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doc: spl: Add info regarding memory reservation

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Add details regarding scheme which need to be followed in SPL and
further stages for those regions which need to be preserved across
bootstages.

Signed-off-by: Devarsh Thakkar <devarsht@ti.com>
Reviewed-by: Simon Glass <sjg@chromium>
This commit is contained in:
Devarsh Thakkar 2023-12-05 21:25:23 +05:30 committed by Tom Rini
parent b557e9f06e
commit aaeb330cac
1 changed files with 28 additions and 0 deletions
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doc/develop/spl.rst
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@ -173,3 +173,31 @@ cflow will spit out a number of warnings as it does not parse
the config files and picks functions based on #ifdef. Parsing the '.i'
files instead introduces another set of headaches. These warnings are
not usually important to understanding the flow, however.
Reserving memory in SPL
-----------------------
If memory needs to be reserved in RAM during SPL stage with the requirement that
the SPL reserved memory remains preserved across further boot stages too
then it needs to be reserved mandatorily starting from end of RAM. This is to
ensure that further stages can simply skip this region before carrying out
further reservations or updating the relocation address.
Also out of these regions which are to be preserved across further stages of
boot, video framebuffer memory region must be reserved first starting from
end of RAM for which helper function spl_reserve_video_from_ram_top is provided
which makes sure that video memory is placed at top of reservation area with
further reservations below it.
The corresponding information of reservation for those regions can be passed to
further boot stages using a bloblist. For e.g. the information for
framebuffer area reserved by SPL can be passed onto U-boot using
BLOBLISTT_U_BOOT_VIDEO.
The further boot stages need to parse each of the bloblist passed from SPL stage
starting from video bloblist and skip this whole SPL reserved memory area from
end of RAM as per the bloblists received, before carrying out further
reservations or updating the relocation address. For e.g, U-boot proper uses
function "setup_relocaddr_from_bloblist" to parse the bloblists passed from
previous stage and skip the memory reserved from previous stage accordingly.