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/*
* 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, { FREQ_83, 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.
*
* Returns the calibration value or 0 if MSR calibration failed.
*/
unsigned long try_msr_calibrate_tsc(void)
{
u32 lo, hi, ratio, freq_id, freq;
unsigned long res;
int cpu_index;
cpu_index = match_cpu(boot_cpu_data.x86, boot_cpu_data.x86_model);
if (cpu_index < 0)
return 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)
goto fail;
/* 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)
goto fail;
/* TSC frequency = maximum resolved freq * maximum resolved bus ratio */
res = freq * ratio;
pr_info("TSC runs at %lu KHz\n", res);
#ifdef CONFIG_X86_LOCAL_APIC
lapic_timer_frequency = (freq * 1000) / HZ;
pr_info("lapic_timer_frequency = %d\n", lapic_timer_frequency);
#endif
return res;
fail:
pr_warn("Fast TSC calibration using MSR failed\n");
return 0;
}
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