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PM / QoS: Introcuce latency tolerance device PM QoS type

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Add a new latency tolerance device PM QoS type to be use for
specifying active state (RPM_ACTIVE) memory access (DMA) latency
tolerance requirements for devices.  It may be used to prevent
hardware from choosing overly aggressive energy-saving operation
modes (causing too much latency to appear) for the whole platform.

This feature reqiures hardware support, so it only will be
available for devices having a new .set_latency_tolerance()
callback in struct dev_pm_info populated, in which case the
routine pointed to by it should implement whatever is necessary
to transfer the effective requirement value to the hardware.

Whenever the effective latency tolerance changes for the device,
its .set_latency_tolerance() callback will be executed and the
effective value will be passed to it.  If that value is negative,
which means that the list of latency tolerance requirements for
the device is empty, the callback is expected to switch the
underlying hardware latency tolerance control mechanism to an
autonomous mode if available.  If that value is PM_QOS_LATENCY_ANY,
in turn, and the hardware supports a special "no requirement"
setting, the callback is expected to use it.  That allows software
to prevent the hardware from automatically updating the device's
latency tolerance in response to its power state changes (e.g. during
transitions from D3cold to D0), which generally may be done in the
autonomous latency tolerance control mode.

If .set_latency_tolerance() is present for the device, a new
pm_qos_latency_tolerance_us attribute will be present in the
devivce's power directory in sysfs.  Then, user space can use
that attribute to specify its latency tolerance requirement for
the device, if any.  Writing "any" to it means "no requirement, but
do not let the hardware control latency tolerance" and writing
"auto" to it allows the hardware to be switched to the autonomous
mode if there are no other requirements from the kernel side in the
device's list.

This changeset includes a fix from Mika Westerberg.

Signed-off-by: Rafael J. Wysocki <[email protected]>
This commit is contained in:
Rafael J. Wysocki 2014-02-11 00:35:38 +01:00
commit 2d984ad132
7 changed files with 277 additions and 44 deletions
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59
Documentation/power/pm_qos_interface.txt
View file

@ -88,17 +88,19 @@ node.
2. PM QoS per-device latency and flags framework
For each device, there are two lists of PM QoS requests. One is maintained
along with the aggregated target of resume latency value and the other is for
PM QoS flags. Values are updated in response to changes of the request list.
For each device, there are three lists of PM QoS requests. Two of them are
maintained along with the aggregated targets of resume latency and active
state latency tolerance (in microseconds) and the third one is for PM QoS flags.
Values are updated in response to changes of the request list.
Target resume latency value is simply the minimum of the request values held in
the parameter list elements. The PM QoS flags aggregate value is a gather
(bitwise OR) of all list elements' values. Two device PM QoS flags are defined
currently: PM_QOS_FLAG_NO_POWER_OFF and PM_QOS_FLAG_REMOTE_WAKEUP.
The target values of resume latency and active state latency tolerance are
simply the minimum of the request values held in the parameter list elements.
The PM QoS flags aggregate value is a gather (bitwise OR) of all list elements'
values. Two device PM QoS flags are defined currently: PM_QOS_FLAG_NO_POWER_OFF
and PM_QOS_FLAG_REMOTE_WAKEUP.
Note: the aggregated target value is implemented in such a way that reading the
aggregated value does not require any locking mechanism.
Note: The aggregated target values are implemented in such a way that reading
the aggregated value does not require any locking mechanism.
From kernel mode the use of this interface is the following:
@ -177,3 +179,42 @@ The callback is called when the aggregated value for any device is changed
int dev_pm_qos_remove_global_notifier(notifier):
Removes the notification callback function from the global notification tree
of the framework.
Active state latency tolerance
This device PM QoS type is used to support systems in which hardware may switch
to energy-saving operation modes on the fly. In those systems, if the operation
mode chosen by the hardware attempts to save energy in an overly aggressive way,
it may cause excess latencies to be visible to software, causing it to miss
certain protocol requirements or target frame or sample rates etc.
If there is a latency tolerance control mechanism for a given device available
to software, the .set_latency_tolerance callback in that device's dev_pm_info
structure should be populated. The routine pointed to by it is should implement
whatever is necessary to transfer the effective requirement value to the
hardware.
Whenever the effective latency tolerance changes for the device, its
.set_latency_tolerance() callback will be executed and the effective value will
be passed to it. If that value is negative, which means that the list of
latency tolerance requirements for the device is empty, the callback is expected
to switch the underlying hardware latency tolerance control mechanism to an
autonomous mode if available. If that value is PM_QOS_LATENCY_ANY, in turn, and
the hardware supports a special "no requirement" setting, the callback is
expected to use it. That allows software to prevent the hardware from
automatically updating the device's latency tolerance in response to its power
state changes (e.g. during transitions from D3cold to D0), which generally may
be done in the autonomous latency tolerance control mode.
If .set_latency_tolerance() is present for the device, sysfs attribute
pm_qos_latency_tolerance_us will be present in the devivce's power directory.
Then, user space can use that attribute to specify its latency tolerance
requirement for the device, if any. Writing "any" to it means "no requirement,
but do not let the hardware control latency tolerance" and writing "auto" to it
allows the hardware to be switched to the autonomous mode if there are no other
requirements from the kernel side in the device's list.
Kernel code can use the functions described above along with the
DEV_PM_QOS_LATENCY_TOLERANCE device PM QoS type to add, remove and update
latency tolerance requirements for devices.