Hi

Currently all stages that need cbmem need an implementation of a
cbmem_top function. On platforms with fully open source coreboot code it
is generally not a problem to link in all stages the code reading the
hardware registers to figur out where the top of lower memory is. On FSP
platforms this proves to be painful and using the value provided by the
FSP-M HOB is to be preferred. Next stages don't have access to this
variable since CAR is gone. EBDA is used to pass it on to next stages.

The problem with this is that EBDA also need to be written on S3 as one
cannot assume it to be still there. Writing things on S3 is always
fragile as it could overwrite other things set up by something else.

One possible solution is to back up the area it's going to write to in
romstage in cbmem and restoring it on the ramstage, but writing to cbmem
on S3 resume is just moving the problem to a different place and likely
makes things even worse...

One other idea I could come up with is to make the cbmem_top pointer an
argument to the relocatable stages needing it (postcar and ramstage).
This would unify all cbmem_top implementation in those stages and should
be more robust on S3 resume as on S3 resume the ramstage is typically
fetched from RAM (cbmem or tseg stage cache).

Any thoughts?

Arthur Heymans
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