x86/tsc: Set LAPIC timer period to crystal clock frequency
authorDaniel Drake <drake@endlessm.com>
Thu, 9 May 2019 05:54:17 +0000 (13:54 +0800)
committerIngo Molnar <mingo@kernel.org>
Thu, 9 May 2019 09:06:49 +0000 (11:06 +0200)
commit2420a0b1798d7a78d1f9b395f09f3c80d92cc588
tree8f74fbf9b6f1d832491369545bcce1e7637fe72e
parent52ae346bd26c7a8b17ea82e9a09671e98c5402b7
x86/tsc: Set LAPIC timer period to crystal clock frequency

The APIC timer calibration (calibrate_APIC_timer()) can be skipped
in cases where we know the APIC timer frequency. On Intel SoCs,
we believe that the APIC is fed by the crystal clock; this would make
sense, and the crystal clock frequency has been verified against the
APIC timer calibration result on ApolloLake, GeminiLake, Kabylake,
CoffeeLake, WhiskeyLake and AmberLake.

Set lapic_timer_period based on the crystal clock frequency
accordingly.

APIC timer calibration would normally be skipped on modern CPUs
by nature of the TSC deadline timer being used instead,
however this change is still potentially useful, e.g. if the
TSC deadline timer has been disabled with a kernel parameter.
calibrate_APIC_timer() uses the legacy timer, but we are seeing
new platforms that omit such legacy functionality, so avoiding
such codepaths is becoming more important.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Daniel Drake <drake@endlessm.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: len.brown@intel.com
Cc: linux@endlessm.com
Cc: rafael.j.wysocki@intel.com
Link: http://lkml.kernel.org/r/20190509055417.13152-3-drake@endlessm.com
Link: https://lkml.kernel.org/r/20190419083533.32388-1-drake@endlessm.com
Link: https://lkml.kernel.org/r/alpine.DEB.2.21.1904031206440.1967@nanos.tec.linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>
arch/x86/kernel/tsc.c