----|| |/
||--+ in ----|
| |\
GND GND
This configuration is normally used as a way to achieve one of two things:
- Level-shifting: to reach a logical level higher than that of the silicon
where the output resides.
- inverse wire-OR on an I/O line, for example a GPIO line, making it possible
for any driving stage on the line to drive it low even if any other output
to the same line is simultaneously driving it high. A special case of this
is driving the SCL and SCA lines of an I2C bus, which is by definition a
wire-OR bus.
Both usecases require that the line be equipped with a pull-up resistor. This
resistor will make the line tend to high level unless one of the transistors on
the rail actively pulls it down.
The level on the line will go as high as the VDD on the pull-up resistor, which
may be higher than the level supported by the transistor, achieveing a
level-shift to the higher VDD.
Integrated electronics often have an output driver stage in the form of a CMOS
"totem-pole" with one N-MOS and one P-MOS transistor where one of them drives
the line high and one of them drives the line low. This is called a push-pull
output. The "totem-pole" looks like so::
VDD
|
OD ||--+
+--/ ---o|| P-MOS-FET
| ||--+
IN --+ +----- out
| ||--+
+--/ ----|| N-MOS-FET
OS ||--+
|
GND
The desired output signal (e.g. coming directly from some GPIO output register)
arrives at IN. The switches named "OD" and "OS" are normally closed, creating
a push-pull circuit.
Consider the little "switches" named "OD" and "OS" that enable/disable the
P-MOS or N-MOS transistor right after the split of the input. As you can see,
either transistor will go totally numb if this switch is open. The totem-pole
is then halved and give high impedance instead of actively driving the line
high or low respectively. That is usually how software-controlled open
drain/source works.
Some GPIO hardware come in open drain / open source configuration. Some are
hard-wired lines that will only support open drain or open source no matter
what: there is only one transistor there. Some are software-configurable:
by flipping a bit in a register the output can be configured as open drain
or open source, in practice by flicking open the switches labeled "OD" and "OS"
in the drawing above.
By disabling the P-MOS transistor, the output can be driven between GND and
high impedance (open drain), and by disabling the N-MOS transistor, the output
can be driven between VDD and high impedance (open source). In the first case,
a pull-up resistor is needed on the outgoing rail to complete the circuit, and
in the second case, a pull-down resistor is needed on the rail.
Hardware that supports open drain or open source or both, can implement a
special callback in the gpio_chip: .set_config() that takes a generic
pinconf packed value telling whether to configure the line as open drain,
open source or push-pull. This will happen in response to the
GPIO_OPEN_DRAIN or GPIO_OPEN_SOURCE flag set in the machine file, or coming
from other hardware descriptions.
If this state can not be configured in hardware, i.e. if the GPIO hardware does
not support open drain/open source in hardware, the GPIO library will instead
use a trick: when a line is set as output, if the line is flagged as open
drain, and the IN output value is low, it will be driven low as usual. But
if the IN output value is set to high, it will instead *NOT* be driven high,
instead it will be switched to input, as input mode is high impedance, thus
achieveing an "open drain emulation" of sorts: electrically the behaviour will
be identical, with the exception of possible hardware glitches when switching
the mode of the line.
For open source configuration the same principle is used, just that instead
of actively driving the line low, it is set to input.
GPIO drivers providing IRQs
---------------------------
It is custom that GPIO drivers (GPIO chips) are also providing interrupts,
most often cascaded off a parent interrupt controller, and in some special
cases the GPIO logic is melded with a SoC's primary interrupt controller.
The IRQ portions of the GPIO block are implemented using an irqchip, using
the header <linux/irq.h>. So basically such a driver is utilizing two sub-
systems simultaneously: gpio and irq.
RT_FULL: a realtime compliant GPIO driver should not use spinlock_t or any
sleepable APIs (like PM runtime) as part of its irq_chip implementation.
* spinlock_t should be replaced with raw_spinlock_t [1].
* If sleepable APIs have to be used, these can be done from the .irq_bus_lock()
and .irq_bus_unlock() callbacks, as these are the only slowpath callbacks
on an irqchip. Create the callbacks if needed [2].
GPIO irqchips usually fall in one of two categories:
* CHAINED GPIO irqchips: these are usually the type that is embedded on
an SoC. This means that there is a fast IRQ flow handler for the GPIOs that
gets called in a chain from the parent IRQ handler, most typically the
system interrupt controller. This means that the GPIO irqchip handler will
be called immediately from the parent irqchip, while holding the IRQs
disabled. The GPIO irqchip will then end up calling something like this
sequence in its interrupt handler::
static irqreturn_t foo_gpio_irq(int irq, void *data)
chained_irq_enter(...);
generic_handle_irq(...);
chained_irq_exit(...);
Chained GPIO irqchips typically can NOT set the .can_sleep flag on
struct gpio_chip, as everything happens directly in the callbacks: no
slow bus traffic like I2C can be used.
RT_FULL: Note, chained IRQ handlers will not be forced threaded on -RT.
As result, spinlock_t or any sleepable APIs (like PM runtime) can't be used
in chained IRQ handler.
If required (and if it can't be converted to the nested threaded GPIO irqchip)
a chained IRQ handler can be converted to generic irq handler and this way
it will be a threaded IRQ handler on -RT and a hard IRQ handler on non-RT
(for example, see [3]).
Know W/A: The generic_handle_irq() is expected to be called with IRQ disabled,
so the IRQ core will complain if it is called from an IRQ handler which is
forced to a thread. The "fake?" raw lock can be used to W/A this problem::
raw_spinlock_t wa_lock;
static irqreturn_t omap_gpio_irq_handler(int irq, void *gpiobank)
unsigned long wa_lock_flags;
raw_spin_lock_irqsave(&bank->wa_lock, wa_lock_flags);
generic_handle_irq(irq_find_mapping(bank->chip.irq.domain, bit));
raw_spin_unlock_irqrestore(&bank->wa_lock, wa_lock_flags);
* GENERIC CHAINED GPIO irqchips: these are the same as "CHAINED GPIO irqchips",
but chained IRQ handlers are not used. Instead GPIO IRQs dispatching is
performed by generic IRQ handler which is configured using request_irq().
The GPIO irqchip will then end up calling something like this sequence in
its interrupt handler::
static irqreturn_t gpio_rcar_irq_handler(int irq, void *dev_id)
for each detected GPIO IRQ
generic_handle_irq(...);
RT_FULL: Such kind of handlers will be forced threaded on -RT, as result IRQ
core will complain that generic_handle_irq() is called with IRQ enabled and
the same W/A as for "CHAINED GPIO irqchips" can be applied.
* NESTED THREADED GPIO irqchips: these are off-chip GPIO expanders and any
other GPIO irqchip residing on the other side of a sleeping bus. Of course
such drivers that need slow bus traffic to read out IRQ status and similar,
traffic which may in turn incur other IRQs to happen, cannot be handled
in a quick IRQ handler with IRQs disabled. Instead they need to spawn a
thread and then mask the parent IRQ line until the interrupt is handled
by the driver. The hallmark of this driver is to call something like
this in its interrupt handler::
static irqreturn_t foo_gpio_irq(int irq, void *data)
...
handle_nested_irq(irq);
The hallmark of threaded GPIO irqchips is that they set the .can_sleep
flag on struct gpio_chip to true, indicating that this chip may sleep
when accessing the GPIOs.
To help out in handling the set-up and management of GPIO irqchips and the
associated irqdomain and resource allocation callbacks, the gpiolib has
some helpers that can be enabled by selecting the GPIOLIB_IRQCHIP Kconfig
symbol:
* gpiochip_irqchip_add(): adds a chained irqchip to a gpiochip. It will pass
the struct gpio_chip* for the chip to all IRQ callbacks, so the callbacks
need to embed the gpio_chip in its state container and obtain a pointer
to the container using container_of().
(See Documentation/driver-model/design-patterns.txt)
* gpiochip_irqchip_add_nested(): adds a nested irqchip to a gpiochip.
Apart from that it works exactly like the chained irqchip.
* gpiochip_set_chained_irqchip(): sets up a chained irq handler for a
gpio_chip from a parent IRQ and passes the struct gpio_chip* as handler
data. (Notice handler data, since the irqchip data is likely used by the
parent irqchip!).
* gpiochip_set_nested_irqchip(): sets up a nested irq handler for a
gpio_chip from a parent IRQ. As the parent IRQ has usually been
explicitly requested by the driver, this does very little more than
mark all the child IRQs as having the other IRQ as parent.
If there is a need to exclude certain GPIOs from the IRQ domain, you can
set .irq.need_valid_mask of the gpiochip before gpiochip_add_data() is
called. This allocates an .irq.valid_mask with as many bits set as there
are GPIOs in the chip. Drivers can exclude GPIOs by clearing bits from this
mask. The mask must be filled in before gpiochip_irqchip_add() or
gpiochip_irqchip_add_nested() is called.
To use the helpers please keep the following in mind:
- Make sure to assign all relevant members of the struct gpio_chip so that
the irqchip can initialize. E.g. .dev and .can_sleep shall be set up
properly.
- Nominally set all handlers to handle_bad_irq() in the setup call and pass
handle_bad_irq() as flow handler parameter in gpiochip_irqchip_add() if it is
expected for GPIO driver that irqchip .set_type() callback have to be called
before using/enabling GPIO IRQ. Then set the handler to handle_level_irq()
and/or handle_edge_irq() in the irqchip .set_type() callback depending on
what your controller supports.
It is legal for any IRQ consumer to request an IRQ from any irqchip no matter
if that is a combined GPIO+IRQ driver. The basic premise is that gpio_chip and
irq_chip are orthogonal, and offering their services independent of each
other.
gpiod_to_irq() is just a convenience function to figure out the IRQ for a
certain GPIO line and should not be relied upon to have been called before
the IRQ is used.
So always prepare the hardware and make it ready for action in respective
callbacks from the GPIO and irqchip APIs. Do not rely on gpiod_to_irq() having
been called first.
This orthogonality leads to ambiguities that we need to solve: if there is
competition inside the subsystem which side is using the resource (a certain
GPIO line and register for example) it needs to deny certain operations and
keep track of usage inside of the gpiolib subsystem. This is why the API
below exists.
Locking IRQ usage
-----------------
Input GPIOs can be used as IRQ signals. When this happens, a driver is requested
to mark the GPIO as being used as an IRQ::
int gpiochip_lock_as_irq(struct gpio_chip *chip, unsigned int offset)
This will prevent the use of non-irq related GPIO APIs until the GPIO IRQ lock
is released::
void gpiochip_unlock_as_irq(struct gpio_chip *chip, unsigned int offset)
When implementing an irqchip inside a GPIO driver, these two functions should
typically be called in the .startup() and .shutdown() callbacks from the
irqchip.
When using the gpiolib irqchip helpers, these callback are automatically
assigned.
Real-Time compliance for GPIO IRQ chips
---------------------------------------
Any provider of irqchips needs to be carefully tailored to support Real Time
preemption. It is desirable that all irqchips in the GPIO subsystem keep this
in mind and does the proper testing to assure they are real time-enabled.
So, pay attention on above " RT_FULL:" notes, please.
The following is a checklist to follow when preparing a driver for real
time-compliance:
- ensure spinlock_t is not used as part irq_chip implementation;
- ensure that sleepable APIs are not used as part irq_chip implementation.
If sleepable APIs have to be used, these can be done from the .irq_bus_lock()
and .irq_bus_unlock() callbacks;
- Chained GPIO irqchips: ensure spinlock_t or any sleepable APIs are not used
from chained IRQ handler;
- Generic chained GPIO irqchips: take care about generic_handle_irq() calls and
apply corresponding W/A;
- Chained GPIO irqchips: get rid of chained IRQ handler and use generic irq
handler if possible :)
- regmap_mmio: Sry, but you are in trouble :( if MMIO regmap is used as for
GPIO IRQ chip implementation;
- Test your driver with the appropriate in-kernel real time test cases for both
level and edge IRQs.
Requesting self-owned GPIO pins
-------------------------------
Sometimes it is useful to allow a GPIO chip driver to request its own GPIO
descriptors through the gpiolib API. Using gpio_request() for this purpose
does not help since it pins the module to the kernel forever (it calls
try_module_get()). A GPIO driver can use the following functions instead
to request and free descriptors without being pinned to the kernel forever::
struct gpio_desc *gpiochip_request_own_desc(struct gpio_desc *desc,
const char *label)
void gpiochip_free_own_desc(struct gpio_desc *desc)
Descriptors requested with gpiochip_request_own_desc() must be released with
gpiochip_free_own_desc().
These functions must be used with care since they do not affect module use
count. Do not use the functions to request gpio descriptors not owned by the
calling driver.
* [1] http://www.spinics.net/lists/linux-omap/msg120425.html
* [2] https://lkml.org/lkml/2015/9/25/494
* [3] https://lkml.org/lkml/2015/9/25/495

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============================
Subsystem drivers using GPIO
============================
Note that standard kernel drivers exist for common GPIO tasks and will provide
the right in-kernel and userspace APIs/ABIs for the job, and that these
drivers can quite easily interconnect with other kernel subsystems using
hardware descriptions such as device tree or ACPI:
- leds-gpio: drivers/leds/leds-gpio.c will handle LEDs connected to GPIO
lines, giving you the LED sysfs interface
- ledtrig-gpio: drivers/leds/trigger/ledtrig-gpio.c will provide a LED trigger,
i.e. a LED will turn on/off in response to a GPIO line going high or low
(and that LED may in turn use the leds-gpio as per above).
- gpio-keys: drivers/input/keyboard/gpio_keys.c is used when your GPIO line
can generate interrupts in response to a key press. Also supports debounce.
- gpio-keys-polled: drivers/input/keyboard/gpio_keys_polled.c is used when your
GPIO line cannot generate interrupts, so it needs to be periodically polled
by a timer.
- gpio_mouse: drivers/input/mouse/gpio_mouse.c is used to provide a mouse with
up to three buttons by simply using GPIOs and no mouse port. You can cut the
mouse cable and connect the wires to GPIO lines or solder a mouse connector
to the lines for a more permanent solution of this type.
- gpio-beeper: drivers/input/misc/gpio-beeper.c is used to provide a beep from
an external speaker connected to a GPIO line.
- extcon-gpio: drivers/extcon/extcon-gpio.c is used when you need to read an
external connector status, such as a headset line for an audio driver or an
HDMI connector. It will provide a better userspace sysfs interface than GPIO.
- restart-gpio: drivers/power/reset/gpio-restart.c is used to restart/reboot
the system by pulling a GPIO line and will register a restart handler so
userspace can issue the right system call to restart the system.
- poweroff-gpio: drivers/power/reset/gpio-poweroff.c is used to power the
system down by pulling a GPIO line and will register a pm_power_off()
callback so that userspace can issue the right system call to power down the
system.
- gpio-gate-clock: drivers/clk/clk-gpio.c is used to control a gated clock
(off/on) that uses a GPIO, and integrated with the clock subsystem.
- i2c-gpio: drivers/i2c/busses/i2c-gpio.c is used to drive an I2C bus
(two wires, SDA and SCL lines) by hammering (bitbang) two GPIO lines. It will
appear as any other I2C bus to the system and makes it possible to connect
drivers for the I2C devices on the bus like any other I2C bus driver.
- spi_gpio: drivers/spi/spi-gpio.c is used to drive an SPI bus (variable number
of wires, at least SCK and optionally MISO, MOSI and chip select lines) using
GPIO hammering (bitbang). It will appear as any other SPI bus on the system
and makes it possible to connect drivers for SPI devices on the bus like
any other SPI bus driver. For example any MMC/SD card can then be connected
to this SPI by using the mmc_spi host from the MMC/SD card subsystem.
- w1-gpio: drivers/w1/masters/w1-gpio.c is used to drive a one-wire bus using
a GPIO line, integrating with the W1 subsystem and handling devices on
the bus like any other W1 device.
- gpio-fan: drivers/hwmon/gpio-fan.c is used to control a fan for cooling the
system, connected to a GPIO line (and optionally a GPIO alarm line),
presenting all the right in-kernel and sysfs interfaces to make your system
not overheat.
- gpio-regulator: drivers/regulator/gpio-regulator.c is used to control a
regulator providing a certain voltage by pulling a GPIO line, integrating
with the regulator subsystem and giving you all the right interfaces.
- gpio-wdt: drivers/watchdog/gpio_wdt.c is used to provide a watchdog timer
that will periodically "ping" a hardware connected to a GPIO line by toggling
it from 1-to-0-to-1. If that hardware does not receive its "ping"
periodically, it will reset the system.
- gpio-nand: drivers/mtd/nand/gpio.c is used to connect a NAND flash chip to
a set of simple GPIO lines: RDY, NCE, ALE, CLE, NWP. It interacts with the
NAND flash MTD subsystem and provides chip access and partition parsing like
any other NAND driving hardware.
- ps2-gpio: drivers/input/serio/ps2-gpio.c is used to drive a PS/2 (IBM) serio
bus, data and clock line, by bit banging two GPIO lines. It will appear as
any other serio bus to the system and makes it possible to connect drivers
for e.g. keyboards and other PS/2 protocol based devices.
Apart from this there are special GPIO drivers in subsystems like MMC/SD to
read card detect and write protect GPIO lines, and in the TTY serial subsystem
to emulate MCTRL (modem control) signals CTS/RTS by using two GPIO lines. The
MTD NOR flash has add-ons for extra GPIO lines too, though the address bus is
usually connected directly to the flash.
Use those instead of talking directly to the GPIOs using sysfs; they integrate
with kernel frameworks better than your userspace code could. Needless to say,
just using the appropriate kernel drivers will simplify and speed up your
embedded hacking in particular by providing ready-made components.

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General Purpose Input/Output (GPIO)
===================================
Contents:
.. toctree::
:maxdepth: 2
intro
driver
consumer
board
drivers-on-gpio
legacy
Core
====
@ -11,15 +23,6 @@ Core
.. kernel-doc:: drivers/gpio/gpiolib.c
:export:
Legacy API
==========
The functions listed in this section are deprecated. The GPIO descriptor based
API described above should be used in new code.
.. kernel-doc:: drivers/gpio/gpiolib-legacy.c
:export:
ACPI support
============

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============
Introduction
============
GPIO Interfaces
===============
The documents in this directory give detailed instructions on how to access
GPIOs in drivers, and how to write a driver for a device that provides GPIOs
itself.
Due to the history of GPIO interfaces in the kernel, there are two different
ways to obtain and use GPIOs:
- The descriptor-based interface is the preferred way to manipulate GPIOs,
and is described by all the files in this directory excepted gpio-legacy.txt.
- The legacy integer-based interface which is considered deprecated (but still
usable for compatibility reasons) is documented in gpio-legacy.txt.
The remainder of this document applies to the new descriptor-based interface.
gpio-legacy.txt contains the same information applied to the legacy
integer-based interface.
What is a GPIO?
===============
A "General Purpose Input/Output" (GPIO) is a flexible software-controlled
digital signal. They are provided from many kinds of chip, and are familiar
to Linux developers working with embedded and custom hardware. Each GPIO
represents a bit connected to a particular pin, or "ball" on Ball Grid Array
(BGA) packages. Board schematics show which external hardware connects to
which GPIOs. Drivers can be written generically, so that board setup code
passes such pin configuration data to drivers.
System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every
non-dedicated pin can be configured as a GPIO; and most chips have at least
several dozen of them. Programmable logic devices (like FPGAs) can easily
provide GPIOs; multifunction chips like power managers, and audio codecs
often have a few such pins to help with pin scarcity on SOCs; and there are
also "GPIO Expander" chips that connect using the I2C or SPI serial buses.
Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS
firmware knowing how they're used).
The exact capabilities of GPIOs vary between systems. Common options:
- Output values are writable (high=1, low=0). Some chips also have
options about how that value is driven, so that for example only one
value might be driven, supporting "wire-OR" and similar schemes for the
other value (notably, "open drain" signaling).
- Input values are likewise readable (1, 0). Some chips support readback
of pins configured as "output", which is very useful in such "wire-OR"
cases (to support bidirectional signaling). GPIO controllers may have
input de-glitch/debounce logic, sometimes with software controls.
- Inputs can often be used as IRQ signals, often edge triggered but
sometimes level triggered. Such IRQs may be configurable as system
wakeup events, to wake the system from a low power state.
- Usually a GPIO will be configurable as either input or output, as needed
by different product boards; single direction ones exist too.
- Most GPIOs can be accessed while holding spinlocks, but those accessed
through a serial bus normally can't. Some systems support both types.
On a given board each GPIO is used for one specific purpose like monitoring
MMC/SD card insertion/removal, detecting card write-protect status, driving
a LED, configuring a transceiver, bit-banging a serial bus, poking a hardware
watchdog, sensing a switch, and so on.
Common GPIO Properties
======================
These properties are met through all the other documents of the GPIO interface
and it is useful to understand them, especially if you need to define GPIO
mappings.
Active-High and Active-Low
--------------------------
It is natural to assume that a GPIO is "active" when its output signal is 1
("high"), and inactive when it is 0 ("low"). However in practice the signal of a
GPIO may be inverted before is reaches its destination, or a device could decide
to have different conventions about what "active" means. Such decisions should
be transparent to device drivers, therefore it is possible to define a GPIO as
being either active-high ("1" means "active", the default) or active-low ("0"
means "active") so that drivers only need to worry about the logical signal and
not about what happens at the line level.
Open Drain and Open Source
--------------------------
Sometimes shared signals need to use "open drain" (where only the low signal
level is actually driven), or "open source" (where only the high signal level is
driven) signaling. That term applies to CMOS transistors; "open collector" is
used for TTL. A pullup or pulldown resistor causes the high or low signal level.
This is sometimes called a "wire-AND"; or more practically, from the negative
logic (low=true) perspective this is a "wire-OR".
One common example of an open drain signal is a shared active-low IRQ line.
Also, bidirectional data bus signals sometimes use open drain signals.
Some GPIO controllers directly support open drain and open source outputs; many
don't. When you need open drain signaling but your hardware doesn't directly
support it, there's a common idiom you can use to emulate it with any GPIO pin
that can be used as either an input or an output:
LOW: gpiod_direction_output(gpio, 0) ... this drives the signal and overrides
the pullup.
HIGH: gpiod_direction_input(gpio) ... this turns off the output, so the pullup
(or some other device) controls the signal.
The same logic can be applied to emulate open source signaling, by driving the
high signal and configuring the GPIO as input for low. This open drain/open
source emulation can be handled transparently by the GPIO framework.
If you are "driving" the signal high but gpiod_get_value(gpio) reports a low
value (after the appropriate rise time passes), you know some other component is
driving the shared signal low. That's not necessarily an error. As one common
example, that's how I2C clocks are stretched: a slave that needs a slower clock
delays the rising edge of SCK, and the I2C master adjusts its signaling rate
accordingly.

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@ -0,0 +1,770 @@
======================
Legacy GPIO Interfaces
======================
This provides an overview of GPIO access conventions on Linux.
These calls use the gpio_* naming prefix. No other calls should use that
prefix, or the related __gpio_* prefix.
What is a GPIO?
===============
A "General Purpose Input/Output" (GPIO) is a flexible software-controlled
digital signal. They are provided from many kinds of chip, and are familiar
to Linux developers working with embedded and custom hardware. Each GPIO
represents a bit connected to a particular pin, or "ball" on Ball Grid Array
(BGA) packages. Board schematics show which external hardware connects to
which GPIOs. Drivers can be written generically, so that board setup code
passes such pin configuration data to drivers.
System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every
non-dedicated pin can be configured as a GPIO; and most chips have at least
several dozen of them. Programmable logic devices (like FPGAs) can easily
provide GPIOs; multifunction chips like power managers, and audio codecs
often have a few such pins to help with pin scarcity on SOCs; and there are
also "GPIO Expander" chips that connect using the I2C or SPI serial busses.
Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS
firmware knowing how they're used).
The exact capabilities of GPIOs vary between systems. Common options:
- Output values are writable (high=1, low=0). Some chips also have
options about how that value is driven, so that for example only one
value might be driven ... supporting "wire-OR" and similar schemes
for the other value (notably, "open drain" signaling).
- Input values are likewise readable (1, 0). Some chips support readback
of pins configured as "output", which is very useful in such "wire-OR"
cases (to support bidirectional signaling). GPIO controllers may have
input de-glitch/debounce logic, sometimes with software controls.
- Inputs can often be used as IRQ signals, often edge triggered but
sometimes level triggered. Such IRQs may be configurable as system
wakeup events, to wake the system from a low power state.
- Usually a GPIO will be configurable as either input or output, as needed
by different product boards; single direction ones exist too.
- Most GPIOs can be accessed while holding spinlocks, but those accessed
through a serial bus normally can't. Some systems support both types.
On a given board each GPIO is used for one specific purpose like monitoring
MMC/SD card insertion/removal, detecting card writeprotect status, driving
a LED, configuring a transceiver, bitbanging a serial bus, poking a hardware
watchdog, sensing a switch, and so on.
GPIO conventions
================
Note that this is called a "convention" because you don't need to do it this
way, and it's no crime if you don't. There **are** cases where portability
is not the main issue; GPIOs are often used for the kind of board-specific
glue logic that may even change between board revisions, and can't ever be
used on a board that's wired differently. Only least-common-denominator
functionality can be very portable. Other features are platform-specific,
and that can be critical for glue logic.
Plus, this doesn't require any implementation framework, just an interface.
One platform might implement it as simple inline functions accessing chip
registers; another might implement it by delegating through abstractions
used for several very different kinds of GPIO controller. (There is some
optional code supporting such an implementation strategy, described later
in this document, but drivers acting as clients to the GPIO interface must
not care how it's implemented.)
That said, if the convention is supported on their platform, drivers should
use it when possible. Platforms must select GPIOLIB if GPIO functionality
is strictly required. Drivers that can't work without
standard GPIO calls should have Kconfig entries which depend on GPIOLIB. The
GPIO calls are available, either as "real code" or as optimized-away stubs,
when drivers use the include file:
#include <linux/gpio.h>
If you stick to this convention then it'll be easier for other developers to
see what your code is doing, and help maintain it.
Note that these operations include I/O barriers on platforms which need to
use them; drivers don't need to add them explicitly.
Identifying GPIOs
-----------------
GPIOs are identified by unsigned integers in the range 0..MAX_INT. That
reserves "negative" numbers for other purposes like marking signals as
"not available on this board", or indicating faults. Code that doesn't
touch the underlying hardware treats these integers as opaque cookies.
Platforms define how they use those integers, and usually #define symbols
for the GPIO lines so that board-specific setup code directly corresponds
to the relevant schematics. In contrast, drivers should only use GPIO
numbers passed to them from that setup code, using platform_data to hold
board-specific pin configuration data (along with other board specific
data they need). That avoids portability problems.
So for example one platform uses numbers 32-159 for GPIOs; while another
uses numbers 0..63 with one set of GPIO controllers, 64-79 with another
type of GPIO controller, and on one particular board 80-95 with an FPGA.
The numbers need not be contiguous; either of those platforms could also
use numbers 2000-2063 to identify GPIOs in a bank of I2C GPIO expanders.
If you want to initialize a structure with an invalid GPIO number, use
some negative number (perhaps "-EINVAL"); that will never be valid. To
test if such number from such a structure could reference a GPIO, you
may use this predicate:
int gpio_is_valid(int number);
A number that's not valid will be rejected by calls which may request
or free GPIOs (see below). Other numbers may also be rejected; for
example, a number might be valid but temporarily unused on a given board.
Whether a platform supports multiple GPIO controllers is a platform-specific
implementation issue, as are whether that support can leave "holes" in the space
of GPIO numbers, and whether new controllers can be added at runtime. Such issues
can affect things including whether adjacent GPIO numbers are both valid.
Using GPIOs
-----------
The first thing a system should do with a GPIO is allocate it, using
the gpio_request() call; see later.
One of the next things to do with a GPIO, often in board setup code when
setting up a platform_device using the GPIO, is mark its direction::
/* set as input or output, returning 0 or negative errno */
int gpio_direction_input(unsigned gpio);
int gpio_direction_output(unsigned gpio, int value);
The return value is zero for success, else a negative errno. It should
be checked, since the get/set calls don't have error returns and since
misconfiguration is possible. You should normally issue these calls from
a task context. However, for spinlock-safe GPIOs it's OK to use them
before tasking is enabled, as part of early board setup.
For output GPIOs, the value provided becomes the initial output value.
This helps avoid signal glitching during system startup.
For compatibility with legacy interfaces to GPIOs, setting the direction
of a GPIO implicitly requests that GPIO (see below) if it has not been
requested already. That compatibility is being removed from the optional
gpiolib framework.
Setting the direction can fail if the GPIO number is invalid, or when
that particular GPIO can't be used in that mode. It's generally a bad
idea to rely on boot firmware to have set the direction correctly, since
it probably wasn't validated to do more than boot Linux. (Similarly,
that board setup code probably needs to multiplex that pin as a GPIO,
and configure pullups/pulldowns appropriately.)
Spinlock-Safe GPIO access
-------------------------
Most GPIO controllers can be accessed with memory read/write instructions.
Those don't need to sleep, and can safely be done from inside hard
(nonthreaded) IRQ handlers and similar contexts.
Use the following calls to access such GPIOs,
for which gpio_cansleep() will always return false (see below)::
/* GPIO INPUT: return zero or nonzero */
int gpio_get_value(unsigned gpio);
/* GPIO OUTPUT */
void gpio_set_value(unsigned gpio, int value);
The values are boolean, zero for low, nonzero for high. When reading the
value of an output pin, the value returned should be what's seen on the
pin ... that won't always match the specified output value, because of
issues including open-drain signaling and output latencies.
The get/set calls have no error returns because "invalid GPIO" should have
been reported earlier from gpio_direction_*(). However, note that not all
platforms can read the value of output pins; those that can't should always
return zero. Also, using these calls for GPIOs that can't safely be accessed
without sleeping (see below) is an error.
Platform-specific implementations are encouraged to optimize the two
calls to access the GPIO value in cases where the GPIO number (and for
output, value) are constant. It's normal for them to need only a couple
of instructions in such cases (reading or writing a hardware register),
and not to need spinlocks. Such optimized calls can make bitbanging
applications a lot more efficient (in both space and time) than spending
dozens of instructions on subroutine calls.
GPIO access that may sleep
--------------------------
Some GPIO controllers must be accessed using message based busses like I2C
or SPI. Commands to read or write those GPIO values require waiting to
get to the head of a queue to transmit a command and get its response.
This requires sleeping, which can't be done from inside IRQ handlers.
Platforms that support this type of GPIO distinguish them from other GPIOs
by returning nonzero from this call (which requires a valid GPIO number,
which should have been previously allocated with gpio_request)::
int gpio_cansleep(unsigned gpio);
To access such GPIOs, a different set of accessors is defined::
/* GPIO INPUT: return zero or nonzero, might sleep */
int gpio_get_value_cansleep(unsigned gpio);
/* GPIO OUTPUT, might sleep */
void gpio_set_value_cansleep(unsigned gpio, int value);
Accessing such GPIOs requires a context which may sleep, for example
a threaded IRQ handler, and those accessors must be used instead of
spinlock-safe accessors without the cansleep() name suffix.
Other than the fact that these accessors might sleep, and will work
on GPIOs that can't be accessed from hardIRQ handlers, these calls act
the same as the spinlock-safe calls.
**IN ADDITION** calls to setup and configure such GPIOs must be made
from contexts which may sleep, since they may need to access the GPIO
controller chip too (These setup calls are usually made from board
setup or driver probe/teardown code, so this is an easy constraint.)::
gpio_direction_input()
gpio_direction_output()
gpio_request()
## gpio_request_one()
## gpio_request_array()
## gpio_free_array()
gpio_free()
gpio_set_debounce()
Claiming and Releasing GPIOs
----------------------------
To help catch system configuration errors, two calls are defined::
/* request GPIO, returning 0 or negative errno.
* non-null labels may be useful for diagnostics.
*/
int gpio_request(unsigned gpio, const char *label);
/* release previously-claimed GPIO */
void gpio_free(unsigned gpio);
Passing invalid GPIO numbers to gpio_request() will fail, as will requesting
GPIOs that have already been claimed with that call. The return value of
gpio_request() must be checked. You should normally issue these calls from
a task context. However, for spinlock-safe GPIOs it's OK to request GPIOs
before tasking is enabled, as part of early board setup.
These calls serve two basic purposes. One is marking the signals which
are actually in use as GPIOs, for better diagnostics; systems may have
several hundred potential GPIOs, but often only a dozen are used on any
given board. Another is to catch conflicts, identifying errors when
(a) two or more drivers wrongly think they have exclusive use of that
signal, or (b) something wrongly believes it's safe to remove drivers
needed to manage a signal that's in active use. That is, requesting a
GPIO can serve as a kind of lock.
Some platforms may also use knowledge about what GPIOs are active for
power management, such as by powering down unused chip sectors and, more
easily, gating off unused clocks.
For GPIOs that use pins known to the pinctrl subsystem, that subsystem should
be informed of their use; a gpiolib driver's .request() operation may call
pinctrl_gpio_request(), and a gpiolib driver's .free() operation may call
pinctrl_gpio_free(). The pinctrl subsystem allows a pinctrl_gpio_request()
to succeed concurrently with a pin or pingroup being "owned" by a device for
pin multiplexing.
Any programming of pin multiplexing hardware that is needed to route the
GPIO signal to the appropriate pin should occur within a GPIO driver's
.direction_input() or .direction_output() operations, and occur after any
setup of an output GPIO's value. This allows a glitch-free migration from a
pin's special function to GPIO. This is sometimes required when using a GPIO
to implement a workaround on signals typically driven by a non-GPIO HW block.
Some platforms allow some or all GPIO signals to be routed to different pins.
Similarly, other aspects of the GPIO or pin may need to be configured, such as
pullup/pulldown. Platform software should arrange that any such details are
configured prior to gpio_request() being called for those GPIOs, e.g. using
the pinctrl subsystem's mapping table, so that GPIO users need not be aware
of these details.
Also note that it's your responsibility to have stopped using a GPIO
before you free it.
Considering in most cases GPIOs are actually configured right after they
are claimed, three additional calls are defined::
/* request a single GPIO, with initial configuration specified by
* 'flags', identical to gpio_request() wrt other arguments and
* return value
*/
int gpio_request_one(unsigned gpio, unsigned long flags, const char *label);
/* request multiple GPIOs in a single call
*/
int gpio_request_array(struct gpio *array, size_t num);
/* release multiple GPIOs in a single call
*/
void gpio_free_array(struct gpio *array, size_t num);
where 'flags' is currently defined to specify the following properties:
* GPIOF_DIR_IN - to configure direction as input
* GPIOF_DIR_OUT - to configure direction as output
* GPIOF_INIT_LOW - as output, set initial level to LOW
* GPIOF_INIT_HIGH - as output, set initial level to HIGH
* GPIOF_OPEN_DRAIN - gpio pin is open drain type.
* GPIOF_OPEN_SOURCE - gpio pin is open source type.
* GPIOF_EXPORT_DIR_FIXED - export gpio to sysfs, keep direction
* GPIOF_EXPORT_DIR_CHANGEABLE - also export, allow changing direction
since GPIOF_INIT_* are only valid when configured as output, so group valid
combinations as:
* GPIOF_IN - configure as input
* GPIOF_OUT_INIT_LOW - configured as output, initial level LOW
* GPIOF_OUT_INIT_HIGH - configured as output, initial level HIGH
When setting the flag as GPIOF_OPEN_DRAIN then it will assume that pins is
open drain type. Such pins will not be driven to 1 in output mode. It is
require to connect pull-up on such pins. By enabling this flag, gpio lib will
make the direction to input when it is asked to set value of 1 in output mode
to make the pin HIGH. The pin is make to LOW by driving value 0 in output mode.
When setting the flag as GPIOF_OPEN_SOURCE then it will assume that pins is
open source type. Such pins will not be driven to 0 in output mode. It is
require to connect pull-down on such pin. By enabling this flag, gpio lib will
make the direction to input when it is asked to set value of 0 in output mode
to make the pin LOW. The pin is make to HIGH by driving value 1 in output mode.
In the future, these flags can be extended to support more properties.
Further more, to ease the claim/release of multiple GPIOs, 'struct gpio' is
introduced to encapsulate all three fields as::
struct gpio {
unsigned gpio;
unsigned long flags;
const char *label;
};
A typical example of usage::
static struct gpio leds_gpios[] = {
{ 32, GPIOF_OUT_INIT_HIGH, "Power LED" }, /* default to ON */
{ 33, GPIOF_OUT_INIT_LOW, "Green LED" }, /* default to OFF */
{ 34, GPIOF_OUT_INIT_LOW, "Red LED" }, /* default to OFF */
{ 35, GPIOF_OUT_INIT_LOW, "Blue LED" }, /* default to OFF */
{ ... },
};
err = gpio_request_one(31, GPIOF_IN, "Reset Button");
if (err)
...
err = gpio_request_array(leds_gpios, ARRAY_SIZE(leds_gpios));
if (err)
...
gpio_free_array(leds_gpios, ARRAY_SIZE(leds_gpios));
GPIOs mapped to IRQs
--------------------
GPIO numbers are unsigned integers; so are IRQ numbers. These make up
two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can
map between them using calls like::
/* map GPIO numbers to IRQ numbers */
int gpio_to_irq(unsigned gpio);
/* map IRQ numbers to GPIO numbers (avoid using this) */
int irq_to_gpio(unsigned irq);
Those return either the corresponding number in the other namespace, or
else a negative errno code if the mapping can't be done. (For example,
some GPIOs can't be used as IRQs.) It is an unchecked error to use a GPIO
number that wasn't set up as an input using gpio_direction_input(), or
to use an IRQ number that didn't originally come from gpio_to_irq().
These two mapping calls are expected to cost on the order of a single
addition or subtraction. They're not allowed to sleep.
Non-error values returned from gpio_to_irq() can be passed to request_irq()
or free_irq(). They will often be stored into IRQ resources for platform
devices, by the board-specific initialization code. Note that IRQ trigger
options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are
system wakeup capabilities.
Non-error values returned from irq_to_gpio() would most commonly be used
with gpio_get_value(), for example to initialize or update driver state
when the IRQ is edge-triggered. Note that some platforms don't support
this reverse mapping, so you should avoid using it.
Emulating Open Drain Signals
----------------------------
Sometimes shared signals need to use "open drain" signaling, where only the
low signal level is actually driven. (That term applies to CMOS transistors;
"open collector" is used for TTL.) A pullup resistor causes the high signal
level. This is sometimes called a "wire-AND"; or more practically, from the
negative logic (low=true) perspective this is a "wire-OR".
One common example of an open drain signal is a shared active-low IRQ line.
Also, bidirectional data bus signals sometimes use open drain signals.
Some GPIO controllers directly support open drain outputs; many don't. When
you need open drain signaling but your hardware doesn't directly support it,
there's a common idiom you can use to emulate it with any GPIO pin that can
be used as either an input or an output:
LOW: gpio_direction_output(gpio, 0) ... this drives the signal
and overrides the pullup.
HIGH: gpio_direction_input(gpio) ... this turns off the output,
so the pullup (or some other device) controls the signal.
If you are "driving" the signal high but gpio_get_value(gpio) reports a low
value (after the appropriate rise time passes), you know some other component
is driving the shared signal low. That's not necessarily an error. As one
common example, that's how I2C clocks are stretched: a slave that needs a
slower clock delays the rising edge of SCK, and the I2C master adjusts its
signaling rate accordingly.
GPIO controllers and the pinctrl subsystem
------------------------------------------
A GPIO controller on a SOC might be tightly coupled with the pinctrl
subsystem, in the sense that the pins can be used by other functions
together with an optional gpio feature. We have already covered the
case where e.g. a GPIO controller need to reserve a pin or set the
direction of a pin by calling any of::
pinctrl_gpio_request()
pinctrl_gpio_free()
pinctrl_gpio_direction_input()
pinctrl_gpio_direction_output()
But how does the pin control subsystem cross-correlate the GPIO
numbers (which are a global business) to a certain pin on a certain
pin controller?
This is done by registering "ranges" of pins, which are essentially
cross-reference tables. These are described in
Documentation/driver-api/pinctl.rst
While the pin allocation is totally managed by the pinctrl subsystem,
gpio (under gpiolib) is still maintained by gpio drivers. It may happen
that different pin ranges in a SoC is managed by different gpio drivers.
This makes it logical to let gpio drivers announce their pin ranges to
the pin ctrl subsystem before it will call 'pinctrl_gpio_request' in order
to request the corresponding pin to be prepared by the pinctrl subsystem
before any gpio usage.
For this, the gpio controller can register its pin range with pinctrl
subsystem. There are two ways of doing it currently: with or without DT.
For with DT support refer to Documentation/devicetree/bindings/gpio/gpio.txt.
For non-DT support, user can call gpiochip_add_pin_range() with appropriate
parameters to register a range of gpio pins with a pinctrl driver. For this
exact name string of pinctrl device has to be passed as one of the
argument to this routine.
What do these conventions omit?
===============================
One of the biggest things these conventions omit is pin multiplexing, since
this is highly chip-specific and nonportable. One platform might not need
explicit multiplexing; another might have just two options for use of any
given pin; another might have eight options per pin; another might be able
to route a given GPIO to any one of several pins. (Yes, those examples all
come from systems that run Linux today.)
Related to multiplexing is configuration and enabling of the pullups or
pulldowns integrated on some platforms. Not all platforms support them,
or support them in the same way; and any given board might use external
pullups (or pulldowns) so that the on-chip ones should not be used.
(When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.)
Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a
platform-specific issue, as are models like (not) having a one-to-one
correspondence between configurable pins and GPIOs.
There are other system-specific mechanisms that are not specified here,
like the aforementioned options for input de-glitching and wire-OR output.
Hardware may support reading or writing GPIOs in gangs, but that's usually
configuration dependent: for GPIOs sharing the same bank. (GPIOs are
commonly grouped in banks of 16 or 32, with a given SOC having several such
banks.) Some systems can trigger IRQs from output GPIOs, or read values
from pins not managed as GPIOs. Code relying on such mechanisms will
necessarily be nonportable.
Dynamic definition of GPIOs is not currently standard; for example, as
a side effect of configuring an add-on board with some GPIO expanders.
GPIO implementor's framework (OPTIONAL)
=======================================
As noted earlier, there is an optional implementation framework making it
easier for platforms to support different kinds of GPIO controller using
the same programming interface. This framework is called "gpiolib".
As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file
will be found there. That will list all the controllers registered through
this framework, and the state of the GPIOs currently in use.
Controller Drivers: gpio_chip
-----------------------------
In this framework each GPIO controller is packaged as a "struct gpio_chip"
with information common to each controller of that type:
- methods to establish GPIO direction
- methods used to access GPIO values
- flag saying whether calls to its methods may sleep
- optional debugfs dump method (showing extra state like pullup config)
- label for diagnostics
There is also per-instance data, which may come from device.platform_data:
the number of its first GPIO, and how many GPIOs it exposes.
The code implementing a gpio_chip should support multiple instances of the
controller, possibly using the driver model. That code will configure each
gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be
rare; use gpiochip_remove() when it is unavoidable.
Most often a gpio_chip is part of an instance-specific structure with state
not exposed by the GPIO interfaces, such as addressing, power management,
and more. Chips such as codecs will have complex non-GPIO state.
Any debugfs dump method should normally ignore signals which haven't been
requested as GPIOs. They can use gpiochip_is_requested(), which returns
either NULL or the label associated with that GPIO when it was requested.
Platform Support
----------------
To force-enable this framework, a platform's Kconfig will "select" GPIOLIB,
else it is up to the user to configure support for GPIO.
It may also provide a custom value for ARCH_NR_GPIOS, so that it better
reflects the number of GPIOs in actual use on that platform, without
wasting static table space. (It should count both built-in/SoC GPIOs and
also ones on GPIO expanders.
If neither of these options are selected, the platform does not support
GPIOs through GPIO-lib and the code cannot be enabled by the user.
Trivial implementations of those functions can directly use framework
code, which always dispatches through the gpio_chip::
#define gpio_get_value __gpio_get_value
#define gpio_set_value __gpio_set_value
#define gpio_cansleep __gpio_cansleep
Fancier implementations could instead define those as inline functions with
logic optimizing access to specific SOC-based GPIOs. For example, if the
referenced GPIO is the constant "12", getting or setting its value could
cost as little as two or three instructions, never sleeping. When such an
optimization is not possible those calls must delegate to the framework
code, costing at least a few dozen instructions. For bitbanged I/O, such
instruction savings can be significant.
For SOCs, platform-specific code defines and registers gpio_chip instances
for each bank of on-chip GPIOs. Those GPIOs should be numbered/labeled to
match chip vendor documentation, and directly match board schematics. They
may well start at zero and go up to a platform-specific limit. Such GPIOs
are normally integrated into platform initialization to make them always be
available, from arch_initcall() or earlier; they can often serve as IRQs.
Board Support
-------------
For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi
function devices, FPGAs or CPLDs -- most often board-specific code handles
registering controller devices and ensures that their drivers know what GPIO
numbers to use with gpiochip_add(). Their numbers often start right after
platform-specific GPIOs.
For example, board setup code could create structures identifying the range
of GPIOs that chip will expose, and passes them to each GPIO expander chip
using platform_data. Then the chip driver's probe() routine could pass that
data to gpiochip_add().
Initialization order can be important. For example, when a device relies on
an I2C-based GPIO, its probe() routine should only be called after that GPIO
becomes available. That may mean the device should not be registered until
calls for that GPIO can work. One way to address such dependencies is for
such gpio_chip controllers to provide setup() and teardown() callbacks to
board specific code; those board specific callbacks would register devices
once all the necessary resources are available, and remove them later when
the GPIO controller device becomes unavailable.
Sysfs Interface for Userspace (OPTIONAL)
========================================
Platforms which use the "gpiolib" implementors framework may choose to
configure a sysfs user interface to GPIOs. This is different from the
debugfs interface, since it provides control over GPIO direction and
value instead of just showing a gpio state summary. Plus, it could be
present on production systems without debugging support.
Given appropriate hardware documentation for the system, userspace could
know for example that GPIO #23 controls the write protect line used to
protect boot loader segments in flash memory. System upgrade procedures
may need to temporarily remove that protection, first importing a GPIO,
then changing its output state, then updating the code before re-enabling
the write protection. In normal use, GPIO #23 would never be touched,
and the kernel would have no need to know about it.
Again depending on appropriate hardware documentation, on some systems
userspace GPIO can be used to determine system configuration data that
standard kernels won't know about. And for some tasks, simple userspace
GPIO drivers could be all that the system really needs.
Note that standard kernel drivers exist for common "LEDs and Buttons"
GPIO tasks: "leds-gpio" and "gpio_keys", respectively. Use those
instead of talking directly to the GPIOs; they integrate with kernel
frameworks better than your userspace code could.
Paths in Sysfs
--------------
There are three kinds of entry in /sys/class/gpio:
- Control interfaces used to get userspace control over GPIOs;
- GPIOs themselves; and
- GPIO controllers ("gpio_chip" instances).
That's in addition to standard files including the "device" symlink.
The control interfaces are write-only:
/sys/class/gpio/
"export" ... Userspace may ask the kernel to export control of
a GPIO to userspace by writing its number to this file.
Example: "echo 19 > export" will create a "gpio19" node
for GPIO #19, if that's not requested by kernel code.
"unexport" ... Reverses the effect of exporting to userspace.
Example: "echo 19 > unexport" will remove a "gpio19"
node exported using the "export" file.
GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
and have the following read/write attributes:
/sys/class/gpio/gpioN/
"direction" ... reads as either "in" or "out". This value may
normally be written. Writing as "out" defaults to
initializing the value as low. To ensure glitch free
operation, values "low" and "high" may be written to
configure the GPIO as an output with that initial value.
Note that this attribute *will not exist* if the kernel
doesn't support changing the direction of a GPIO, or
it was exported by kernel code that didn't explicitly
allow userspace to reconfigure this GPIO's direction.
"value" ... reads as either 0 (low) or 1 (high). If the GPIO
is configured as an output, this value may be written;
any nonzero value is treated as high.
If the pin can be configured as interrupt-generating interrupt
and if it has been configured to generate interrupts (see the
description of "edge"), you can poll(2) on that file and
poll(2) will return whenever the interrupt was triggered. If
you use poll(2), set the events POLLPRI and POLLERR. If you
use select(2), set the file descriptor in exceptfds. After
poll(2) returns, either lseek(2) to the beginning of the sysfs
file and read the new value or close the file and re-open it
to read the value.
"edge" ... reads as either "none", "rising", "falling", or
"both". Write these strings to select the signal edge(s)
that will make poll(2) on the "value" file return.
This file exists only if the pin can be configured as an
interrupt generating input pin.
"active_low" ... reads as either 0 (false) or 1 (true). Write
any nonzero value to invert the value attribute both
for reading and writing. Existing and subsequent
poll(2) support configuration via the edge attribute
for "rising" and "falling" edges will follow this
setting.
GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
controller implementing GPIOs starting at #42) and have the following
read-only attributes:
/sys/class/gpio/gpiochipN/
"base" ... same as N, the first GPIO managed by this chip
"label" ... provided for diagnostics (not always unique)
"ngpio" ... how many GPIOs this manges (N to N + ngpio - 1)
Board documentation should in most cases cover what GPIOs are used for
what purposes. However, those numbers are not always stable; GPIOs on
a daughtercard might be different depending on the base board being used,
or other cards in the stack. In such cases, you may need to use the
gpiochip nodes (possibly in conjunction with schematics) to determine
the correct GPIO number to use for a given signal.
Exporting from Kernel code
--------------------------
Kernel code can explicitly manage exports of GPIOs which have already been
requested using gpio_request()::
/* export the GPIO to userspace */
int gpio_export(unsigned gpio, bool direction_may_change);
/* reverse gpio_export() */
void gpio_unexport();
/* create a sysfs link to an exported GPIO node */
int gpio_export_link(struct device *dev, const char *name,
unsigned gpio)
After a kernel driver requests a GPIO, it may only be made available in
the sysfs interface by gpio_export(). The driver can control whether the
signal direction may change. This helps drivers prevent userspace code
from accidentally clobbering important system state.
This explicit exporting can help with debugging (by making some kinds
of experiments easier), or can provide an always-there interface that's
suitable for documenting as part of a board support package.
After the GPIO has been exported, gpio_export_link() allows creating
symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can
use this to provide the interface under their own device in sysfs with
a descriptive name.
API Reference
=============
The functions listed in this section are deprecated. The GPIO descriptor based
API should be used in new code.
.. kernel-doc:: drivers/gpio/gpiolib-legacy.c
:export:

2
Documentation/driver-api/index.rst
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@ -44,7 +44,7 @@ available subsections can be seen below.
uio-howto
firmware/index
pinctl
gpio
gpio/index
misc_devices
dmaengine/index
slimbus