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    Subject[tip:x86/timers] x86/tsc: Enumerate BXT tsc_khz via CPUID
    Commit-ID:  ff4c86635ee12461fd3bd911d7d5253394da8f9d
    Gitweb: http://git.kernel.org/tip/ff4c86635ee12461fd3bd911d7d5253394da8f9d
    Author: Len Brown <len.brown@intel.com>
    AuthorDate: Fri, 17 Jun 2016 01:22:52 -0400
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Mon, 11 Jul 2016 21:30:13 +0200

    x86/tsc: Enumerate BXT tsc_khz via CPUID

    Hard code the BXT crystal clock (aka ART - Always Running Timer)
    to 19.200 MHz, and use CPUID leaf 0x15 to determine the BXT TSC frequency.

    Use tsc_khz to sanity check BXT cpu_khz,
    which can be erroneous in some configurations.

    (I simplified the original patch from Bin Gao.)

    Original-From: Bin Gao <bin.gao@intel.com>
    Signed-off-by: Len Brown <len.brown@intel.com>
    Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Link: http://lkml.kernel.org/r/bf4e7c175acd6d09719c47c319b10ff1f0627ff8.1466138954.git.len.brown@intel.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    arch/x86/kernel/tsc.c | 16 +++++++++++++++-
    1 file changed, 15 insertions(+), 1 deletion(-)

    diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
    index e1496b7..2a952fc 100644
    --- a/arch/x86/kernel/tsc.c
    +++ b/arch/x86/kernel/tsc.c
    @@ -693,7 +693,11 @@ unsigned long native_calibrate_tsc(void)
    switch (boot_cpu_data.x86_model) {
    case 0x4E: /* SKL */
    case 0x5E: /* SKL */
    - crystal_khz = 24000; /* 24 MHz */
    + crystal_khz = 24000; /* 24.0 MHz */
    + break;
    + case 0x5C: /* BXT */
    + crystal_khz = 19200; /* 19.2 MHz */
    + break;
    }
    }

    @@ -895,6 +899,8 @@ int recalibrate_cpu_khz(void)
    tsc_khz = x86_platform.calibrate_tsc();
    if (tsc_khz == 0)
    tsc_khz = cpu_khz;
    + else if (abs(cpu_khz - tsc_khz) * 10 > tsc_khz)
    + cpu_khz = tsc_khz;
    cpu_data(0).loops_per_jiffy = cpufreq_scale(cpu_data(0).loops_per_jiffy,
    cpu_khz_old, cpu_khz);

    @@ -1302,8 +1308,16 @@ void __init tsc_init(void)

    cpu_khz = x86_platform.calibrate_cpu();
    tsc_khz = x86_platform.calibrate_tsc();
    +
    + /*
    + * Trust non-zero tsc_khz as authorative,
    + * and use it to sanity check cpu_khz,
    + * which will be off if system timer is off.
    + */
    if (tsc_khz == 0)
    tsc_khz = cpu_khz;
    + else if (abs(cpu_khz - tsc_khz) * 10 > tsc_khz)
    + cpu_khz = tsc_khz;

    if (!tsc_khz) {
    mark_tsc_unstable("could not calculate TSC khz");
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