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>
diff --git a/src/soc/amd/stoneyridge/monotonic_timer.c b/src/soc/amd/stoneyridge/monotonic_timer.c
index 06e0c93..bd1e4e7 100644
--- a/src/soc/amd/stoneyridge/monotonic_timer.c
+++ b/src/soc/amd/stoneyridge/monotonic_timer.c
@@ -12,6 +12,8 @@
mono_time_set_usecs(mt, timestamp_get());
}
+/* The TSC has a non-constant rate before the microcode update is applied, so it can't be used
+ in timestamp_get before that. Instead, the Performance Time Stamp Counter is used. */
uint64_t timestamp_get(void)
{
msr_t msr;