[M] Change in coreboot[master]: arch/x86/postcar_loader: Don't add postcar to stage cache

[Reka Norman (Code Review)]

Attention is currently required from: Subrata Banik, Reka Norman, Nick Vaccaro, Julius Werner, Angel Pons, Karthikeyan Ramasubramanian.

Hello build bot (Jenkins), Subrata Banik, Kangheui Won, Nick Vaccaro, Julius Werner, Angel Pons, Arthur Heymans, Karthikeyan Ramasubramanian,

I'd like you to reexamine a change. Please visit

https://review.coreboot.org/c/coreboot/+/68641

to look at the new patch set (#3).

Change subject: arch/x86/postcar_loader: Don't add postcar to stage cache
......................................................................

arch/x86/postcar_loader: Don't add postcar to stage cache

In romstage, CBMEM and TSEG are not yet cached, so reading/writing to
the stage cache is slow. This means there isn't really any benefit to
adding postcar to the stage cache:
 - During boot, adding it to the stage cache adds a few ms of boot time,
   since writing to uncached memory is slow.
 - During S3 resume, reading from stage cache takes roughly the same
   time as reading from SPI, so we get little or no savings anyway.

E.g. on nivviks, this change saves 6 ms of boot time and 2 ms of S3
resume time.

Adding postcar to stage cache:
Boot:
  rmodule_stage_load()     - 4 ms
  stage_cache_add()        - 6 ms
S3 resume:
  stage_cache_load_stage() - 6 ms

Not adding postcar to stage cache:
Boot:
  rmodule_stage_load()     - 4 ms
S3 resume:
  rmodule_stage_load()     - 4 ms

Savings:
Boot time      - 6 ms
S3 resume time - 2 ms

------------------------------------
Savings on other devices:

nivviks anahera kindred treeya
rmodule_stage_load()          4.0     1.4     0.8    2.8
stage_cache_add()             6.0     2.8     1.5    2.0
stage_cache_load_stage()      6.0     2.8     1.5    2.0

Boot time savings             6.0     2.8     1.5    2.0
S3 resume time savings        2.0     1.4     0.7   -0.8

Note AMD platforms from zork onwards don't use postcar due to PSP
verstage.

BUG=b:247940538
TEST=On nivviks, boot and check S3 resume still works. Add logging to
check postcar is added to stage cache when POSTCAR_IN_STAGE_CACHE is
selected, and not added otherwise.

Change-Id: I3614c0874a6c71d13606b0b782ea445692d88bb1
Signed-off-by: Reka Norman 
---
M src/arch/x86/Kconfig
M src/arch/x86/postcar_loader.c
2 files changed, 70 insertions(+), 2 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/41/68641/3