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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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