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>

1 files changed, 15 insertions, 1 deletions

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index e1496b79c28a..2a952fcb1516 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");