soc/amd/stoneyridge/monotonic_time: add comment that we can't use TSC

Due to a non-constant TSC rate before the microcode update is applied,
the Performance Time Stamp Counter is used instead. To clarify this, add
a comment to the timestamp_get implementation. See commit 24079323d4d8
("soc/amd/stoneyridge: provide alternate monotonic timer") and the
description of the TscInvariant bit in CPUID Fn8000_0007_EDX Advanced
Power Management Information in the public version of BKDG #55072 Rev
3.09 for more details.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I824b372c36fa6f3eb912469b235a9474f6a58ff5
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74018
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>

0 files changed, 0 insertions, 0 deletions