x86, tsc: Add static (MSR) TSC calibration on Intel Atom SoCs

Bin Gao · H. Peter Anvin · commit 7da7c1561366 · 2014-01-15T22:28:48.000-08:00
On SoCs that have the calibration MSRs available, either there is no PIT, HPET or PMTIMER to calibrate against, or the PIT/HPET/PMTIMER is driven from the same clock as the TSC, so calibration is redundant and just slows down the boot. TSC rate is caculated by this formula: <maximum core-clock to bus-clock ratio> * <maximum resolved frequency> The ratio and the resolved frequency ID can be obtained from MSR. See Intel 64 and IA-32 System Programming Guid section 16.12 and 30.11.5 for details. Signed-off-by: Bin Gao <bin.gao@intel.com> Signed-off-by: H. Peter Anvin <hpa@linux.intel.com> Link: http://lkml.kernel.org/n/tip-rgm7xmg7k6qnjlw3ynkcjsmh@git.kernel.org
diff --git a/arch/x86/include/asm/tsc.h b/arch/x86/include/asm/tsc.h
@@ -65,4 +65,7 @@ extern int notsc_setup(char *);
 extern void tsc_save_sched_clock_state(void);
 extern void tsc_restore_sched_clock_state(void);
 
+/* MSR based TSC calibration for Intel Atom SoC platforms */
+int try_msr_calibrate_tsc(unsigned long *fast_calibrate);
+
 #endif /* _ASM_X86_TSC_H */
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
@@ -32,7 +32,7 @@ obj-$(CONFIG_X86_64)	+= vsyscall_emu_64.o
 obj-y			+= bootflag.o e820.o
 obj-y			+= pci-dma.o quirks.o topology.o kdebugfs.o
 obj-y			+= alternative.o i8253.o pci-nommu.o hw_breakpoint.o
-obj-y			+= tsc.o io_delay.o rtc.o
+obj-y			+= tsc.o tsc_msr.o io_delay.o rtc.o
 obj-y			+= pci-iommu_table.o
 obj-y			+= resource.o
 
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
@@ -419,6 +419,16 @@ unsigned long native_calibrate_tsc(void)
 	unsigned long flags, latch, ms, fast_calibrate;
 	int hpet = is_hpet_enabled(), i, loopmin;
 
+	/* Calibrate TSC using MSR for Intel Atom SoCs */
+	local_irq_save(flags);
+	i = try_msr_calibrate_tsc(&fast_calibrate);
+	local_irq_restore(flags);
+	if (i >= 0) {
+		if (i == 0)
+			pr_warn("Fast TSC calibration using MSR failed\n");
+		return fast_calibrate;
+	}
+
 	local_irq_save(flags);
 	fast_calibrate = quick_pit_calibrate();
 	local_irq_restore(flags);
diff --git a/arch/x86/kernel/tsc_msr.c b/arch/x86/kernel/tsc_msr.c
@@ -0,0 +1,125 @@
+/*
+ * tsc_msr.c - MSR based TSC calibration on Intel Atom SoC platforms.
+ *
+ * TSC in Intel Atom SoC runs at a constant rate which can be figured
+ * by this formula:
+ * <maximum core-clock to bus-clock ratio> * <maximum resolved frequency>
+ * See Intel 64 and IA-32 System Programming Guid section 16.12 and 30.11.5
+ * for details.
+ * Especially some Intel Atom SoCs don't have PIT(i8254) or HPET, so MSR
+ * based calibration is the only option.
+ *
+ *
+ * Copyright (C) 2013 Intel Corporation
+ * Author: Bin Gao <bin.gao@intel.com>
+ *
+ * This file is released under the GPLv2.
+ */
+
+#include <linux/kernel.h>
+#include <asm/processor.h>
+#include <asm/setup.h>
+#include <asm/apic.h>
+#include <asm/param.h>
+
+/* CPU reference clock frequency: in KHz */
+#define FREQ_83		83200
+#define FREQ_100	99840
+#define FREQ_133	133200
+#define FREQ_166	166400
+
+#define MAX_NUM_FREQS	8
+
+/*
+ * According to Intel 64 and IA-32 System Programming Guide,
+ * if MSR_PERF_STAT[31] is set, the maximum resolved bus ratio can be
+ * read in MSR_PLATFORM_ID[12:8], otherwise in MSR_PERF_STAT[44:40].
+ * Unfortunately some Intel Atom SoCs aren't quite compliant to this,
+ * so we need manually differentiate SoC families. This is what the
+ * field msr_plat does.
+ */
+struct freq_desc {
+	u8 x86_family;	/* CPU family */
+	u8 x86_model;	/* model */
+	u8 msr_plat;	/* 1: use MSR_PLATFORM_INFO, 0: MSR_IA32_PERF_STATUS */
+	u32 freqs[MAX_NUM_FREQS];
+};
+
+static struct freq_desc freq_desc_tables[] = {
+	/* PNW */
+	{ 6, 0x27, 0, { 0, 0, 0, 0, 0, FREQ_100, 0, FREQ_83 } },
+	/* CLV+ */
+	{ 6, 0x35, 0, { 0, FREQ_133, 0, 0, 0, FREQ_100, 0, FREQ_83 } },
+	/* TNG */
+	{ 6, 0x4a, 1, { 0, FREQ_100, FREQ_133, 0, 0, 0, 0, 0 } },
+	/* VLV2 */
+	{ 6, 0x37, 1, { 0, FREQ_100, FREQ_133, FREQ_166, 0, 0, 0, 0 } },
+	/* ANN */
+	{ 6, 0x5a, 1, { FREQ_83, FREQ_100, FREQ_133, FREQ_100, 0, 0, 0, 0 } },
+};
+
+static int match_cpu(u8 family, u8 model)
+{
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(freq_desc_tables); i++) {
+		if ((family == freq_desc_tables[i].x86_family) &&
+			(model == freq_desc_tables[i].x86_model))
+			return i;
+	}
+
+	return -1;
+}
+
+/* Map CPU reference clock freq ID(0-7) to CPU reference clock freq(KHz) */
+#define id_to_freq(cpu_index, freq_id) \
+	(freq_desc_tables[cpu_index].freqs[freq_id])
+
+/*
+ * Do MSR calibration only for known/supported CPUs.
+ * Return values:
+ * -1: CPU is unknown/unsupported for MSR based calibration
+ *  0: CPU is known/supported, but calibration failed
+ *  1: CPU is known/supported, and calibration succeeded
+ */
+int try_msr_calibrate_tsc(unsigned long *fast_calibrate)
+{
+	int cpu_index;
+	u32 lo, hi, ratio, freq_id, freq;
+
+	cpu_index = match_cpu(boot_cpu_data.x86, boot_cpu_data.x86_model);
+	if (cpu_index < 0)
+		return -1;
+
+	*fast_calibrate = 0;
+
+	if (freq_desc_tables[cpu_index].msr_plat) {
+		rdmsr(MSR_PLATFORM_INFO, lo, hi);
+		ratio = (lo >> 8) & 0x1f;
+	} else {
+		rdmsr(MSR_IA32_PERF_STATUS, lo, hi);
+		ratio = (hi >> 8) & 0x1f;
+	}
+	pr_info("Maximum core-clock to bus-clock ratio: 0x%x\n", ratio);
+
+	if (!ratio)
+		return 0;
+
+	/* Get FSB FREQ ID */
+	rdmsr(MSR_FSB_FREQ, lo, hi);
+	freq_id = lo & 0x7;
+	freq = id_to_freq(cpu_index, freq_id);
+	pr_info("Resolved frequency ID: %u, frequency: %u KHz\n",
+				freq_id, freq);
+	if (!freq)
+		return 0;
+
+	/* TSC frequency = maximum resolved freq * maximum resolved bus ratio */
+	*fast_calibrate = freq * ratio;
+	pr_info("TSC runs at %lu KHz\n", *fast_calibrate);
+
+	lapic_timer_frequency = (freq * 1000) / HZ;
+	pr_info("lapic_timer_frequency = %d\n", lapic_timer_frequency);
+
+	return 1;
+}
