[PATCH v2 3/3] x86/tsc: set LAPIC timer period to crystal clock frequency

[Daniel Drake]

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.

Link: https://lkml.kernel.org/r/20190419083533.32388-1-drake@xxxxxxxxxxxx
Link: https://lkml.kernel.org/r/alpine.DEB.2.21.1904031206440.1967@xxxxxxxxxxxxxxxxxxxxxxx
Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Daniel Drake <drake@xxxxxxxxxxxx>
---

Notes:
    v2:
     - remove unnecessary braces
     - use newly renamed variable lapic_timer_period

 arch/x86/kernel/tsc.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 6e6d933fb99c..8f47c4862c56 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -671,6 +671,16 @@ unsigned long native_calibrate_tsc(void)
 	if (boot_cpu_data.x86_model == INTEL_FAM6_ATOM_GOLDMONT)
 		setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
 
+#ifdef CONFIG_X86_LOCAL_APIC
+	/*
+	 * The local APIC appears to be fed by the core crystal clock
+	 * (which sounds entirely sensible). We can set the global
+	 * lapic_timer_period here to avoid having to calibrate the APIC
+	 * timer later.
+	 */
+	lapic_timer_period = crystal_khz * 1000 / HZ;
+#endif
+
 	return crystal_khz * ebx_numerator / eax_denominator;
 }
 
-- 
2.20.1