// SPDX-License-Identifier: GPL-2.0-only
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/sched/clock.h>
#include <linux/init.h>
#include <linux/export.h>
#include <linux/timer.h>
#include <linux/acpi_pmtmr.h>
#include <linux/cpufreq.h>
#include <linux/delay.h>
#include <linux/clocksource.h>
#include <linux/percpu.h>
#include <linux/timex.h>
#include <linux/static_key.h>

#include <asm/hpet.h>
#include <asm/timer.h>
#include <asm/vgtod.h>
#include <asm/time.h>
#include <asm/delay.h>
#include <asm/hypervisor.h>
#include <asm/nmi.h>
#include <asm/x86_init.h>
#include <asm/geode.h>
#include <asm/apic.h>
#include <asm/intel-family.h>
#include <asm/i8259.h>
#include <asm/uv/uv.h>

unsigned int __read_mostly cpu_khz;     /* TSC clocks / usec, not used here */
EXPORT_SYMBOL(cpu_khz);

unsigned int __read_mostly tsc_khz;
EXPORT_SYMBOL(tsc_khz);

#define KHZ     1000

/*
 * TSC can be unstable due to cpufreq or due to unsynced TSCs
 */
static int __read_mostly tsc_unstable;
static unsigned int __initdata tsc_early_khz;

static DEFINE_STATIC_KEY_FALSE(__use_tsc);

int tsc_clocksource_reliable;

static u32 art_to_tsc_numerator;
static u32 art_to_tsc_denominator;
static u64 art_to_tsc_offset;
struct clocksource *art_related_clocksource;

struct cyc2ns {
        struct cyc2ns_data data[2];     /*  0 + 2*16 = 32 */
        seqcount_latch_t   seq;         /* 32 + 4    = 36 */

}; /* fits one cacheline */

static DEFINE_PER_CPU_ALIGNED(struct cyc2ns, cyc2ns);

static int __init tsc_early_khz_setup(char *buf)
{
        return kstrtouint(buf, 0, &tsc_early_khz);
}
early_param("tsc_early_khz", tsc_early_khz_setup);

__always_inline void cyc2ns_read_begin(struct cyc2ns_data *data)
{
        int seq, idx;

        preempt_disable_notrace();

        do {
                seq = this_cpu_read(cyc2ns.seq.seqcount.sequence);
                idx = seq & 1;

                data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset);
                data->cyc2ns_mul    = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul);
                data->cyc2ns_shift  = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift);

        } while (unlikely(seq != this_cpu_read(cyc2ns.seq.seqcount.sequence)));
}

__always_inline void cyc2ns_read_end(void)
{
        preempt_enable_notrace();
}

/*
 * Accelerators for sched_clock()
 * convert from cycles(64bits) => nanoseconds (64bits)
 *  basic equation:
 *              ns = cycles / (freq / ns_per_sec)
 *              ns = cycles * (ns_per_sec / freq)
 *              ns = cycles * (10^9 / (cpu_khz * 10^3))
 *              ns = cycles * (10^6 / cpu_khz)
 *
 *      Then we use scaling math (suggested by george@mvista.com) to get:
 *              ns = cycles * (10^6 * SC / cpu_khz) / SC
 *              ns = cycles * cyc2ns_scale / SC
 *
 *      And since SC is a constant power of two, we can convert the div
 *  into a shift. The larger SC is, the more accurate the conversion, but
 *  cyc2ns_scale needs to be a 32-bit value so that 32-bit multiplication
 *  (64-bit result) can be used.
 *
 *  We can use khz divisor instead of mhz to keep a better precision.
 *  (mathieu.desnoyers@polymtl.ca)
 *
 *                      -johnstul@us.ibm.com "math is hard, lets go shopping!"
 */

static __always_inline unsigned long long cycles_2_ns(unsigned long long cyc)
{
        struct cyc2ns_data data;
        unsigned long long ns;

        cyc2ns_read_begin(&data);

        ns = data.cyc2ns_offset;
        ns += mul_u64_u32_shr(cyc, data.cyc2ns_mul, data.cyc2ns_shift);

        cyc2ns_read_end();

        return ns;
}

static void __set_cyc2ns_scale(unsigned long khz, int cpu, unsigned long long tsc_now)
{
        unsigned long long ns_now;
        struct cyc2ns_data data;
        struct cyc2ns *c2n;

        ns_now = cycles_2_ns(tsc_now);

        /*
         * Compute a new multiplier as per the above comment and ensure our
         * time function is continuous; see the comment near struct
         * cyc2ns_data.
         */
        clocks_calc_mult_shift(&data.cyc2ns_mul, &data.cyc2ns_shift, khz,
                               NSEC_PER_MSEC, 0);

        /*
         * cyc2ns_shift is exported via arch_perf_update_userpage() where it is
         * not expected to be greater than 31 due to the original published
         * conversion algorithm shifting a 32-bit value (now specifies a 64-bit
         * value) - refer perf_event_mmap_page documentation in perf_event.h.
         */
        if (data.cyc2ns_shift == 32) {
                data.cyc2ns_shift = 31;
                data.cyc2ns_mul >>= 1;
        }

        data.cyc2ns_offset = ns_now -
                mul_u64_u32_shr(tsc_now, data.cyc2ns_mul, data.cyc2ns_shift);

        c2n = per_cpu_ptr(&cyc2ns, cpu);

        raw_write_seqcount_latch(&c2n->seq);
        c2n->data[0] = data;
        raw_write_seqcount_latch(&c2n->seq);
        c2n->data[1] = data;
}

static void set_cyc2ns_scale(unsigned long khz, int cpu, unsigned long long tsc_now)
{
        unsigned long flags;

        local_irq_save(flags);
        sched_clock_idle_sleep_event();

        if (khz)
                __set_cyc2ns_scale(khz, cpu, tsc_now);

        sched_clock_idle_wakeup_event();
        local_irq_restore(flags);
}

/*
 * Initialize cyc2ns for boot cpu
 */
static void __init cyc2ns_init_boot_cpu(void)
{
        struct cyc2ns *c2n = this_cpu_ptr(&cyc2ns);

        seqcount_latch_init(&c2n->seq);
        __set_cyc2ns_scale(tsc_khz, smp_processor_id(), rdtsc());
}

/*
 * Secondary CPUs do not run through tsc_init(), so set up
 * all the scale factors for all CPUs, assuming the same
 * speed as the bootup CPU.
 */
static void __init cyc2ns_init_secondary_cpus(void)
{
        unsigned int cpu, this_cpu = smp_processor_id();
        struct cyc2ns *c2n = this_cpu_ptr(&cyc2ns);
        struct cyc2ns_data *data = c2n->data;

        for_each_possible_cpu(cpu) {
                if (cpu != this_cpu) {
                        seqcount_latch_init(&c2n->seq);
                        c2n = per_cpu_ptr(&cyc2ns, cpu);
                        c2n->data[0] = data[0];
                        c2n->data[1] = data[1];
                }
        }
}

/*
 * Scheduler clock - returns current time in nanosec units.
 */
u64 native_sched_clock(void)
{
        if (static_branch_likely(&__use_tsc)) {
                u64 tsc_now = rdtsc();

                /* return the value in ns */
                return cycles_2_ns(tsc_now);
        }

        /*
         * Fall back to jiffies if there's no TSC available:
         * ( But note that we still use it if the TSC is marked
         *   unstable. We do this because unlike Time Of Day,
         *   the scheduler clock tolerates small errors and it's
         *   very important for it to be as fast as the platform
         *   can achieve it. )
         */

        /* No locking but a rare wrong value is not a big deal: */
        return (jiffies_64 - INITIAL_JIFFIES) * (1000000000 / HZ);
}

/*
 * Generate a sched_clock if you already have a TSC value.
 */
u64 native_sched_clock_from_tsc(u64 tsc)
{
        return cycles_2_ns(tsc);
}

/* We need to define a real function for sched_clock, to override the
   weak default version */
#ifdef CONFIG_PARAVIRT
unsigned long long sched_clock(void)
{
        return paravirt_sched_clock();
}

bool using_native_sched_clock(void)
{
        return pv_ops.time.sched_clock == native_sched_clock;
}
#else
unsigned long long
sched_clock(void) __attribute__((alias("native_sched_clock")));

bool using_native_sched_clock(void) { return true; }
#endif

int check_tsc_unstable(void)
{
        return tsc_unstable;
}
EXPORT_SYMBOL_GPL(check_tsc_unstable);

#ifdef CONFIG_X86_TSC
int __init notsc_setup(char *str)
{
        mark_tsc_unstable("boot parameter notsc");
        return 1;
}
#else
/*
 * disable flag for tsc. Takes effect by clearing the TSC cpu flag
 * in cpu/common.c
 */
int __init notsc_setup(char *str)
{
        setup_clear_cpu_cap(X86_FEATURE_TSC);
        return 1;
}
#endif

__setup("notsc", notsc_setup);

static int no_sched_irq_time;
static int no_tsc_watchdog;

static int __init tsc_setup(char *str)
{
        if (!strcmp(str, "reliable"))
                tsc_clocksource_reliable = 1;
        if (!strncmp(str, "noirqtime", 9))
                no_sched_irq_time = 1;
        if (!strcmp(str, "unstable"))
                mark_tsc_unstable("boot parameter");
        if (!strcmp(str, "nowatchdog"))
                no_tsc_watchdog = 1;
        return 1;
}

__setup("tsc=", tsc_setup);

#define MAX_RETRIES             5
#define TSC_DEFAULT_THRESHOLD   0x20000

/*
 * Read TSC and the reference counters. Take care of any disturbances
 */
static u64 tsc_read_refs(u64 *p, int hpet)
{
        u64 t1, t2;
        u64 thresh = tsc_khz ? tsc_khz >> 5 : TSC_DEFAULT_THRESHOLD;
        int i;

        for (i = 0; i < MAX_RETRIES; i++) {
                t1 = get_cycles();
                if (hpet)
                        *p = hpet_readl(HPET_COUNTER) & 0xFFFFFFFF;
                else
                        *p = acpi_pm_read_early();
                t2 = get_cycles();
                if ((t2 - t1) < thresh)
                        return t2;
        }
        return ULLONG_MAX;
}

/*
 * Calculate the TSC frequency from HPET reference
 */
static unsigned long calc_hpet_ref(u64 deltatsc, u64 hpet1, u64 hpet2)
{
        u64 tmp;

        if (hpet2 < hpet1)
                hpet2 += 0x100000000ULL;
        hpet2 -= hpet1;
        tmp = ((u64)hpet2 * hpet_readl(HPET_PERIOD));
        do_div(tmp, 1000000);
        deltatsc = div64_u64(deltatsc, tmp);

        return (unsigned long) deltatsc;
}

/*
 * Calculate the TSC frequency from PMTimer reference
 */
static unsigned long calc_pmtimer_ref(u64 deltatsc, u64 pm1, u64 pm2)
{
        u64 tmp;

        if (!pm1 && !pm2)
                return ULONG_MAX;

        if (pm2 < pm1)
                pm2 += (u64)ACPI_PM_OVRRUN;
        pm2 -= pm1;
        tmp = pm2 * 1000000000LL;
        do_div(tmp, PMTMR_TICKS_PER_SEC);
        do_div(deltatsc, tmp);

        return (unsigned long) deltatsc;
}

#define CAL_MS          10
#define CAL_LATCH       (PIT_TICK_RATE / (1000 / CAL_MS))
#define CAL_PIT_LOOPS   1000

#define CAL2_MS         50
#define CAL2_LATCH      (PIT_TICK_RATE / (1000 / CAL2_MS))
#define CAL2_PIT_LOOPS  5000


/*
 * Try to calibrate the TSC against the Programmable
 * Interrupt Timer and return the frequency of the TSC
 * in kHz.
 *
 * Return ULONG_MAX on failure to calibrate.
 */
static unsigned long pit_calibrate_tsc(u32 latch, unsigned long ms, int loopmin)
{
        u64 tsc, t1, t2, delta;
        unsigned long tscmin, tscmax;
        int pitcnt;

        if (!has_legacy_pic()) {
                /*
                 * Relies on tsc_early_delay_calibrate() to have given us semi
                 * usable udelay(), wait for the same 50ms we would have with
                 * the PIT loop below.
                 */
                udelay(10 * USEC_PER_MSEC);
                udelay(10 * USEC_PER_MSEC);
                udelay(10 * USEC_PER_MSEC);
                udelay(10 * USEC_PER_MSEC);
                udelay(10 * USEC_PER_MSEC);
                return ULONG_MAX;
        }

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

        /*
         * Setup CTC channel 2* for mode 0, (interrupt on terminal
         * count mode), binary count. Set the latch register to 50ms
         * (LSB then MSB) to begin countdown.
         */
        outb(0xb0, 0x43);
        outb(latch & 0xff, 0x42);
        outb(latch >> 8, 0x42);

        tsc = t1 = t2 = get_cycles();

        pitcnt = 0;
        tscmax = 0;
        tscmin = ULONG_MAX;
        while ((inb(0x61) & 0x20) == 0) {
                t2 = get_cycles();
                delta = t2 - tsc;
                tsc = t2;
                if ((unsigned long) delta < tscmin)
                        tscmin = (unsigned int) delta;
                if ((unsigned long) delta > tscmax)
                        tscmax = (unsigned int) delta;
                pitcnt++;
        }

        /*
         * Sanity checks:
         *
         * If we were not able to read the PIT more than loopmin
         * times, then we have been hit by a massive SMI
         *
         * If the maximum is 10 times larger than the minimum,
         * then we got hit by an SMI as well.
         */
        if (pitcnt < loopmin || tscmax > 10 * tscmin)
                return ULONG_MAX;

        /* Calculate the PIT value */
        delta = t2 - t1;
        do_div(delta, ms);
        return delta;
}

/*
 * 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 frequency.
 */
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 = get_cycles();
        }
        *deltap = get_cycles() - 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 quick_pit_calibrate(void)
{
        int i;
        u64 tsc, delta;
        unsigned long d1, d2;

        if (!has_legacy_pic())
                return 0;

        /* 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;

                        delta -= tsc;

                        /*
                         * Extrapolate the error and fail fast if the error will
                         * never be below 500 ppm.
                         */
                        if (i == 1 &&
                            d1 + d2 >= (delta * MAX_QUICK_PIT_ITERATIONS) >> 11)
                                return 0;

                        /*
                         * Iterate until the error is less than 500 ppm
                         */
                        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;
                }
        }
        pr_info("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;
        do_div(delta, i*256*1000);
        pr_info("Fast TSC calibration using PIT\n");
        return delta;
}

/**
 * native_calibrate_tsc
 * Determine TSC frequency via CPUID, else return 0.
 */
unsigned long native_calibrate_tsc(void)
{
        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);

        if (ebx_numerator == 0 || eax_denominator == 0)
                return 0;

        crystal_khz = ecx_hz / 1000;

        /*
         * Denverton SoCs don't report crystal clock, and also don't support
         * CPUID.0x16 for the calculation below, so hardcode the 25MHz crystal
         * clock.
         */
        if (crystal_khz == 0 &&
                        boot_cpu_data.x86_model == INTEL_FAM6_ATOM_GOLDMONT_D)
                crystal_khz = 25000;

        /*
         * TSC frequency reported directly by CPUID is a "hardware reported"
         * frequency and is the most accurate one so far we have. This
         * is considered a known frequency.
         */
        if (crystal_khz != 0)
                setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);

        /*
         * Some Intel SoCs like Skylake and Kabylake don't report the crystal
         * clock, but we can easily calculate it to a high degree of accuracy
         * by considering the crystal ratio and the CPU speed.
         */
        if (crystal_khz == 0 && boot_cpu_data.cpuid_level >= 0x16) {
                unsigned int eax_base_mhz, ebx, ecx, edx;

                cpuid(0x16, &eax_base_mhz, &ebx, &ecx, &edx);
                crystal_khz = eax_base_mhz * 1000 *
                        eax_denominator / ebx_numerator;
        }

        if (crystal_khz == 0)
                return 0;

        /*
         * For Atom SoCs TSC is the only reliable clocksource.
         * Mark TSC reliable so no watchdog on it.
         */
        if (boot_cpu_data.x86_model == INTEL_FAM6_ATOM_GOLDMONT)
                setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);

#ifdef CONFIG_X86_LOCAL_APIC
        /*
         * The local APIC appears to be fed by the core crystal clock
         * (which sounds entirely sensible). We can set the global
         * lapic_timer_period here to avoid having to calibrate the APIC
         * timer later.
         */
        lapic_timer_period = crystal_khz * 1000 / HZ;
#endif

        return crystal_khz * ebx_numerator / eax_denominator;
}

static unsigned long cpu_khz_from_cpuid(void)
{
        unsigned int eax_base_mhz, ebx_max_mhz, ecx_bus_mhz, edx;

        if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)
                return 0;

        if (boot_cpu_data.cpuid_level < 0x16)
                return 0;

        eax_base_mhz = ebx_max_mhz = ecx_bus_mhz = edx = 0;

        cpuid(0x16, &eax_base_mhz, &ebx_max_mhz, &ecx_bus_mhz, &edx);

        return eax_base_mhz * 1000;
}

/*
 * calibrate cpu using pit, hpet, and ptimer methods. They are available
 * later in boot after acpi is initialized.
 */
static unsigned long pit_hpet_ptimer_calibrate_cpu(void)
{
        u64 tsc1, tsc2, delta, ref1, ref2;
        unsigned long tsc_pit_min = ULONG_MAX, tsc_ref_min = ULONG_MAX;
        unsigned long flags, latch, ms;
        int hpet = is_hpet_enabled(), i, loopmin;

        /*
         * Run 5 calibration loops to get the lowest frequency value
         * (the best estimate). We use two different calibration modes
         * here:
         *
         * 1) PIT loop. We set the PIT Channel 2 to oneshot mode and
         * load a timeout of 50ms. We read the time right after we
         * started the timer and wait until the PIT count down reaches
         * zero. In each wait loop iteration we read the TSC and check
         * the delta to the previous read. We keep track of the min
         * and max values of that delta. The delta is mostly defined
         * by the IO time of the PIT access, so we can detect when
         * any disturbance happened between the two reads. If the
         * maximum time is significantly larger than the minimum time,
         * then we discard the result and have another try.
         *
         * 2) Reference counter. If available we use the HPET or the
         * PMTIMER as a reference to check the sanity of that value.
         * We use separate TSC readouts and check inside of the
         * reference read for any possible disturbance. We dicard
         * disturbed values here as well. We do that around the PIT
         * calibration delay loop as we have to wait for a certain
         * amount of time anyway.
         */

        /* Preset PIT loop values */
        latch = CAL_LATCH;
        ms = CAL_MS;
        loopmin = CAL_PIT_LOOPS;

        for (i = 0; i < 3; i++) {
                unsigned long tsc_pit_khz;

                /*
                 * Read the start value and the reference count of
                 * hpet/pmtimer when available. Then do the PIT
                 * calibration, which will take at least 50ms, and
                 * read the end value.
                 */
                local_irq_save(flags);
                tsc1 = tsc_read_refs(&ref1, hpet);
                tsc_pit_khz = pit_calibrate_tsc(latch, ms, loopmin);
                tsc2 = tsc_read_refs(&ref2, hpet);
                local_irq_restore(flags);

                /* Pick the lowest PIT TSC calibration so far */
                tsc_pit_min = min(tsc_pit_min, tsc_pit_khz);

                /* hpet or pmtimer available ? */
                if (ref1 == ref2)
                        continue;

                /* Check, whether the sampling was disturbed */
                if (tsc1 == ULLONG_MAX || tsc2 == ULLONG_MAX)
                        continue;

                tsc2 = (tsc2 - tsc1) * 1000000LL;
                if (hpet)
                        tsc2 = calc_hpet_ref(tsc2, ref1, ref2);
                else
                        tsc2 = calc_pmtimer_ref(tsc2, ref1, ref2);

                tsc_ref_min = min(tsc_ref_min, (unsigned long) tsc2);

                /* Check the reference deviation */
                delta = ((u64) tsc_pit_min) * 100;
                do_div(delta, tsc_ref_min);

                /*
                 * If both calibration results are inside a 10% window
                 * then we can be sure, that the calibration
                 * succeeded. We break out of the loop right away. We
                 * use the reference value, as it is more precise.
                 */
                if (delta >= 90 && delta <= 110) {
                        pr_info("PIT calibration matches %s. %d loops\n",
                                hpet ? "HPET" : "PMTIMER", i + 1);
                        return tsc_ref_min;
                }

                /*
                 * Check whether PIT failed more than once. This
                 * happens in virtualized environments. We need to
                 * give the virtual PC a slightly longer timeframe for
                 * the HPET/PMTIMER to make the result precise.
                 */
                if (i == 1 && tsc_pit_min == ULONG_MAX) {
                        latch = CAL2_LATCH;
                        ms = CAL2_MS;
                        loopmin = CAL2_PIT_LOOPS;
                }
        }

        /*
         * Now check the results.
         */
        if (tsc_pit_min == ULONG_MAX) {
                /* PIT gave no useful value */
                pr_warn("Unable to calibrate against PIT\n");

                /* We don't have an alternative source, disable TSC */
                if (!hpet && !ref1 && !ref2) {
                        pr_notice("No reference (HPET/PMTIMER) available\n");
                        return 0;
                }

                /* The alternative source failed as well, disable TSC */
                if (tsc_ref_min == ULONG_MAX) {
                        pr_warn("HPET/PMTIMER calibration failed\n");
                        return 0;
                }

                /* Use the alternative source */
                pr_info("using %s reference calibration\n",
                        hpet ? "HPET" : "PMTIMER");

                return tsc_ref_min;
        }

        /* We don't have an alternative source, use the PIT calibration value */
        if (!hpet && !ref1 && !ref2) {
                pr_info("Using PIT calibration value\n");
                return tsc_pit_min;
        }

        /* The alternative source failed, use the PIT calibration value */
        if (tsc_ref_min == ULONG_MAX) {
                pr_warn("HPET/PMTIMER calibration failed. Using PIT calibration.\n");
                return tsc_pit_min;
        }

        /*
         * The calibration values differ too much. In doubt, we use
         * the PIT value as we know that there are PMTIMERs around
         * running at double speed. At least we let the user know:
         */
        pr_warn("PIT calibration deviates from %s: %lu %lu\n",
                hpet ? "HPET" : "PMTIMER", tsc_pit_min, tsc_ref_min);
        pr_info("Using PIT calibration value\n");
        return tsc_pit_min;
}

/**
 * native_calibrate_cpu_early - can calibrate the cpu early in boot
 */
unsigned long native_calibrate_cpu_early(void)
{
        unsigned long flags, fast_calibrate = cpu_khz_from_cpuid();

        if (!fast_calibrate)
                fast_calibrate = cpu_khz_from_msr();
        if (!fast_calibrate) {
                local_irq_save(flags);
                fast_calibrate = quick_pit_calibrate();
                local_irq_restore(flags);
        }
        return fast_calibrate;
}


/**
 * native_calibrate_cpu - calibrate the cpu
 */
static unsigned long native_calibrate_cpu(void)
{
        unsigned long tsc_freq = native_calibrate_cpu_early();

        if (!tsc_freq)
                tsc_freq = pit_hpet_ptimer_calibrate_cpu();

        return tsc_freq;
}

void recalibrate_cpu_khz(void)
{
#ifndef CONFIG_SMP
        unsigned long cpu_khz_old = cpu_khz;

        if (!boot_cpu_has(X86_FEATURE_TSC))
                return;

        cpu_khz = x86_platform.calibrate_cpu();
        tsc_khz = x86_platform.calibrate_tsc();
        if (tsc_khz == 0)
                tsc_khz = cpu_khz;
        else if (abs(cpu_khz - tsc_khz) * 10 > tsc_khz)
                cpu_khz = tsc_khz;
        cpu_data(0).loops_per_jiffy = cpufreq_scale(cpu_data(0).loops_per_jiffy,
                                                    cpu_khz_old, cpu_khz);
#endif
}

EXPORT_SYMBOL(recalibrate_cpu_khz);


static unsigned long long cyc2ns_suspend;

void tsc_save_sched_clock_state(void)
{
        if (!sched_clock_stable())
                return;

        cyc2ns_suspend = sched_clock();
}

/*
 * Even on processors with invariant TSC, TSC gets reset in some the
 * ACPI system sleep states. And in some systems BIOS seem to reinit TSC to
 * arbitrary value (still sync'd across cpu's) during resume from such sleep
 * states. To cope up with this, recompute the cyc2ns_offset for each cpu so
 * that sched_clock() continues from the point where it was left off during
 * suspend.
 */
void tsc_restore_sched_clock_state(void)
{
        unsigned long long offset;
        unsigned long flags;
        int cpu;

        if (!sched_clock_stable())
                return;

        local_irq_save(flags);

        /*
         * We're coming out of suspend, there's no concurrency yet; don't
         * bother being nice about the RCU stuff, just write to both
         * data fields.
         */

        this_cpu_write(cyc2ns.data[0].cyc2ns_offset, 0);
        this_cpu_write(cyc2ns.data[1].cyc2ns_offset, 0);

        offset = cyc2ns_suspend - sched_clock();

        for_each_possible_cpu(cpu) {
                per_cpu(cyc2ns.data[0].cyc2ns_offset, cpu) = offset;
                per_cpu(cyc2ns.data[1].cyc2ns_offset, cpu) = offset;
        }

        local_irq_restore(flags);
}

#ifdef CONFIG_CPU_FREQ
/*
 * Frequency scaling support. Adjust the TSC based timer when the CPU frequency
 * changes.
 *
 * NOTE: On SMP the situation is not fixable in general, so simply mark the TSC
 * as unstable and give up in those cases.
 *
 * Should fix up last_tsc too. Currently gettimeofday in the
 * first tick after the change will be slightly wrong.
 */

static unsigned int  ref_freq;
static unsigned long loops_per_jiffy_ref;
static unsigned long tsc_khz_ref;

static int time_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
                                void *data)
{
        struct cpufreq_freqs *freq = data;

        if (num_online_cpus() > 1) {
                mark_tsc_unstable("cpufreq changes on SMP");
                return 0;
        }

        if (!ref_freq) {
                ref_freq = freq->old;
                loops_per_jiffy_ref = boot_cpu_data.loops_per_jiffy;
                tsc_khz_ref = tsc_khz;
        }

        if ((val == CPUFREQ_PRECHANGE  && freq->old < freq->new) ||
            (val == CPUFREQ_POSTCHANGE && freq->old > freq->new)) {
                boot_cpu_data.loops_per_jiffy =

                        cpufreq_scale(loops_per_jiffy_ref, ref_freq, freq->new);

                tsc_khz = cpufreq_scale(tsc_khz_ref, ref_freq, freq->new);
                if (!(freq->flags & CPUFREQ_CONST_LOOPS))
                        mark_tsc_unstable("cpufreq changes");

                set_cyc2ns_scale(tsc_khz, freq->policy->cpu, rdtsc());
        }

        return 0;
}

static struct notifier_block time_cpufreq_notifier_block = {
        .notifier_call  = time_cpufreq_notifier
};

static int __init cpufreq_register_tsc_scaling(void)
{
        if (!boot_cpu_has(X86_FEATURE_TSC))
                return 0;
        if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
                return 0;
        cpufreq_register_notifier(&time_cpufreq_notifier_block,
                                CPUFREQ_TRANSITION_NOTIFIER);
        return 0;
}

core_initcall(cpufreq_register_tsc_scaling);

#endif /* CONFIG_CPU_FREQ */

#define ART_CPUID_LEAF (0x15)
#define ART_MIN_DENOMINATOR (1)


/*
 * If ART is present detect the numerator:denominator to convert to TSC
 */
static void __init detect_art(void)
{
        unsigned int unused[2];

        if (boot_cpu_data.cpuid_level < ART_CPUID_LEAF)
                return;

        /*
         * Don't enable ART in a VM, non-stop TSC and TSC_ADJUST required,
         * and the TSC counter resets must not occur asynchronously.
         */
        if (boot_cpu_has(X86_FEATURE_HYPERVISOR) ||
            !boot_cpu_has(X86_FEATURE_NONSTOP_TSC) ||
            !boot_cpu_has(X86_FEATURE_TSC_ADJUST) ||
            tsc_async_resets)
                return;

        cpuid(ART_CPUID_LEAF, &art_to_tsc_denominator,
              &art_to_tsc_numerator, unused, unused+1);

        if (art_to_tsc_denominator < ART_MIN_DENOMINATOR)
                return;

        rdmsrl(MSR_IA32_TSC_ADJUST, art_to_tsc_offset);

        /* Make this sticky over multiple CPU init calls */
        setup_force_cpu_cap(X86_FEATURE_ART);
}


/* clocksource code */

static void tsc_resume(struct clocksource *cs)
{
        tsc_verify_tsc_adjust(true);
}

/*
 * We used to compare the TSC to the cycle_last value in the clocksource
 * structure to avoid a nasty time-warp. This can be observed in a
 * very small window right after one CPU updated cycle_last under
 * xtime/vsyscall_gtod lock and the other CPU reads a TSC value which
 * is smaller than the cycle_last reference value due to a TSC which
 * is slighty behind. This delta is nowhere else observable, but in
 * that case it results in a forward time jump in the range of hours
 * due to the unsigned delta calculation of the time keeping core
 * code, which is necessary to support wrapping clocksources like pm
 * timer.
 *
 * This sanity check is now done in the core timekeeping code.
 * checking the result of read_tsc() - cycle_last for being negative.
 * That works because CLOCKSOURCE_MASK(64) does not mask out any bit.
 */
static u64 read_tsc(struct clocksource *cs)
{
        return (u64)rdtsc_ordered();
}

static void tsc_cs_mark_unstable(struct clocksource *cs)
{
        if (tsc_unstable)
                return;

        tsc_unstable = 1;
        if (using_native_sched_clock())
                clear_sched_clock_stable();
        disable_sched_clock_irqtime();
        pr_info("Marking TSC unstable due to clocksource watchdog\n");
}

static void tsc_cs_tick_stable(struct clocksource *cs)
{
        if (tsc_unstable)
                return;

        if (using_native_sched_clock())
                sched_clock_tick_stable();
}

static int tsc_cs_enable(struct clocksource *cs)
{
        vclocks_set_used(VDSO_CLOCKMODE_TSC);
        return 0;
}

/*
 * .mask MUST be CLOCKSOURCE_MASK(64). See comment above read_tsc()
 */
static struct clocksource clocksource_tsc_early = {
        .name                   = "tsc-early",
        .rating                 = 299,
        .read                   = read_tsc,
        .mask                   = CLOCKSOURCE_MASK(64),
        .flags                  = CLOCK_SOURCE_IS_CONTINUOUS |
                                  CLOCK_SOURCE_MUST_VERIFY,
        .vdso_clock_mode        = VDSO_CLOCKMODE_TSC,
        .enable                 = tsc_cs_enable,
        .resume                 = tsc_resume,
        .mark_unstable          = tsc_cs_mark_unstable,
        .tick_stable            = tsc_cs_tick_stable,
        .list                   = LIST_HEAD_INIT(clocksource_tsc_early.list),
};

/*
 * Must mark VALID_FOR_HRES early such that when we unregister tsc_early
 * this one will immediately take over. We will only register if TSC has
 * been found good.
 */
static struct clocksource clocksource_tsc = {
        .name                   = "tsc",
        .rating                 = 300,
        .read                   = read_tsc,
        .mask                   = CLOCKSOURCE_MASK(64),
        .flags                  = CLOCK_SOURCE_IS_CONTINUOUS |
                                  CLOCK_SOURCE_VALID_FOR_HRES |
                                  CLOCK_SOURCE_MUST_VERIFY,
        .vdso_clock_mode        = VDSO_CLOCKMODE_TSC,
        .enable                 = tsc_cs_enable,
        .resume                 = tsc_resume,
        .mark_unstable          = tsc_cs_mark_unstable,
        .tick_stable            = tsc_cs_tick_stable,
        .list                   = LIST_HEAD_INIT(clocksource_tsc.list),
};

void mark_tsc_unstable(char *reason)
{
        if (tsc_unstable)
                return;

        tsc_unstable = 1;
        if (using_native_sched_clock())
                clear_sched_clock_stable();
        disable_sched_clock_irqtime();
        pr_info("Marking TSC unstable due to %s\n", reason);

        clocksource_mark_unstable(&clocksource_tsc_early);
        clocksource_mark_unstable(&clocksource_tsc);
}

EXPORT_SYMBOL_GPL(mark_tsc_unstable);

static void __init check_system_tsc_reliable(void)
{
#if defined(CONFIG_MGEODEGX1) || defined(CONFIG_MGEODE_LX) || defined(CONFIG_X86_GENERIC)
        if (is_geode_lx()) {
                /* RTSC counts during suspend */
#define RTSC_SUSP 0x100
                unsigned long res_low, res_high;

                rdmsr_safe(MSR_GEODE_BUSCONT_CONF0, &res_low, &res_high);
                /* Geode_LX - the OLPC CPU has a very reliable TSC */
                if (res_low & RTSC_SUSP)
                        tsc_clocksource_reliable = 1;
        }
#endif
        if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE))
                tsc_clocksource_reliable = 1;
}

/*
 * Make an educated guess if the TSC is trustworthy and synchronized
 * over all CPUs.
 */
int unsynchronized_tsc(void)
{
        if (!boot_cpu_has(X86_FEATURE_TSC) || tsc_unstable)
                return 1;

#ifdef CONFIG_SMP
        if (apic_is_clustered_box())
                return 1;
#endif

        if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
                return 0;

        if (tsc_clocksource_reliable)
                return 0;
        /*
         * Intel systems are normally all synchronized.
         * Exceptions must mark TSC as unstable:
         */
        if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL) {
                /* assume multi socket systems are not synchronized: */
                if (num_possible_cpus() > 1)
                        return 1;
        }

        return 0;
}

/*
 * Convert ART to TSC given numerator/denominator found in detect_art()
 */
struct system_counterval_t convert_art_to_tsc(u64 art)
{
        u64 tmp, res, rem;

        rem = do_div(art, art_to_tsc_denominator);

        res = art * art_to_tsc_numerator;
        tmp = rem * art_to_tsc_numerator;

        do_div(tmp, art_to_tsc_denominator);
        res += tmp + art_to_tsc_offset;

        return (struct system_counterval_t) {.cs = art_related_clocksource,
                        .cycles = res};
}
EXPORT_SYMBOL(convert_art_to_tsc);

/**
 * convert_art_ns_to_tsc() - Convert ART in nanoseconds to TSC.
 * @art_ns: ART (Always Running Timer) in unit of nanoseconds
 *
 * PTM requires all timestamps to be in units of nanoseconds. When user
 * software requests a cross-timestamp, this function converts system timestamp
 * to TSC.
 *
 * This is valid when CPU feature flag X86_FEATURE_TSC_KNOWN_FREQ is set
 * indicating the tsc_khz is derived from CPUID[15H]. Drivers should check
 * that this flag is set before conversion to TSC is attempted.
 *
 * Return:
 * struct system_counterval_t - system counter value with the pointer to the
 *      corresponding clocksource
 *      @cycles:        System counter value
 *      @cs:            Clocksource corresponding to system counter value. Used
 *                      by timekeeping code to verify comparibility of two cycle
 *                      values.
 */

struct system_counterval_t convert_art_ns_to_tsc(u64 art_ns)
{
        u64 tmp, res, rem;

        rem = do_div(art_ns, USEC_PER_SEC);

        res = art_ns * tsc_khz;
        tmp = rem * tsc_khz;

        do_div(tmp, USEC_PER_SEC);
        res += tmp;

        return (struct system_counterval_t) { .cs = art_related_clocksource,
                                              .cycles = res};
}
EXPORT_SYMBOL(convert_art_ns_to_tsc);


static void tsc_refine_calibration_work(struct work_struct *work);
static DECLARE_DELAYED_WORK(tsc_irqwork, tsc_refine_calibration_work);
/**
 * tsc_refine_calibration_work - Further refine tsc freq calibration
 * @work - ignored.
 *
 * This functions uses delayed work over a period of a
 * second to further refine the TSC freq value. Since this is
 * timer based, instead of loop based, we don't block the boot
 * process while this longer calibration is done.
 *
 * If there are any calibration anomalies (too many SMIs, etc),
 * or the refined calibration is off by 1% of the fast early
 * calibration, we throw out the new calibration and use the
 * early calibration.
 */
static void tsc_refine_calibration_work(struct work_struct *work)
{
        static u64 tsc_start = ULLONG_MAX, ref_start;
        static int hpet;
        u64 tsc_stop, ref_stop, delta;
        unsigned long freq;
        int cpu;

        /* Don't bother refining TSC on unstable systems */
        if (tsc_unstable)
                goto unreg;

        /*
         * Since the work is started early in boot, we may be
         * delayed the first time we expire. So set the workqueue
         * again once we know timers are working.
         */
        if (tsc_start == ULLONG_MAX) {
restart:
                /*
                 * Only set hpet once, to avoid mixing hardware
                 * if the hpet becomes enabled later.
                 */
                hpet = is_hpet_enabled();
                tsc_start = tsc_read_refs(&ref_start, hpet);
                schedule_delayed_work(&tsc_irqwork, HZ);
                return;
        }

        tsc_stop = tsc_read_refs(&ref_stop, hpet);

        /* hpet or pmtimer available ? */
        if (ref_start == ref_stop)
                goto out;

        /* Check, whether the sampling was disturbed */
        if (tsc_stop == ULLONG_MAX)
                goto restart;

        delta = tsc_stop - tsc_start;
        delta *= 1000000LL;
        if (hpet)
                freq = calc_hpet_ref(delta, ref_start, ref_stop);
        else
                freq = calc_pmtimer_ref(delta, ref_start, ref_stop);

        /* Make sure we're within 1% */
        if (abs(tsc_khz - freq) > tsc_khz/100)
                goto out;

        tsc_khz = freq;
        pr_info("Refined TSC clocksource calibration: %lu.%03lu MHz\n",
                (unsigned long)tsc_khz / 1000,
                (unsigned long)tsc_khz % 1000);

        /* Inform the TSC deadline clockevent devices about the recalibration */
        lapic_update_tsc_freq();

        /* Update the sched_clock() rate to match the clocksource one */
        for_each_possible_cpu(cpu)
                set_cyc2ns_scale(tsc_khz, cpu, tsc_stop);

out:
        if (tsc_unstable)
                goto unreg;

        if (boot_cpu_has(X86_FEATURE_ART))
                art_related_clocksource = &clocksource_tsc;
        clocksource_register_khz(&clocksource_tsc, tsc_khz);
unreg:
        clocksource_unregister(&clocksource_tsc_early);
}


static int __init init_tsc_clocksource(void)
{
        if (!boot_cpu_has(X86_FEATURE_TSC) || !tsc_khz)
                return 0;

        if (tsc_unstable)
                goto unreg;

        if (tsc_clocksource_reliable || no_tsc_watchdog)
                clocksource_tsc.flags &= ~CLOCK_SOURCE_MUST_VERIFY;

        if (boot_cpu_has(X86_FEATURE_NONSTOP_TSC_S3))
                clocksource_tsc.flags |= CLOCK_SOURCE_SUSPEND_NONSTOP;

        /*
         * When TSC frequency is known (retrieved via MSR or CPUID), we skip
         * the refined calibration and directly register it as a clocksource.
         */
        if (boot_cpu_has(X86_FEATURE_TSC_KNOWN_FREQ)) {
                if (boot_cpu_has(X86_FEATURE_ART))
                        art_related_clocksource = &clocksource_tsc;
                clocksource_register_khz(&clocksource_tsc, tsc_khz);
unreg:
                clocksource_unregister(&clocksource_tsc_early);
                return 0;
        }

        schedule_delayed_work(&tsc_irqwork, 0);
        return 0;
}
/*
 * We use device_initcall here, to ensure we run after the hpet
 * is fully initialized, which may occur at fs_initcall time.
 */
device_initcall(init_tsc_clocksource);

static bool __init determine_cpu_tsc_frequencies(bool early)
{
        /* Make sure that cpu and tsc are not already calibrated */
        WARN_ON(cpu_khz || tsc_khz);

        if (early) {
                cpu_khz = x86_platform.calibrate_cpu();
                if (tsc_early_khz)
                        tsc_khz = tsc_early_khz;
                else
                        tsc_khz = x86_platform.calibrate_tsc();
        } else {
                /* We should not be here with non-native cpu calibration */
                WARN_ON(x86_platform.calibrate_cpu != native_calibrate_cpu);
                cpu_khz = pit_hpet_ptimer_calibrate_cpu();
        }

        /*
         * Trust non-zero tsc_khz as authoritative,
         * and use it to sanity check cpu_khz,
         * which will be off if system timer is off.
         */
        if (tsc_khz == 0)
                tsc_khz = cpu_khz;
        else if (abs(cpu_khz - tsc_khz) * 10 > tsc_khz)
                cpu_khz = tsc_khz;

        if (tsc_khz == 0)
                return false;

        pr_info("Detected %lu.%03lu MHz processor\n",
                (unsigned long)cpu_khz / KHZ,
                (unsigned long)cpu_khz % KHZ);

        if (cpu_khz != tsc_khz) {
                pr_info("Detected %lu.%03lu MHz TSC",
                        (unsigned long)tsc_khz / KHZ,
                        (unsigned long)tsc_khz % KHZ);
        }
        return true;
}

static unsigned long __init get_loops_per_jiffy(void)
{
        u64 lpj = (u64)tsc_khz * KHZ;

        do_div(lpj, HZ);
        return lpj;
}

static void __init tsc_enable_sched_clock(void)
{
        /* Sanitize TSC ADJUST before cyc2ns gets initialized */
        tsc_store_and_check_tsc_adjust(true);
        cyc2ns_init_boot_cpu();
        static_branch_enable(&__use_tsc);
}

void __init tsc_early_init(void)
{
        if (!boot_cpu_has(X86_FEATURE_TSC))
                return;
        /* Don't change UV TSC multi-chassis synchronization */
        if (is_early_uv_system())
                return;
        if (!determine_cpu_tsc_frequencies(true))
                return;
        loops_per_jiffy = get_loops_per_jiffy();

        tsc_enable_sched_clock();
}

void __init tsc_init(void)
{
        /*
         * native_calibrate_cpu_early can only calibrate using methods that are
         * available early in boot.
         */
        if (x86_platform.calibrate_cpu == native_calibrate_cpu_early)
                x86_platform.calibrate_cpu = native_calibrate_cpu;

        if (!boot_cpu_has(X86_FEATURE_TSC)) {
                setup_clear_cpu_cap(X86_FEATURE_TSC_DEADLINE_TIMER);
                return;
        }

        if (!tsc_khz) {
                /* We failed to determine frequencies earlier, try again */
                if (!determine_cpu_tsc_frequencies(false)) {
                        mark_tsc_unstable("could not calculate TSC khz");
                        setup_clear_cpu_cap(X86_FEATURE_TSC_DEADLINE_TIMER);
                        return;
                }
                tsc_enable_sched_clock();
        }

        cyc2ns_init_secondary_cpus();

        if (!no_sched_irq_time)
                enable_sched_clock_irqtime();

        lpj_fine = get_loops_per_jiffy();
        use_tsc_delay();

        check_system_tsc_reliable();

        if (unsynchronized_tsc()) {
                mark_tsc_unstable("TSCs unsynchronized");
                return;
        }

        if (tsc_clocksource_reliable || no_tsc_watchdog)
                clocksource_tsc_early.flags &= ~CLOCK_SOURCE_MUST_VERIFY;

        clocksource_register_khz(&clocksource_tsc_early, tsc_khz);
        detect_art();
}

#ifdef CONFIG_SMP
/*
 * If we have a constant TSC and are using the TSC for the delay loop,
 * we can skip clock calibration if another cpu in the same socket has already
 * been calibrated. This assumes that CONSTANT_TSC applies to all
 * cpus in the socket - this should be a safe assumption.
 */
unsigned long calibrate_delay_is_known(void)
{
        int sibling, cpu = smp_processor_id();
        int constant_tsc = cpu_has(&cpu_data(cpu), X86_FEATURE_CONSTANT_TSC);
        const struct cpumask *mask = topology_core_cpumask(cpu);

        if (!constant_tsc || !mask)
                return 0;

        sibling = cpumask_any_but(mask, cpu);
        if (sibling < nr_cpu_ids)
                return cpu_data(sibling).loops_per_jiffy;
        return 0;
}
#endif