Commit 604dc917 authored May 09, 2019 by Daniel Drake Committed by Ingo Molnar May 09, 2019

x86/tsc: Use CPUID.0x16 to calculate missing crystal frequency

native_calibrate_tsc() had a data mapping Intel CPU families
and crystal clock speed, but hardcoded tables are not ideal, and this
approach was already problematic at least in the Skylake X case, as
seen in commit:

  b5112030 ("x86/tsc: Fix erroneous TSC rate on Skylake Xeon")

By examining CPUID data from http://instlatx64.atw.hu/

 and units
in the lab, we have found that 3 different scenarios need to be dealt
with, and we can eliminate most of the hardcoded data using an approach a
little more advanced than before:

 1. ApolloLake, GeminiLake, CannonLake (and presumably all new chipsets
    from this point) report the crystal frequency directly via CPUID.0x15.
    That's definitive data that we can rely upon.

 2. Skylake, Kabylake and all variants of those two chipsets report a
    crystal frequency of zero, however we can calculate the crystal clock
    speed by condidering data from CPUID.0x16.

    This method correctly distinguishes between the two crystal clock
    frequencies present on different Skylake X variants that caused
    headaches before.

    As the calculations do not quite match the previously-hardcoded values
    in some cases (e.g. 23913043Hz instead of 24MHz), TSC refinement is
    enabled on all platforms where we had to calculate the crystal
    frequency in this way.

 3. Denverton (GOLDMONT_X) reports a crystal frequency of zero and does
    not support CPUID.0x16, so we leave this entry hardcoded.

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/20190509055417.13152-1-drake@endlessm.com
Link: https://lkml.kernel.org/r/20190419083533.32388-1-drake@endlessm.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent ffa6f55e

arch/x86/kernel/tsc.c

+27 −20

Original line number	Diff line number	Diff line
		@@ -631,32 +631,39 @@ unsigned long native_calibrate_tsc(void)

		crystal_khz = ecx_hz / 1000;

		if (crystal_khz == 0) {
		switch (boot_cpu_data.x86_model) {
		case INTEL_FAM6_SKYLAKE_MOBILE:
		case INTEL_FAM6_SKYLAKE_DESKTOP:
		case INTEL_FAM6_KABYLAKE_MOBILE:
		case INTEL_FAM6_KABYLAKE_DESKTOP:
		crystal_khz = 24000; /* 24.0 MHz */
		break;
		case INTEL_FAM6_ATOM_GOLDMONT_X:
		crystal_khz = 25000; /* 25.0 MHz */
		break;
		case INTEL_FAM6_ATOM_GOLDMONT:
		crystal_khz = 19200; /* 19.2 MHz */
		break;
		}
		}
		/*
		* Denverton SoCs don't report crystal clock, and also don't support
		* CPUID.0x16 for the calculation below, so hardcode the 25MHz crystal
		* clock.
		*/
		if (crystal_khz == 0 &&
		boot_cpu_data.x86_model == INTEL_FAM6_ATOM_GOLDMONT_X)
		crystal_khz = 25000;

		if (crystal_khz == 0)
		return 0;
		/*
		* TSC frequency determined by CPUID is a "hardware reported"
		* TSC frequency reported directly by CPUID is a "hardware reported"
		* frequency and is the most accurate one so far we have. This
		* is considered a known frequency.
		*/
		if (crystal_khz != 0)
		setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);

		/*
		* Some Intel SoCs like Skylake and Kabylake don't report the crystal
		* clock, but we can easily calculate it to a high degree of accuracy
		* by considering the crystal ratio and the CPU speed.
		*/
		if (crystal_khz == 0 && boot_cpu_data.cpuid_level >= 0x16) {
		unsigned int eax_base_mhz, ebx, ecx, edx;

		cpuid(0x16, &eax_base_mhz, &ebx, &ecx, &edx);
		crystal_khz = eax_base_mhz * 1000 *
		eax_denominator / ebx_numerator;
		}

		if (crystal_khz == 0)
		return 0;

		/*
		* For Atom SoCs TSC is the only reliable clocksource.
		* Mark TSC reliable so no watchdog on it.

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