x86: Add ramstage CBFS cache scratchpad support

Having a CBFS cache scratchpad offers a generic way to decompress CBFS
files through the cbfs_map() function without having to reserve a
per-file specific memory region.

This commit introduces the x86 `RAMSTAGE_CBFS_CACHE_SIZE' Kconfig to
set a ramstage CBFS cache size.  A cache size of zero disables the
CBFS cache feature.  The default size is 16 KB which seems a
reasonable minimal value large enough to satisfy basic needs such as
the decompression of a small configuration file.  This setting can be
adjusted depending on the platform needs and capabilities.

To support S3 suspend/resume use-case, the CBFS cache memory cannot be
released to the operating system. There are two options to meet this
requirement:

1. Define a static CBFS cache buffer (located in the .bss section)
2. Create a new CBMEM entry

Option #2 seems more powerful but considering that:

1. The CBFS cache is actually not a cache but just a scratch pad
   designed to be isolated between stages
2. postcar is a very short stage not really needing CBFS cache
3. The static initialization of the `cbfs_cache' global
   variable (cf. src/lib/cbfs.c) offers a simple and robust design

=> It is simpler to use a static buffer and limit the support to
ramstage.

Since some AMD SoCs (cf. `SOC_AMD_COMMON_BLOCK_NONCAR' Kconfig) define
a `_cbfs_cache' region, an extra `POSTRAM_CBFS_CACHE_IN_BSS' Kconfig
must be set to enable the use of a static buffer as the CBFS cache
scratchpad.

TEST=Decompression of vbt.bin in ramstage on rex using cbfs_map()

Change-Id: I7fbb1b51cda9f84842992e365b16c5ced1010b89
Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77885
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

0 files changed, 0 insertions, 0 deletions