airport: remove useless return in a function returning void

[linux/fpc-iii.git] / arch / x86 / kernel / tsc.c

#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/timer.h>
#include <linux/acpi_pmtmr.h>
#include <linux/cpufreq.h>
#include <linux/dmi.h>
#include <linux/delay.h>
#include <linux/clocksource.h>
#include <linux/percpu.h>

#include <asm/hpet.h>
#include <asm/timer.h>
#include <asm/vgtod.h>
#include <asm/time.h>
#include <asm/delay.h>

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

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

/* native_sched_clock() is called before tsc_init(), so
   we must start with the TSC soft disabled to prevent
   erroneous rdtsc usage on !cpu_has_tsc processors */
static int tsc_disabled = -1;

/*
 * Scheduler clock - returns current time in nanosec units.
 */
u64 native_sched_clock(void)
{
        u64 this_offset;

        /*
         * 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 achive it. )
         */
        if (unlikely(tsc_disabled)) {
                /* No locking but a rare wrong value is not a big deal: */
                return (jiffies_64 - INITIAL_JIFFIES) * (1000000000 / HZ);
        }

        /* read the Time Stamp Counter: */
        rdtscll(this_offset);

        /* return the value in ns */
        return __cycles_2_ns(this_offset);
}

/* 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();
}
#else
unsigned long long
sched_clock(void) __attribute__((alias("native_sched_clock")));
#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)
{
        printk(KERN_WARNING "notsc: Kernel compiled with CONFIG_X86_TSC, "
                        "cannot disable TSC completely.\n");
        tsc_disabled = 1;
        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);

#define MAX_RETRIES     5
#define SMI_TRESHOLD    50000

/*
 * Read TSC and the reference counters. Take care of SMI disturbance
 */
static u64 tsc_read_refs(u64 *p, int hpet)
{
        u64 t1, t2;
        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) < SMI_TRESHOLD)
                        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);
        do_div(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       (CLOCK_TICK_RATE / (1000 / CAL_MS))
#define CAL_PIT_LOOPS   1000

#define CAL2_MS         50
#define CAL2_LATCH      (CLOCK_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;

        /* 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 frequencty.
 */
static inline int pit_expect_msb(unsigned char val)
{
        int count = 0;

        for (count = 0; count < 50000; count++) {
                /* Ignore LSB */
                inb(0x42);
                if (inb(0x42) != val)
                        break;
        }
        return count > 50;
}

/*
 * How many MSB values do we want to see? We aim for a
 * 15ms calibration, which assuming a 2us counter read
 * error should give us roughly 150 ppm precision for
 * the calibration.
 */
#define QUICK_PIT_MS 15
#define QUICK_PIT_ITERATIONS (QUICK_PIT_MS * PIT_TICK_RATE / 1000 / 256)

static unsigned long quick_pit_calibrate(void)
{
        /* 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);

        if (pit_expect_msb(0xff)) {
                int i;
                u64 t1, t2, delta;
                unsigned char expect = 0xfe;

                t1 = get_cycles();
                for (i = 0; i < QUICK_PIT_ITERATIONS; i++, expect--) {
                        if (!pit_expect_msb(expect))
                                goto failed;
                }
                t2 = get_cycles();

                /*
                 * Make sure we can rely on the second TSC timestamp:
                 */
                if (!pit_expect_msb(expect))
                        goto failed;

                /*
                 * Ok, if we get here, then we've seen the
                 * MSB of the PIT decrement QUICK_PIT_ITERATIONS
                 * times, and each MSB had many hits, so we never
                 * had any sudden jumps.
                 *
                 * As a result, we can depend on there not being
                 * any odd delays anywhere, and the TSC reads are
                 * reliable.
                 *
                 * kHz = ticks / time-in-seconds / 1000;
                 * kHz = (t2 - t1) / (QPI * 256 / PIT_TICK_RATE) / 1000
                 * kHz = ((t2 - t1) * PIT_TICK_RATE) / (QPI * 256 * 1000)
                 */
                delta = (t2 - t1)*PIT_TICK_RATE;
                do_div(delta, QUICK_PIT_ITERATIONS*256*1000);
                printk("Fast TSC calibration using PIT\n");
                return delta;
        }
failed:
        return 0;
}

/**
 * native_calibrate_tsc - calibrate the tsc on boot
 */
unsigned long native_calibrate_tsc(void)
{
        u64 tsc1, tsc2, delta, ref1, ref2;
        unsigned long tsc_pit_min = ULONG_MAX, tsc_ref_min = ULONG_MAX;
        unsigned long flags, latch, ms, fast_calibrate;
        int hpet = is_hpet_enabled(), i, loopmin;

        local_irq_save(flags);
        fast_calibrate = quick_pit_calibrate();
        local_irq_restore(flags);
        if (fast_calibrate)
                return fast_calibrate;

        /*
         * 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 a
         * SMI/SMM disturbance happend 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 a SMI/SMM 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 (!hpet && !ref1 && !ref2)
                        continue;

                /* Check, whether the sampling was disturbed by an SMI */
                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) {
                        printk(KERN_INFO
                               "TSC: 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 */
                printk(KERN_WARNING "TSC: Unable to calibrate against PIT\n");

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

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

                /* Use the alternative source */
                printk(KERN_INFO "TSC: 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) {
                printk(KERN_INFO "TSC: Using PIT calibration value\n");
                return tsc_pit_min;
        }

        /* The alternative source failed, use the PIT calibration value */
        if (tsc_ref_min == ULONG_MAX) {
                printk(KERN_WARNING "TSC: 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:
         */
        printk(KERN_WARNING "TSC: PIT calibration deviates from %s: %lu %lu.\n",
               hpet ? "HPET" : "PMTIMER", tsc_pit_min, tsc_ref_min);
        printk(KERN_INFO "TSC: Using PIT calibration value\n");
        return tsc_pit_min;
}

#ifdef CONFIG_X86_32
/* Only called from the Powernow K7 cpu freq driver */
int recalibrate_cpu_khz(void)
{
#ifndef CONFIG_SMP
        unsigned long cpu_khz_old = cpu_khz;

        if (cpu_has_tsc) {
                tsc_khz = calibrate_tsc();
                cpu_khz = tsc_khz;
                cpu_data(0).loops_per_jiffy =
                        cpufreq_scale(cpu_data(0).loops_per_jiffy,
                                        cpu_khz_old, cpu_khz);
                return 0;
        } else
                return -ENODEV;
#else
        return -ENODEV;
#endif
}

EXPORT_SYMBOL(recalibrate_cpu_khz);

#endif /* CONFIG_X86_32 */

/* 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.
 *
 *  We can use khz divisor instead of mhz to keep a better precision, since
 *  cyc2ns_scale is limited to 10^6 * 2^10, which fits in 32 bits.
 *  (mathieu.desnoyers@polymtl.ca)
 *
 *                      -johnstul@us.ibm.com "math is hard, lets go shopping!"
 */

DEFINE_PER_CPU(unsigned long, cyc2ns);

static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
{
        unsigned long long tsc_now, ns_now;
        unsigned long flags, *scale;

        local_irq_save(flags);
        sched_clock_idle_sleep_event();

        scale = &per_cpu(cyc2ns, cpu);

        rdtscll(tsc_now);
        ns_now = __cycles_2_ns(tsc_now);

        if (cpu_khz)
                *scale = (NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR)/cpu_khz;

        sched_clock_idle_wakeup_event(0);
        local_irq_restore(flags);
}

#ifdef CONFIG_CPU_FREQ

/* Frequency scaling support. Adjust the TSC based timer when the cpu frequency
 * changes.
 *
 * RED-PEN: On SMP we assume all CPUs run with the same frequency.  It's
 * not that important because current Opteron setups do not support
 * scaling on SMP anyroads.
 *
 * 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;
        unsigned long *lpj, dummy;

        if (cpu_has(&cpu_data(freq->cpu), X86_FEATURE_CONSTANT_TSC))
                return 0;

        lpj = &dummy;
        if (!(freq->flags & CPUFREQ_CONST_LOOPS))
#ifdef CONFIG_SMP
                lpj = &cpu_data(freq->cpu).loops_per_jiffy;
#else
        lpj = &boot_cpu_data.loops_per_jiffy;
#endif

        if (!ref_freq) {
                ref_freq = freq->old;
                loops_per_jiffy_ref = *lpj;
                tsc_khz_ref = tsc_khz;
        }
        if ((val == CPUFREQ_PRECHANGE  && freq->old < freq->new) ||
                        (val == CPUFREQ_POSTCHANGE && freq->old > freq->new) ||
                        (val == CPUFREQ_RESUMECHANGE)) {
                *lpj =  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->cpu);

        return 0;
}

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

static int __init cpufreq_tsc(void)
{
        if (!cpu_has_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_tsc);

#endif /* CONFIG_CPU_FREQ */

/* clocksource code */

static struct clocksource clocksource_tsc;

/*
 * We 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.
 */
static cycle_t read_tsc(void)
{
        cycle_t ret = (cycle_t)get_cycles();

        return ret >= clocksource_tsc.cycle_last ?
                ret : clocksource_tsc.cycle_last;
}

#ifdef CONFIG_X86_64
static cycle_t __vsyscall_fn vread_tsc(void)
{
        cycle_t ret = (cycle_t)vget_cycles();

        return ret >= __vsyscall_gtod_data.clock.cycle_last ?
                ret : __vsyscall_gtod_data.clock.cycle_last;
}
#endif

static struct clocksource clocksource_tsc = {
        .name                   = "tsc",
        .rating                 = 300,
        .read                   = read_tsc,
        .mask                   = CLOCKSOURCE_MASK(64),
        .shift                  = 22,
        .flags                  = CLOCK_SOURCE_IS_CONTINUOUS |
                                  CLOCK_SOURCE_MUST_VERIFY,
#ifdef CONFIG_X86_64
        .vread                  = vread_tsc,
#endif
};

void mark_tsc_unstable(char *reason)
{
        if (!tsc_unstable) {
                tsc_unstable = 1;
                printk("Marking TSC unstable due to %s\n", reason);
                /* Change only the rating, when not registered */
                if (clocksource_tsc.mult)
                        clocksource_change_rating(&clocksource_tsc, 0);
                else
                        clocksource_tsc.rating = 0;
        }
}

EXPORT_SYMBOL_GPL(mark_tsc_unstable);

static int __init dmi_mark_tsc_unstable(const struct dmi_system_id *d)
{
        printk(KERN_NOTICE "%s detected: marking TSC unstable.\n",
                        d->ident);
        tsc_unstable = 1;
        return 0;
}

/* List of systems that have known TSC problems */
static struct dmi_system_id __initdata bad_tsc_dmi_table[] = {
        {
                .callback = dmi_mark_tsc_unstable,
                .ident = "IBM Thinkpad 380XD",
                .matches = {
                        DMI_MATCH(DMI_BOARD_VENDOR, "IBM"),
                        DMI_MATCH(DMI_BOARD_NAME, "2635FA0"),
                },
        },
        {}
};

/*
 * Geode_LX - the OLPC CPU has a possibly a very reliable TSC
 */
#ifdef CONFIG_MGEODE_LX
/* RTSC counts during suspend */
#define RTSC_SUSP 0x100

static void __init check_geode_tsc_reliable(void)
{
        unsigned long res_low, res_high;

        rdmsr_safe(MSR_GEODE_BUSCONT_CONF0, &res_low, &res_high);
        if (res_low & RTSC_SUSP)
                clocksource_tsc.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
}
#else
static inline void check_geode_tsc_reliable(void) { }
#endif

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

#ifdef CONFIG_X86_SMP
        if (apic_is_clustered_box())
                return 1;
#endif

        if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
                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)
                        tsc_unstable = 1;
        }

        return tsc_unstable;
}

static void __init init_tsc_clocksource(void)
{
        clocksource_tsc.mult = clocksource_khz2mult(tsc_khz,
                        clocksource_tsc.shift);
        /* lower the rating if we already know its unstable: */
        if (check_tsc_unstable()) {
                clocksource_tsc.rating = 0;
                clocksource_tsc.flags &= ~CLOCK_SOURCE_IS_CONTINUOUS;
        }
        clocksource_register(&clocksource_tsc);
}

void __init tsc_init(void)
{
        u64 lpj;
        int cpu;

        if (!cpu_has_tsc)
                return;

        tsc_khz = calibrate_tsc();
        cpu_khz = tsc_khz;

        if (!tsc_khz) {
                mark_tsc_unstable("could not calculate TSC khz");
                return;
        }

#ifdef CONFIG_X86_64
        if (cpu_has(&boot_cpu_data, X86_FEATURE_CONSTANT_TSC) &&
                        (boot_cpu_data.x86_vendor == X86_VENDOR_AMD))
                cpu_khz = calibrate_cpu();
#endif

        printk("Detected %lu.%03lu MHz processor.\n",
                        (unsigned long)cpu_khz / 1000,
                        (unsigned long)cpu_khz % 1000);

        /*
         * 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. (cpufreq notifiers will fix this
         * up if their speed diverges)
         */
        for_each_possible_cpu(cpu)
                set_cyc2ns_scale(cpu_khz, cpu);

        if (tsc_disabled > 0)
                return;

        /* now allow native_sched_clock() to use rdtsc */
        tsc_disabled = 0;

        lpj = ((u64)tsc_khz * 1000);
        do_div(lpj, HZ);
        lpj_fine = lpj;

        use_tsc_delay();
        /* Check and install the TSC clocksource */
        dmi_check_system(bad_tsc_dmi_table);

        if (unsynchronized_tsc())
                mark_tsc_unstable("TSCs unsynchronized");

        check_geode_tsc_reliable();
        init_tsc_clocksource();
}