Change in coreboot[master]: libpayload/x86: Try to discover invariant TSC rate

[Patrick Georgi (Code Review)]

Patrick Georgi has submitted this change. ( https://review.coreboot.org/c/coreboot/+/34170 )

Change subject: libpayload/x86: Try to discover invariant TSC rate
......................................................................

libpayload/x86: Try to discover invariant TSC rate

We can skip the PIT-based TSC calibration if we can derive the invariant
TSC rate from CPUID/MSR data. This is necessary if the PIT is disabled,
which is the default, for instance, on Coffee Lake CPUs.

This implementation should cover all Intel Core i processors at least.
For older processors, we fall back to the PIT calibration.

Change-Id: Ic6607ee2a8b41c2be9dc1bb4f1e23e652bb33889
Signed-off-by: Nico Huber 
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34170
Tested-by: build bot (Jenkins) 
Reviewed-by: Angel Pons 
Reviewed-by: Michael Niewöhner 
---
M payloads/libpayload/arch/x86/timer.c
1 file changed, 116 insertions(+), 3 deletions(-)

Approvals:
  build bot (Jenkins): Verified
  Angel Pons: Looks good to me, approved
  Michael Niewöhner: Looks good to me, but someone else must approve

diff --git a/payloads/libpayload/arch/x86/timer.c b/payloads/libpayload/arch/x86/timer.c
index 1ff2cd6..6dcfd5b 100644
--- a/payloads/libpayload/arch/x86/timer.c
+++ b/payloads/libpayload/arch/x86/timer.c
@@ -33,6 +33,10 @@

#include <libpayload.h>
 #include <arch/rdtsc.h>
+#include <arch/cpuid.h>
+#include <arch/msr.h>
+
+#define MSR_PLATFORM_INFO		0xce

/**
  * @ingroup arch
@@ -41,11 +45,11 @@
 uint32_t cpu_khz;

/**
- * Calculate the speed of the processor for use in delays.
+ * @brief Measure the speed of the processor for use in delays
  *
  * @return The CPU speed in kHz.
  */
-unsigned int get_cpu_speed(void)
+static unsigned int calibrate_pit(void)
 {
    unsigned long long start, end;
    const uint32_t clock_rate = 1193182; // 1.193182 MHz
@@ -71,7 +75,116 @@
     * clock_rate / (interval * 1000). Multiply that by the number of
     * measured clocks to get the kHz value.
     */
-	cpu_khz = (end - start) * clock_rate / (1000 * interval);
+	return (end - start) * clock_rate / (1000 * interval);
+}
+
+/**
+ * @brief Calculates the core clock frequency via CPUID 0x15
+ *
+ * Newer Intel CPUs report their core clock in CPUID leaf 0x15. Early models
+ * supporting this leaf didn't provide the nominal crystal frequency in ecx,
+ * hence we use hard coded values for them.
+ */
+static int get_cpu_khz_xtal(void)
+{
+	uint32_t ecx, edx, num, denom;
+	uint64_t nominal;
+
+	if (cpuid_max() < 0x15)
+		return -1;
+	cpuid(0x15, denom, num, ecx, edx);
+
+	if (denom == 0 || num == 0)
+		return -1;
+
+	if (ecx != 0) {
+		nominal = ecx;
+	} else {
+		if (cpuid_family() != 6)
+			return -1;
+
+		switch (cpuid_model()) {
+		case SKYLAKE_U_Y:
+		case SKYLAKE_S_H:
+		case KABYLAKE_U_Y:
+		case KABYLAKE_S_H:
+			nominal = 24000000;
+			break;
+		case APOLLOLAKE:
+			nominal = 19200000;
+			break;
+		default:
+			return -1;
+		}
+	}
+
+	return nominal * num / denom / 1000;
+}
+
+/**
+ * @brief Returns three times the bus clock in kHz
+ *
+ * The result of calculations with the returned value shall be divided by 3.
+ * This helps to avoid rounding errors.
+ */
+static int get_bus_khz_x3(void)
+{
+	if (cpuid_family() != 6)
+		return -1;
+
+	switch (cpuid_model()) {
+	case NEHALEM:
+		return 400 * 1000; /* 133 MHz */
+	case SANDYBRIDGE:
+	case IVYBRIDGE:
+	case HASWELL:
+	case HASWELL_U:
+	case HASWELL_GT3E:
+	case BROADWELL:
+	case BROADWELL_U:
+		return 300 * 1000; /* 100 MHz */
+	default:
+		return -1;
+	}
+}
+
+/**
+ * @brief Returns the calculated CPU frequency
+ *
+ * Over the years, multiple ways to discover the CPU frequency have been
+ * exposed through CPUID and MSRs. Try the most recent and accurate first
+ * (crystal information in CPUID leaf 0x15) and then fall back to older
+ * methods.
+ *
+ * This should cover all Intel Core i processors at least. For older
+ * processors we fall back to the PIT calibration.
+ */
+static int get_cpu_khz_fast(void)
+{
+	/* Try core crystal clock frequency first (supposed to be more accurate). */
+	const int cpu_khz_xtal = get_cpu_khz_xtal();
+	if (cpu_khz_xtal > 0)
+		return cpu_khz_xtal;
+
+	/* Try `bus clock * speedstep multiplier`. */
+	const int bus_x3 = get_bus_khz_x3();
+	if (bus_x3 <= 0)
+		return -1;
+	/*
+	 * Systems with an invariant TSC report the multiplier (maximum
+	 * non-turbo ratio) in MSR_PLATFORM_INFO[15:8].
+	 */
+	const unsigned int mult = _rdmsr(MSR_PLATFORM_INFO) >> 8 & 0xff;
+	return bus_x3 * mult / 3;
+}
+
+unsigned int get_cpu_speed(void)
+{
+	const int cpu_khz_fast = get_cpu_khz_fast();
+	if (cpu_khz_fast > 0)
+		cpu_khz = (unsigned int)cpu_khz_fast;
+	else
+		cpu_khz = calibrate_pit();

return cpu_khz;
 }