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arm64: Add simple earlyprintk support

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This patch adds support for "earlyprintk=" parameter on the kernel
command line. The format is:

  earlyprintk=<name>[,<addr>][,<options>]

where <name> is the name of the (UART) device, e.g. "pl011", <addr> is
the I/O address. The <options> aren't currently used.

The mapping of the earlyprintk device is done very early during kernel
boot and there are restrictions on which functions it can call. A
special early_io_map() function is added which creates the mapping from
the pre-defined EARLY_IOBASE to the device I/O address passed via the
kernel parameter. The pgd entry corresponding to EARLY_IOBASE is
pre-populated in head.S during kernel boot.

Only PL011 is currently supported and it is assumed that the interface
is already initialised by the boot loader before the kernel is started.

Signed-off-by: Catalin Marinas <[email protected]>
Acked-by: Arnd Bergmann <[email protected]>
This commit is contained in:
Catalin Marinas 2012-10-23 14:55:08 +01:00
commit 2475ff9d2c
9 changed files with 187 additions and 2 deletions
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Documentation/arm64/memory.txt
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@ -35,6 +35,8 @@ ffffffbc00000000 ffffffbdffffffff 8GB vmemmap
ffffffbe00000000 ffffffbffbbfffff ~8GB [guard, future vmmemap]
ffffffbffbc00000 ffffffbffbdfffff 2MB earlyprintk device
ffffffbffbe00000 ffffffbffbe0ffff 64KB PCI I/O space
ffffffbbffff0000 ffffffbcffffffff ~2MB [guard]