soc/intel/mtl: Skip RW CBFS ucode update if RO is locked

This patch eliminates coreboot from loading microcode from RW CBFS
(when the RO descriptor is locked, which indicates a fixed RO image)
because the kernel can already patch the microcode on BSPs and APs
while booting to OS.

This may be a chance to lower the burden on the AP FW side because
patching microcode on in-field devices is subject to firmware updates,
which are rarely published and, if required, must go through the
firmware qualification testing procedure (which is costly, unlike
kernel updates for ucode updates).

1. The FIT loads the necessary microcode from the RO during reset.
2. Reloading microcode from RW CBFS impacts boot time
   (~60ms, core-dependent).
3. The kernel can still load microcode updates.

ChromeOS devices leverage RO+RW-A/RW-B booting. The RO's microcode is
sufficient for initial boot, and the kernel can apply updates later.

BUG=none
TEST=Verified boot optimization; in-field devices skip RW-CBFS microcode
loading when RO is locked.

Change-Id: Ia859809970406fca3fa14e6fa8e766ab16d94c8a
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80567
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: YH Lin <yueherngl@google.com>

0 files changed, 0 insertions, 0 deletions