Commit 05bdebd5 authored by Max Filippov's avatar Max Filippov
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xtensa: document boot parameter passing 



Add booting.txt to Documentation/xtensa with description of boot
parameters representation and passing to the kernel.

Signed-off-by: default avatarMax Filippov <jcmvbkbc@gmail.com>
parent 8834f560

Documentation/xtensa/booting.txt

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Passing boot parameters to the kernel.



Boot parameters are represented as a TLV list in the memory. Please see

arch/xtensa/include/asm/bootparam.h for definition of the bp_tag structure and

tag value constants. First entry in the list must have type BP_TAG_FIRST, last

entry must have type BP_TAG_LAST. The address of the first list entry is

passed to the kernel in the register a2. The address type depends on MMU type:

- For configurations without MMU, with region protection or with MPU the

  address must be the physical address.

- For configurations with region translarion MMU or with MMUv3 and CONFIG_MMU=n

  the address must be a valid address in the current mapping. The kernel will

  not change the mapping on its own.

- For configurations with MMUv2 the address must be a virtual address in the

  default virtual mapping (0xd0000000..0xffffffff).

- For configurations with MMUv3 and CONFIG_MMU=y the address may be either a

  virtual or physical address. In either case it must be within the default

  virtual mapping. It is considered physical if it is within the range of

  physical addresses covered by the default KSEG mapping (XCHAL_KSEG_PADDR..

  XCHAL_KSEG_PADDR + XCHAL_KSEG_SIZE), otherwise it is considered virtual.