drivers/timer/tsc_timer.c - platform/external/u-boot - Gitiles

 // SPDX-License-Identifier: GPL-2.0+
 /*
  * Copyright (c) 2012 The Chromium OS Authors.
  *
  * TSC calibration codes are adapted from Linux kernel
  * arch/x86/kernel/tsc_msr.c and arch/x86/kernel/tsc.c
  */

 #include <common.h>
 #include <bootstage.h>
 #include <dm.h>
 #include <log.h>
 #include <malloc.h>
 #include <time.h>
 #include <timer.h>
 #include <asm/cpu.h>
 #include <asm/io.h>
 #include <asm/i8254.h>
 #include <asm/ibmpc.h>
 #include <asm/msr.h>
 #include <asm/u-boot-x86.h>
 #include <linux/delay.h>

 #define MAX_NUM_FREQS	9

 #define INTEL_FAM6_SKYLAKE_MOBILE	0x4E
 #define INTEL_FAM6_ATOM_GOLDMONT	0x5C /* Apollo Lake */
 #define INTEL_FAM6_SKYLAKE_DESKTOP	0x5E
 #define INTEL_FAM6_ATOM_GOLDMONT_X	0x5F /* Denverton */
 #define INTEL_FAM6_KABYLAKE_MOBILE	0x8E
 #define INTEL_FAM6_KABYLAKE_DESKTOP	0x9E

 DECLARE_GLOBAL_DATA_PTR;

 /*
  * native_calibrate_tsc
  * Determine TSC frequency via CPUID, else return 0.
  */
 static unsigned long native_calibrate_tsc(void)
 {
 	struct cpuid_result tsc_info;
 	unsigned int crystal_freq;

 	if (gd->arch.x86_vendor != X86_VENDOR_INTEL)
 		return 0;

 	if (cpuid_eax(0) < 0x15)
 		return 0;

 	tsc_info = cpuid(0x15);

 	if (tsc_info.ebx == 0 || tsc_info.eax == 0)
 		return 0;

 	crystal_freq = tsc_info.ecx / 1000;
 	if (!CONFIG_IS_ENABLED(X86_TSC_TIMER_NATIVE) && !crystal_freq) {
 		switch (gd->arch.x86_model) {
 		case INTEL_FAM6_SKYLAKE_MOBILE:
 		case INTEL_FAM6_SKYLAKE_DESKTOP:
 		case INTEL_FAM6_KABYLAKE_MOBILE:
 		case INTEL_FAM6_KABYLAKE_DESKTOP:
 			crystal_freq = 24000;	/* 24.0 MHz */
 			break;
 		case INTEL_FAM6_ATOM_GOLDMONT_X:
 			crystal_freq = 25000;	/* 25.0 MHz */
 			break;
 		case INTEL_FAM6_ATOM_GOLDMONT:
 			crystal_freq = 19200;	/* 19.2 MHz */
 			break;
 		default:
 			return 0;
 		}
 	}

 	return (crystal_freq * tsc_info.ebx / tsc_info.eax) / 1000;
 }

 static unsigned long cpu_mhz_from_cpuid(void)
 {
 	if (gd->arch.x86_vendor != X86_VENDOR_INTEL)
 		return 0;

 	if (cpuid_eax(0) < 0x16)
 		return 0;

 	return cpuid_eax(0x16);
 }

 /*
  * 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 */
 	/* 2: use 100MHz, 1: use MSR_PLATFORM_INFO, 0: MSR_IA32_PERF_STATUS */
 	u8 msr_plat;
 	u32 freqs[MAX_NUM_FREQS];
 };

 static struct freq_desc freq_desc_tables[] = {
 	/* PNW */
 	{ 6, 0x27, 0, { 0, 0, 0, 0, 0, 99840, 0, 83200, 0 } },
 	/* CLV+ */
 	{ 6, 0x35, 0, { 0, 133200, 0, 0, 0, 99840, 0, 83200, 0 } },
 	/* TNG - Intel Atom processor Z3400 series */
 	{ 6, 0x4a, 1, { 0, 100000, 133300, 0, 0, 0, 0, 0, 0 } },
 	/* VLV2 - Intel Atom processor E3000, Z3600, Z3700 series */
 	{ 6, 0x37, 1, { 83300, 100000, 133300, 116700, 80000, 0, 0, 0, 0 } },
 	/* ANN - Intel Atom processor Z3500 series */
 	{ 6, 0x5a, 1, { 83300, 100000, 133300, 100000, 0, 0, 0, 0, 0 } },
 	/* AMT - Intel Atom processor X7-Z8000 and X5-Z8000 series */
 	{ 6, 0x4c, 1, { 83300, 100000, 133300, 116700,
 			80000, 93300, 90000, 88900, 87500 } },
 	/* Ivybridge */
 	{ 6, 0x3a, 2, { 0, 0, 0, 0, 0, 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])

 /*
  * TSC on Intel Atom SoCs capable of determining TSC frequency by MSR is
  * reliable and the frequency is known (provided by HW).
  *
  * On these platforms PIT/HPET is generally not available so calibration won't
  * work at all and there is no other clocksource to act as a watchdog for the
  * TSC, so we have no other choice than to trust it.
  *
  * Returns the TSC frequency in MHz or 0 if HW does not provide it.
  */
 static unsigned long __maybe_unused cpu_mhz_from_msr(void)
 {
 	u32 lo, hi, ratio, freq_id, freq;
 	unsigned long res;
 	int cpu_index;

 	if (gd->arch.x86_vendor != X86_VENDOR_INTEL)
 		return 0;

 	cpu_index = match_cpu(gd->arch.x86, gd->arch.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) & 0xff;
 	} else {
 		rdmsr(MSR_IA32_PERF_STATUS, lo, hi);
 		ratio = (hi >> 8) & 0x1f;
 	}
 	debug("Maximum core-clock to bus-clock ratio: 0x%x\n", ratio);

 	if (freq_desc_tables[cpu_index].msr_plat == 2) {
 		/* TODO: Figure out how best to deal with this */
 		freq = 100000;
 		debug("Using frequency: %u KHz\n", freq);
 	} else {
 		/* Get FSB FREQ ID */
 		rdmsr(MSR_FSB_FREQ, lo, hi);
 		freq_id = lo & 0x7;
 		freq = id_to_freq(cpu_index, freq_id);
 		debug("Resolved frequency ID: %u, frequency: %u KHz\n",
 		      freq_id, freq);
 	}

 	/* TSC frequency = maximum resolved freq * maximum resolved bus ratio */
 	res = freq * ratio / 1000;
 	debug("TSC runs at %lu MHz\n", res);

 	return res;
 }

 /*
  * This reads the current MSB of the PIT counter, and
  * checks if we are running on sufficiently fast and
  * non-virtualized hardware.
  *
  * Our expectations are:
  *
  *  - the PIT is running at roughly 1.19MHz
  *
  *  - each IO is going to take about 1us on real hardware,
  *    but we allow it to be much faster (by a factor of 10) or
  *    _slightly_ slower (ie we allow up to a 2us read+counter
  *    update - anything else implies a unacceptably slow CPU
  *    or PIT for the fast calibration to work.
  *
  *  - with 256 PIT ticks to read the value, we have 214us to
  *    see the same MSB (and overhead like doing a single TSC
  *    read per MSB value etc).
  *
  *  - We're doing 2 reads per loop (LSB, MSB), and we expect
  *    them each to take about a microsecond on real hardware.
  *    So we expect a count value of around 100. But we'll be
  *    generous, and accept anything over 50.
  *
  *  - if the PIT is stuck, and we see *many* more reads, we
  *    return early (and the next caller of pit_expect_msb()
  *    then consider it a failure when they don't see the
  *    next expected value).
  *
  * These expectations mean that we know that we have seen the
  * transition from one expected value to another with a fairly
  * high accuracy, and we didn't miss any events. We can thus
  * use the TSC value at the transitions to calculate a pretty
  * good value for the TSC frequencty.
  */
 static inline int pit_verify_msb(unsigned char val)
 {
 	/* Ignore LSB */
 	inb(0x42);
 	return inb(0x42) == val;
 }

 static inline int pit_expect_msb(unsigned char val, u64 *tscp,
 				 unsigned long *deltap)
 {
 	int count;
 	u64 tsc = 0, prev_tsc = 0;

 	for (count = 0; count < 50000; count++) {
 		if (!pit_verify_msb(val))
 			break;
 		prev_tsc = tsc;
 		tsc = rdtsc();
 	}
 	*deltap = rdtsc() - prev_tsc;
 	*tscp = tsc;

 	/*
 	 * We require _some_ success, but the quality control
 	 * will be based on the error terms on the TSC values.
 	 */
 	return count > 5;
 }

 /*
  * How many MSB values do we want to see? We aim for
  * a maximum error rate of 500ppm (in practice the
  * real error is much smaller), but refuse to spend
  * more than 50ms on it.
  */
 #define MAX_QUICK_PIT_MS 50
 #define MAX_QUICK_PIT_ITERATIONS (MAX_QUICK_PIT_MS * PIT_TICK_RATE / 1000 / 256)

 static unsigned long __maybe_unused quick_pit_calibrate(void)
 {
 	int i;
 	u64 tsc, delta;
 	unsigned long d1, d2;

 	/* Set the Gate high, disable speaker */
 	outb((inb(0x61) & ~0x02) | 0x01, 0x61);

 	/*
 	 * Counter 2, mode 0 (one-shot), binary count
 	 *
 	 * NOTE! Mode 2 decrements by two (and then the
 	 * output is flipped each time, giving the same
 	 * final output frequency as a decrement-by-one),
 	 * so mode 0 is much better when looking at the
 	 * individual counts.
 	 */
 	outb(0xb0, 0x43);

 	/* Start at 0xffff */
 	outb(0xff, 0x42);
 	outb(0xff, 0x42);

 	/*
 	 * The PIT starts counting at the next edge, so we
 	 * need to delay for a microsecond. The easiest way
 	 * to do that is to just read back the 16-bit counter
 	 * once from the PIT.
 	 */
 	pit_verify_msb(0);

 	if (pit_expect_msb(0xff, &tsc, &d1)) {
 		for (i = 1; i <= MAX_QUICK_PIT_ITERATIONS; i++) {
 			if (!pit_expect_msb(0xff-i, &delta, &d2))
 				break;

 			/*
 			 * Iterate until the error is less than 500 ppm
 			 */
 			delta -= tsc;
 			if (d1+d2 >= delta >> 11)
 				continue;

 			/*
 			 * Check the PIT one more time to verify that
 			 * all TSC reads were stable wrt the PIT.
 			 *
 			 * This also guarantees serialization of the
 			 * last cycle read ('d2') in pit_expect_msb.
 			 */
 			if (!pit_verify_msb(0xfe - i))
 				break;
 			goto success;
 		}
 	}
 	debug("Fast TSC calibration failed\n");
 	return 0;

 success:
 	/*
 	 * Ok, if we get here, then we've seen the
 	 * MSB of the PIT decrement 'i' times, and the
 	 * error has shrunk to less than 500 ppm.
 	 *
 	 * As a result, we can depend on there not being
 	 * any odd delays anywhere, and the TSC reads are
 	 * reliable (within the error).
 	 *
 	 * kHz = ticks / time-in-seconds / 1000;
 	 * kHz = (t2 - t1) / (I * 256 / PIT_TICK_RATE) / 1000
 	 * kHz = ((t2 - t1) * PIT_TICK_RATE) / (I * 256 * 1000)
 	 */
 	delta *= PIT_TICK_RATE;
 	delta /= (i*256*1000);
 	debug("Fast TSC calibration using PIT\n");
 	return delta / 1000;
 }

 /* Get the speed of the TSC timer in MHz */
 unsigned notrace long get_tbclk_mhz(void)
 {
 	return get_tbclk() / 1000000;
 }

 static ulong get_ms_timer(void)
 {
 	return (get_ticks() * 1000) / get_tbclk();
 }

 ulong get_timer(ulong base)
 {
 	return get_ms_timer() - base;
 }

 ulong notrace timer_get_us(void)
 {
 	return get_ticks() / get_tbclk_mhz();
 }

 ulong timer_get_boot_us(void)
 {
 	return timer_get_us();
 }

 void __udelay(unsigned long usec)
 {
 	u64 now = get_ticks();
 	u64 stop;

 	stop = now + usec * get_tbclk_mhz();

 	while ((int64_t)(stop - get_ticks()) > 0)
 #if defined(CONFIG_QEMU) && defined(CONFIG_SMP)
 		/*
 		 * Add a 'pause' instruction on qemu target,
 		 * to give other VCPUs a chance to run.
 		 */
 		asm volatile("pause");
 #else
 		;
 #endif
 }

 static u64 tsc_timer_get_count(struct udevice *dev)
 {
 	u64 now_tick = rdtsc();

 	return now_tick - gd->arch.tsc_base;
 }

 static void tsc_timer_ensure_setup(bool early)
 {
 	if (gd->arch.tsc_inited)
 		return;
 	if (IS_ENABLED(CONFIG_X86_TSC_READ_BASE))
 		gd->arch.tsc_base = rdtsc();

 	if (!gd->arch.clock_rate) {
 		unsigned long fast_calibrate;

 		fast_calibrate = native_calibrate_tsc();
 		if (fast_calibrate)
 			goto done;

 		/* Reduce code size by dropping other methods */
 		if (CONFIG_IS_ENABLED(X86_TSC_TIMER_NATIVE))
 			panic("no timer");

 		fast_calibrate = cpu_mhz_from_cpuid();
 		if (fast_calibrate)
 			goto done;

 		fast_calibrate = cpu_mhz_from_msr();
 		if (fast_calibrate)
 			goto done;

 		fast_calibrate = quick_pit_calibrate();
 		if (fast_calibrate)
 			goto done;

 		if (early)
 			fast_calibrate = CONFIG_X86_TSC_TIMER_EARLY_FREQ;
 		else
 			return;

 done:
 		gd->arch.clock_rate = fast_calibrate * 1000000;
 	}
 	gd->arch.tsc_inited = true;
 }

 static int tsc_timer_probe(struct udevice *dev)
 {
 	struct timer_dev_priv *uc_priv = dev_get_uclass_priv(dev);

 	/* Try hardware calibration first */
 	tsc_timer_ensure_setup(false);
 	if (!gd->arch.clock_rate) {
 		/*
 		 * Use the clock frequency specified in the
 		 * device tree as last resort
 		 */
 		if (!uc_priv->clock_rate)
 			panic("TSC frequency is ZERO");
 	} else {
 		uc_priv->clock_rate = gd->arch.clock_rate;
 	}

 	return 0;
 }

 unsigned long notrace timer_early_get_rate(void)
 {
 	/*
 	 * When TSC timer is used as the early timer, be warned that the timer
 	 * clock rate can only be calibrated via some hardware ways. Specifying
 	 * it in the device tree won't work for the early timer.
 	 */
 	tsc_timer_ensure_setup(true);

 	return gd->arch.clock_rate;
 }

 u64 notrace timer_early_get_count(void)
 {
 	tsc_timer_ensure_setup(true);

 	return rdtsc() - gd->arch.tsc_base;
 }

 static const struct timer_ops tsc_timer_ops = {
 	.get_count = tsc_timer_get_count,
 };

 static const struct udevice_id tsc_timer_ids[] = {
 	{ .compatible = "x86,tsc-timer", },
 	{ }
 };

 U_BOOT_DRIVER(tsc_timer) = {
 	.name	= "tsc_timer",
 	.id	= UCLASS_TIMER,
 	.of_match = tsc_timer_ids,
 	.probe = tsc_timer_probe,
 	.ops	= &tsc_timer_ops,
 };
	// SPDX-License-Identifier: GPL-2.0+
	/*
	* Copyright (c) 2012 The Chromium OS Authors.
	*
	* TSC calibration codes are adapted from Linux kernel
	* arch/x86/kernel/tsc_msr.c and arch/x86/kernel/tsc.c
	*/

	#include <common.h>
	#include <bootstage.h>
	#include <dm.h>
	#include <log.h>
	#include <malloc.h>
	#include <time.h>
	#include <timer.h>
	#include <asm/cpu.h>
	#include <asm/io.h>
	#include <asm/i8254.h>
	#include <asm/ibmpc.h>
	#include <asm/msr.h>
	#include <asm/u-boot-x86.h>
	#include <linux/delay.h>

	#define MAX_NUM_FREQS 9

	#define INTEL_FAM6_SKYLAKE_MOBILE 0x4E
	#define INTEL_FAM6_ATOM_GOLDMONT 0x5C /* Apollo Lake */
	#define INTEL_FAM6_SKYLAKE_DESKTOP 0x5E
	#define INTEL_FAM6_ATOM_GOLDMONT_X 0x5F /* Denverton */
	#define INTEL_FAM6_KABYLAKE_MOBILE 0x8E
	#define INTEL_FAM6_KABYLAKE_DESKTOP 0x9E

	DECLARE_GLOBAL_DATA_PTR;

	/*
	* native_calibrate_tsc
	* Determine TSC frequency via CPUID, else return 0.
	*/
	static unsigned long native_calibrate_tsc(void)
	{
	struct cpuid_result tsc_info;
	unsigned int crystal_freq;

	if (gd->arch.x86_vendor != X86_VENDOR_INTEL)
	return 0;

	if (cpuid_eax(0) < 0x15)
	return 0;

	tsc_info = cpuid(0x15);

	if (tsc_info.ebx == 0 \|\| tsc_info.eax == 0)
	return 0;

	crystal_freq = tsc_info.ecx / 1000;
	if (!CONFIG_IS_ENABLED(X86_TSC_TIMER_NATIVE) && !crystal_freq) {
	switch (gd->arch.x86_model) {
	case INTEL_FAM6_SKYLAKE_MOBILE:
	case INTEL_FAM6_SKYLAKE_DESKTOP:
	case INTEL_FAM6_KABYLAKE_MOBILE:
	case INTEL_FAM6_KABYLAKE_DESKTOP:
	crystal_freq = 24000; /* 24.0 MHz */
	break;
	case INTEL_FAM6_ATOM_GOLDMONT_X:
	crystal_freq = 25000; /* 25.0 MHz */
	break;
	case INTEL_FAM6_ATOM_GOLDMONT:
	crystal_freq = 19200; /* 19.2 MHz */
	break;
	default:
	return 0;
	}
	}

	return (crystal_freq * tsc_info.ebx / tsc_info.eax) / 1000;
	}

	static unsigned long cpu_mhz_from_cpuid(void)
	{
	if (gd->arch.x86_vendor != X86_VENDOR_INTEL)
	return 0;

	if (cpuid_eax(0) < 0x16)
	return 0;

	return cpuid_eax(0x16);
	}

	/*
	* 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 */
	/* 2: use 100MHz, 1: use MSR_PLATFORM_INFO, 0: MSR_IA32_PERF_STATUS */
	u8 msr_plat;
	u32 freqs[MAX_NUM_FREQS];
	};

	static struct freq_desc freq_desc_tables[] = {
	/* PNW */
	{ 6, 0x27, 0, { 0, 0, 0, 0, 0, 99840, 0, 83200, 0 } },
	/* CLV+ */
	{ 6, 0x35, 0, { 0, 133200, 0, 0, 0, 99840, 0, 83200, 0 } },
	/* TNG - Intel Atom processor Z3400 series */
	{ 6, 0x4a, 1, { 0, 100000, 133300, 0, 0, 0, 0, 0, 0 } },
	/* VLV2 - Intel Atom processor E3000, Z3600, Z3700 series */
	{ 6, 0x37, 1, { 83300, 100000, 133300, 116700, 80000, 0, 0, 0, 0 } },
	/* ANN - Intel Atom processor Z3500 series */
	{ 6, 0x5a, 1, { 83300, 100000, 133300, 100000, 0, 0, 0, 0, 0 } },
	/* AMT - Intel Atom processor X7-Z8000 and X5-Z8000 series */
	{ 6, 0x4c, 1, { 83300, 100000, 133300, 116700,
	80000, 93300, 90000, 88900, 87500 } },
	/* Ivybridge */
	{ 6, 0x3a, 2, { 0, 0, 0, 0, 0, 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])

	/*
	* TSC on Intel Atom SoCs capable of determining TSC frequency by MSR is
	* reliable and the frequency is known (provided by HW).
	*
	* On these platforms PIT/HPET is generally not available so calibration won't
	* work at all and there is no other clocksource to act as a watchdog for the
	* TSC, so we have no other choice than to trust it.
	*
	* Returns the TSC frequency in MHz or 0 if HW does not provide it.
	*/
	static unsigned long __maybe_unused cpu_mhz_from_msr(void)
	{
	u32 lo, hi, ratio, freq_id, freq;
	unsigned long res;
	int cpu_index;

	if (gd->arch.x86_vendor != X86_VENDOR_INTEL)
	return 0;

	cpu_index = match_cpu(gd->arch.x86, gd->arch.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) & 0xff;
	} else {
	rdmsr(MSR_IA32_PERF_STATUS, lo, hi);
	ratio = (hi >> 8) & 0x1f;
	}
	debug("Maximum core-clock to bus-clock ratio: 0x%x\n", ratio);

	if (freq_desc_tables[cpu_index].msr_plat == 2) {
	/* TODO: Figure out how best to deal with this */
	freq = 100000;
	debug("Using frequency: %u KHz\n", freq);
	} else {
	/* Get FSB FREQ ID */
	rdmsr(MSR_FSB_FREQ, lo, hi);
	freq_id = lo & 0x7;
	freq = id_to_freq(cpu_index, freq_id);
	debug("Resolved frequency ID: %u, frequency: %u KHz\n",
	freq_id, freq);
	}

	/* TSC frequency = maximum resolved freq * maximum resolved bus ratio */
	res = freq * ratio / 1000;
	debug("TSC runs at %lu MHz\n", res);

	return res;
	}

	/*
	* This reads the current MSB of the PIT counter, and
	* checks if we are running on sufficiently fast and
	* non-virtualized hardware.
	*
	* Our expectations are:
	*
	* - the PIT is running at roughly 1.19MHz
	*
	* - each IO is going to take about 1us on real hardware,
	* but we allow it to be much faster (by a factor of 10) or
	* _slightly_ slower (ie we allow up to a 2us read+counter
	* update - anything else implies a unacceptably slow CPU
	* or PIT for the fast calibration to work.
	*
	* - with 256 PIT ticks to read the value, we have 214us to
	* see the same MSB (and overhead like doing a single TSC
	* read per MSB value etc).
	*
	* - We're doing 2 reads per loop (LSB, MSB), and we expect
	* them each to take about a microsecond on real hardware.
	* So we expect a count value of around 100. But we'll be
	* generous, and accept anything over 50.
	*
	* - if the PIT is stuck, and we see many more reads, we
	* return early (and the next caller of pit_expect_msb()
	* then consider it a failure when they don't see the
	* next expected value).
	*
	* These expectations mean that we know that we have seen the
	* transition from one expected value to another with a fairly
	* high accuracy, and we didn't miss any events. We can thus
	* use the TSC value at the transitions to calculate a pretty
	* good value for the TSC frequencty.
	*/
	static inline int pit_verify_msb(unsigned char val)
	{
	/* Ignore LSB */
	inb(0x42);
	return inb(0x42) == val;
	}

	static inline int pit_expect_msb(unsigned char val, u64 *tscp,
	unsigned long *deltap)
	{
	int count;
	u64 tsc = 0, prev_tsc = 0;

	for (count = 0; count < 50000; count++) {
	if (!pit_verify_msb(val))
	break;
	prev_tsc = tsc;
	tsc = rdtsc();
	}
	*deltap = rdtsc() - prev_tsc;
	*tscp = tsc;

	/*
	* We require _some_ success, but the quality control
	* will be based on the error terms on the TSC values.
	*/
	return count > 5;
	}

	/*
	* How many MSB values do we want to see? We aim for
	* a maximum error rate of 500ppm (in practice the
	* real error is much smaller), but refuse to spend
	* more than 50ms on it.
	*/
	#define MAX_QUICK_PIT_MS 50
	#define MAX_QUICK_PIT_ITERATIONS (MAX_QUICK_PIT_MS * PIT_TICK_RATE / 1000 / 256)

	static unsigned long __maybe_unused quick_pit_calibrate(void)
	{
	int i;
	u64 tsc, delta;
	unsigned long d1, d2;

	/* Set the Gate high, disable speaker */
	outb((inb(0x61) & ~0x02) \| 0x01, 0x61);

	/*
	* Counter 2, mode 0 (one-shot), binary count
	*
	* NOTE! Mode 2 decrements by two (and then the
	* output is flipped each time, giving the same
	* final output frequency as a decrement-by-one),
	* so mode 0 is much better when looking at the
	* individual counts.
	*/
	outb(0xb0, 0x43);

	/* Start at 0xffff */
	outb(0xff, 0x42);
	outb(0xff, 0x42);

	/*
	* The PIT starts counting at the next edge, so we
	* need to delay for a microsecond. The easiest way
	* to do that is to just read back the 16-bit counter
	* once from the PIT.
	*/
	pit_verify_msb(0);

	if (pit_expect_msb(0xff, &tsc, &d1)) {
	for (i = 1; i <= MAX_QUICK_PIT_ITERATIONS; i++) {
	if (!pit_expect_msb(0xff-i, &delta, &d2))
	break;

	/*
	* Iterate until the error is less than 500 ppm
	*/
	delta -= tsc;
	if (d1+d2 >= delta >> 11)
	continue;

	/*
	* Check the PIT one more time to verify that
	* all TSC reads were stable wrt the PIT.
	*
	* This also guarantees serialization of the
	* last cycle read ('d2') in pit_expect_msb.
	*/
	if (!pit_verify_msb(0xfe - i))
	break;
	goto success;
	}
	}
	debug("Fast TSC calibration failed\n");
	return 0;

	success:
	/*
	* Ok, if we get here, then we've seen the
	* MSB of the PIT decrement 'i' times, and the
	* error has shrunk to less than 500 ppm.
	*
	* As a result, we can depend on there not being
	* any odd delays anywhere, and the TSC reads are
	* reliable (within the error).
	*
	* kHz = ticks / time-in-seconds / 1000;
	* kHz = (t2 - t1) / (I * 256 / PIT_TICK_RATE) / 1000
	* kHz = ((t2 - t1) * PIT_TICK_RATE) / (I * 256 * 1000)
	*/
	delta *= PIT_TICK_RATE;
	delta /= (i2561000);
	debug("Fast TSC calibration using PIT\n");
	return delta / 1000;
	}

	/* Get the speed of the TSC timer in MHz */
	unsigned notrace long get_tbclk_mhz(void)
	{
	return get_tbclk() / 1000000;
	}

	static ulong get_ms_timer(void)
	{
	return (get_ticks() * 1000) / get_tbclk();
	}

	ulong get_timer(ulong base)
	{
	return get_ms_timer() - base;
	}

	ulong notrace timer_get_us(void)
	{
	return get_ticks() / get_tbclk_mhz();
	}

	ulong timer_get_boot_us(void)
	{
	return timer_get_us();
	}

	void __udelay(unsigned long usec)
	{
	u64 now = get_ticks();
	u64 stop;

	stop = now + usec * get_tbclk_mhz();

	while ((int64_t)(stop - get_ticks()) > 0)
	#if defined(CONFIG_QEMU) && defined(CONFIG_SMP)
	/*
	* Add a 'pause' instruction on qemu target,
	* to give other VCPUs a chance to run.
	*/
	asm volatile("pause");
	#else
	;
	#endif
	}

	static u64 tsc_timer_get_count(struct udevice *dev)
	{
	u64 now_tick = rdtsc();

	return now_tick - gd->arch.tsc_base;
	}

	static void tsc_timer_ensure_setup(bool early)
	{
	if (gd->arch.tsc_inited)
	return;
	if (IS_ENABLED(CONFIG_X86_TSC_READ_BASE))
	gd->arch.tsc_base = rdtsc();

	if (!gd->arch.clock_rate) {
	unsigned long fast_calibrate;

	fast_calibrate = native_calibrate_tsc();
	if (fast_calibrate)
	goto done;

	/* Reduce code size by dropping other methods */
	if (CONFIG_IS_ENABLED(X86_TSC_TIMER_NATIVE))
	panic("no timer");

	fast_calibrate = cpu_mhz_from_cpuid();
	if (fast_calibrate)
	goto done;

	fast_calibrate = cpu_mhz_from_msr();
	if (fast_calibrate)
	goto done;

	fast_calibrate = quick_pit_calibrate();
	if (fast_calibrate)
	goto done;

	if (early)
	fast_calibrate = CONFIG_X86_TSC_TIMER_EARLY_FREQ;
	else
	return;

	done:
	gd->arch.clock_rate = fast_calibrate * 1000000;
	}
	gd->arch.tsc_inited = true;
	}

	static int tsc_timer_probe(struct udevice *dev)
	{
	struct timer_dev_priv *uc_priv = dev_get_uclass_priv(dev);

	/* Try hardware calibration first */
	tsc_timer_ensure_setup(false);
	if (!gd->arch.clock_rate) {
	/*
	* Use the clock frequency specified in the
	* device tree as last resort
	*/
	if (!uc_priv->clock_rate)
	panic("TSC frequency is ZERO");
	} else {
	uc_priv->clock_rate = gd->arch.clock_rate;
	}

	return 0;
	}

	unsigned long notrace timer_early_get_rate(void)
	{
	/*
	* When TSC timer is used as the early timer, be warned that the timer
	* clock rate can only be calibrated via some hardware ways. Specifying
	* it in the device tree won't work for the early timer.
	*/
	tsc_timer_ensure_setup(true);

	return gd->arch.clock_rate;
	}

	u64 notrace timer_early_get_count(void)
	{
	tsc_timer_ensure_setup(true);

	return rdtsc() - gd->arch.tsc_base;
	}

	static const struct timer_ops tsc_timer_ops = {
	.get_count = tsc_timer_get_count,
	};

	static const struct udevice_id tsc_timer_ids[] = {
	{ .compatible = "x86,tsc-timer", },
	{ }
	};

	U_BOOT_DRIVER(tsc_timer) = {
	.name = "tsc_timer",
	.id = UCLASS_TIMER,
	.of_match = tsc_timer_ids,
	.probe = tsc_timer_probe,
	.ops = &tsc_timer_ops,
	};