x86/tsc: Enumerate SKL cpu_khz and tsc_khz via CPUID

Skylake CPU base-frequency and TSC frequency may differ
by up to 2%.

Enumerate CPU and TSC frequencies separately, allowing
cpu_khz and tsc_khz to differ.

The existing CPU frequency calibration mechanism is unchanged.
However, CPUID extensions are preferred, when available.

CPUID.0x16 is preferred over MSR and timer calibration
for CPU frequency discovery.

CPUID.0x15 takes precedence over CPU-frequency
for TSC frequency discovery.

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/b27ec289fd005833b27d694d9c2dbb716c5cdff7.1466138954.git.len.brown@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 68 insertions, 7 deletions

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 35a3976c19cc..e1496b79c28a 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -239,7 +239,7 @@ static inline unsigned long long cycles_2_ns(unsigned long long cyc)
 	return ns;
 }
 
-static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
+static void set_cyc2ns_scale(unsigned long khz, int cpu)
 {
 	unsigned long long tsc_now, ns_now;
 	struct cyc2ns_data *data;
@@ -248,7 +248,7 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
 	local_irq_save(flags);
 	sched_clock_idle_sleep_event();
 
-	if (!cpu_khz)
+	if (!khz)
 		goto done;
 
 	data = cyc2ns_write_begin(cpu);
@@ -261,7 +261,7 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
 	 * time function is continuous; see the comment near struct
 	 * cyc2ns_data.
 	 */
-	clocks_calc_mult_shift(&data->cyc2ns_mul, &data->cyc2ns_shift, cpu_khz,
+	clocks_calc_mult_shift(&data->cyc2ns_mul, &data->cyc2ns_shift, khz,
 			       NSEC_PER_MSEC, 0);
 
 	/*
@@ -665,15 +665,72 @@ success:
 }
 
 /**
- * native_calibrate_tsc - calibrate the tsc on boot
+ * native_calibrate_tsc
+ * Determine TSC frequency via CPUID, else return 0.
  */
 unsigned long native_calibrate_tsc(void)
 {
+	unsigned int eax_denominator, ebx_numerator, ecx_hz, edx;
+	unsigned int crystal_khz;
+
+	if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)
+		return 0;
+
+	if (boot_cpu_data.cpuid_level < 0x15)
+		return 0;
+
+	eax_denominator = ebx_numerator = ecx_hz = edx = 0;
+
+	/* CPUID 15H TSC/Crystal ratio, plus optionally Crystal Hz */
+	cpuid(0x15, &eax_denominator, &ebx_numerator, &ecx_hz, &edx);
+
+	if (ebx_numerator == 0 || eax_denominator == 0)
+		return 0;
+
+	crystal_khz = ecx_hz / 1000;
+
+	if (crystal_khz == 0) {
+		switch (boot_cpu_data.x86_model) {
+		case 0x4E:	/* SKL */
+		case 0x5E:	/* SKL */
+			crystal_khz = 24000;	/* 24 MHz */
+		}
+	}
+
+	return crystal_khz * ebx_numerator / eax_denominator;
+}
+
+static unsigned long cpu_khz_from_cpuid(void)
+{
+	unsigned int eax_base_mhz, ebx_max_mhz, ecx_bus_mhz, edx;
+
+	if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)
+		return 0;
+
+	if (boot_cpu_data.cpuid_level < 0x16)
+		return 0;
+
+	eax_base_mhz = ebx_max_mhz = ecx_bus_mhz = edx = 0;
+
+	cpuid(0x16, &eax_base_mhz, &ebx_max_mhz, &ecx_bus_mhz, &edx);
+
+	return eax_base_mhz * 1000;
+}
+
+/**
+ * native_calibrate_cpu - calibrate the cpu on boot
+ */
+unsigned long native_calibrate_cpu(void)
+{
 	u64 tsc1, tsc2, delta, ref1, ref2;
 	unsigned long tsc_pit_min = ULONG_MAX, tsc_ref_min = ULONG_MAX;
 	unsigned long flags, latch, ms, fast_calibrate;
 	int hpet = is_hpet_enabled(), i, loopmin;
 
+	fast_calibrate = cpu_khz_from_cpuid();
+	if (fast_calibrate)
+		return fast_calibrate;
+
 	fast_calibrate = cpu_khz_from_msr();
 	if (fast_calibrate)
 		return fast_calibrate;
@@ -834,8 +891,10 @@ int recalibrate_cpu_khz(void)
 	if (!boot_cpu_has(X86_FEATURE_TSC))
 		return -ENODEV;
 
+	cpu_khz = x86_platform.calibrate_cpu();
 	tsc_khz = x86_platform.calibrate_tsc();
-	cpu_khz = tsc_khz;
+	if (tsc_khz == 0)
+		tsc_khz = cpu_khz;
 	cpu_data(0).loops_per_jiffy = cpufreq_scale(cpu_data(0).loops_per_jiffy,
 						    cpu_khz_old, cpu_khz);
 
@@ -1241,8 +1300,10 @@ void __init tsc_init(void)
 		return;
 	}
 
+	cpu_khz = x86_platform.calibrate_cpu();
 	tsc_khz = x86_platform.calibrate_tsc();
-	cpu_khz = tsc_khz;
+	if (tsc_khz == 0)
+		tsc_khz = cpu_khz;
 
 	if (!tsc_khz) {
 		mark_tsc_unstable("could not calculate TSC khz");
@@ -1262,7 +1323,7 @@ void __init tsc_init(void)
 	 */
 	for_each_possible_cpu(cpu) {
 		cyc2ns_init(cpu);
-		set_cyc2ns_scale(cpu_khz, cpu);
+		set_cyc2ns_scale(tsc_khz, cpu);
 	}
 
 	if (tsc_disabled > 0)