[v2 6/9] x86/tsc: use cpuid to determine TSC frequency

[Pavel Tatashin]

One of the newer methods to determine TSC frequency, is to use one of cpuid
extensions to get TSC/Crystal ratio. This method is preferred on CPUs that
implements it. This patch adds a new function calibrate_tsc_early() that
can be called early in boot to determine the TSC by using this method.

Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
---
 arch/x86/kernel/tsc.c | 39 ++++++++++++++++++++++++++-------------
 1 file changed, 26 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 5add503..1c9fc23 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -26,6 +26,9 @@
 #include <asm/apic.h>
 #include <asm/intel-family.h>
 
+/* CPUID 15H TSC/Crystal ratio, plus optionally Crystal Hz */
+#define CPUID_TSC_LEAF		0x15
+
 unsigned int __read_mostly cpu_khz;	/* TSC clocks / usec, not used here */
 EXPORT_SYMBOL(cpu_khz);
 
@@ -671,24 +674,16 @@ static unsigned long quick_pit_calibrate(bool early_boot)
 }
 
 /**
- * native_calibrate_tsc
- * Determine TSC frequency via CPUID, else return 0.
+ * The caller already checked that TSC leaf capability can be read from cpuid
  */
-unsigned long native_calibrate_tsc(void)
+static unsigned long calibrate_tsc_early(int model)
 {
 	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);
+	cpuid(CPUID_TSC_LEAF, &eax_denominator, &ebx_numerator, &ecx_hz, &edx);
 
 	if (ebx_numerator == 0 || eax_denominator == 0)
 		return 0;
@@ -696,7 +691,7 @@ unsigned long native_calibrate_tsc(void)
 	crystal_khz = ecx_hz / 1000;
 
 	if (crystal_khz == 0) {
-		switch (boot_cpu_data.x86_model) {
+		switch (model) {
 		case INTEL_FAM6_SKYLAKE_MOBILE:
 		case INTEL_FAM6_SKYLAKE_DESKTOP:
 		case INTEL_FAM6_KABYLAKE_MOBILE:
@@ -713,6 +708,24 @@ unsigned long native_calibrate_tsc(void)
 		}
 	}
 
+	return crystal_khz * ebx_numerator / eax_denominator;
+}
+
+/**
+ * native_calibrate_tsc
+ * Determine TSC frequency via CPUID, else return 0.
+ */
+unsigned long native_calibrate_tsc(void)
+{
+	unsigned int tsc_khz;
+
+	if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)
+		return 0;
+
+	if (boot_cpu_data.cpuid_level < CPUID_TSC_LEAF)
+		return 0;
+
+	tsc_khz = calibrate_tsc_early(boot_cpu_data.x86_model);
 	/*
 	 * TSC frequency determined by CPUID is a "hardware reported"
 	 * frequency and is the most accurate one so far we have. This
@@ -727,7 +740,7 @@ unsigned long native_calibrate_tsc(void)
 	if (boot_cpu_data.x86_model == INTEL_FAM6_ATOM_GOLDMONT)
 		setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
 
-	return crystal_khz * ebx_numerator / eax_denominator;
+	return tsc_khz;
 }
 
 static unsigned long cpu_khz_from_cpuid(void)
-- 
1.8.3.1