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soc/intel/pmc: add a note about legacy OSes/payloads to PM Timer Kconfig 

Since ACPI 5.0A it is allowed to disable the ACPI Timer, when the
according FADT flag `ACPI_FADT_PLATFORM_CLOCK` is unset.

Starting with Skylake, most platforms (except Xeon-SP) support PM Timer
emulation, so even legacy OSes and payloads should work fine with the
hardware PM Timer disabled. However, when the `TMR_STS` functionality
is required, some legacy OSes might still not work (properly).

Add a note about this to the Kconfig help.

Change-Id: I53f1814113902124779ed85da030374439570688
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58018
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Lance Zhao
This commit is contained in:
Michael Niewöhner 2021-09-25 00:10:30 +02:00 committed by Nico Huber
parent 5c259642c2
commit c47fc40022
1 changed files with 7 additions and 0 deletions
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7
src/soc/intel/common/block/pmc/Kconfig
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@ -76,4 +76,11 @@ config USE_PM_ACPI_TIMER
Disabling this option also stops the hardware TCO timer and makes
the TCO watchdog unavailable.
Note: On platforms without uCode PM Timer emulation, legacy OSes
or payloads with ACPI version < 5.0A might not work without
PM ACPI timer.
(Legacy) software requiring `TMR_STS` (for timer overflow
interrupts) will not work with this option disabled.
endif # PM_ACPI_TIMER_OPTIONAL