smaps: teach smaps_pte_range() about THP pmds

[linux-2.6.git] / arch / x86 / kernel / apb_timer.c

/*
 * apb_timer.c: Driver for Langwell APB timers
 *
 * (C) Copyright 2009 Intel Corporation
 * Author: Jacob Pan (jacob.jun.pan@intel.com)
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; version 2
 * of the License.
 *
 * Note:
 * Langwell is the south complex of Intel Moorestown MID platform. There are
 * eight external timers in total that can be used by the operating system.
 * The timer information, such as frequency and addresses, is provided to the
 * OS via SFI tables.
 * Timer interrupts are routed via FW/HW emulated IOAPIC independently via
 * individual redirection table entries (RTE).
 * Unlike HPET, there is no master counter, therefore one of the timers are
 * used as clocksource. The overall allocation looks like:
 *  - timer 0 - NR_CPUs for per cpu timer
 *  - one timer for clocksource
 *  - one timer for watchdog driver.
 * It is also worth notice that APB timer does not support true one-shot mode,
 * free-running mode will be used here to emulate one-shot mode.
 * APB timer can also be used as broadcast timer along with per cpu local APIC
 * timer, but by default APB timer has higher rating than local APIC timers.
 */

#include <linux/clocksource.h>
#include <linux/clockchips.h>
#include <linux/delay.h>
#include <linux/errno.h>
#include <linux/init.h>
#include <linux/sysdev.h>
#include <linux/slab.h>
#include <linux/pm.h>
#include <linux/pci.h>
#include <linux/sfi.h>
#include <linux/interrupt.h>
#include <linux/cpu.h>
#include <linux/irq.h>

#include <asm/fixmap.h>
#include <asm/apb_timer.h>
#include <asm/mrst.h>

#define APBT_MASK                       CLOCKSOURCE_MASK(32)
#define APBT_SHIFT                      22
#define APBT_CLOCKEVENT_RATING          110
#define APBT_CLOCKSOURCE_RATING         250
#define APBT_MIN_DELTA_USEC             200

#define EVT_TO_APBT_DEV(evt) container_of(evt, struct apbt_dev, evt)
#define APBT_CLOCKEVENT0_NUM   (0)
#define APBT_CLOCKEVENT1_NUM   (1)
#define APBT_CLOCKSOURCE_NUM   (2)

static unsigned long apbt_address;
static int apb_timer_block_enabled;
static void __iomem *apbt_virt_address;
static int phy_cs_timer_id;

/*
 * Common DW APB timer info
 */
static uint64_t apbt_freq;

static void apbt_set_mode(enum clock_event_mode mode,
                          struct clock_event_device *evt);
static int apbt_next_event(unsigned long delta,
                           struct clock_event_device *evt);
static cycle_t apbt_read_clocksource(struct clocksource *cs);
static void apbt_restart_clocksource(struct clocksource *cs);

struct apbt_dev {
        struct clock_event_device evt;
        unsigned int num;
        int cpu;
        unsigned int irq;
        unsigned int tick;
        unsigned int count;
        unsigned int flags;
        char name[10];
};

static DEFINE_PER_CPU(struct apbt_dev, cpu_apbt_dev);

#ifdef CONFIG_SMP
static unsigned int apbt_num_timers_used;
static struct apbt_dev *apbt_devs;
#endif

static  inline unsigned long apbt_readl_reg(unsigned long a)
{
        return readl(apbt_virt_address + a);
}

static inline void apbt_writel_reg(unsigned long d, unsigned long a)
{
        writel(d, apbt_virt_address + a);
}

static inline unsigned long apbt_readl(int n, unsigned long a)
{
        return readl(apbt_virt_address + a + n * APBTMRS_REG_SIZE);
}

static inline void apbt_writel(int n, unsigned long d, unsigned long a)
{
        writel(d, apbt_virt_address + a + n * APBTMRS_REG_SIZE);
}

static inline void apbt_set_mapping(void)
{
        struct sfi_timer_table_entry *mtmr;

        if (apbt_virt_address) {
                pr_debug("APBT base already mapped\n");
                return;
        }
        mtmr = sfi_get_mtmr(APBT_CLOCKEVENT0_NUM);
        if (mtmr == NULL) {
                printk(KERN_ERR "Failed to get MTMR %d from SFI\n",
                       APBT_CLOCKEVENT0_NUM);
                return;
        }
        apbt_address = (unsigned long)mtmr->phys_addr;
        if (!apbt_address) {
                printk(KERN_WARNING "No timer base from SFI, use default\n");
                apbt_address = APBT_DEFAULT_BASE;
        }
        apbt_virt_address = ioremap_nocache(apbt_address, APBT_MMAP_SIZE);
        if (apbt_virt_address) {
                pr_debug("Mapped APBT physical addr %p at virtual addr %p\n",\
                         (void *)apbt_address, (void *)apbt_virt_address);
        } else {
                pr_debug("Failed mapping APBT phy address at %p\n",\
                         (void *)apbt_address);
                goto panic_noapbt;
        }
        apbt_freq = mtmr->freq_hz / USEC_PER_SEC;
        sfi_free_mtmr(mtmr);

        /* Now figure out the physical timer id for clocksource device */
        mtmr = sfi_get_mtmr(APBT_CLOCKSOURCE_NUM);
        if (mtmr == NULL)
                goto panic_noapbt;

        /* Now figure out the physical timer id */
        phy_cs_timer_id = (unsigned int)(mtmr->phys_addr & 0xff)
                / APBTMRS_REG_SIZE;
        pr_debug("Use timer %d for clocksource\n", phy_cs_timer_id);
        return;

panic_noapbt:
        panic("Failed to setup APB system timer\n");

}

static inline void apbt_clear_mapping(void)
{
        iounmap(apbt_virt_address);
        apbt_virt_address = NULL;
}

/*
 * APBT timer interrupt enable / disable
 */
static inline int is_apbt_capable(void)
{
        return apbt_virt_address ? 1 : 0;
}

static struct clocksource clocksource_apbt = {
        .name           = "apbt",
        .rating         = APBT_CLOCKSOURCE_RATING,
        .read           = apbt_read_clocksource,
        .mask           = APBT_MASK,
        .shift          = APBT_SHIFT,
        .flags          = CLOCK_SOURCE_IS_CONTINUOUS,
        .resume         = apbt_restart_clocksource,
};

/* boot APB clock event device */
static struct clock_event_device apbt_clockevent = {
        .name           = "apbt0",
        .features       = CLOCK_EVT_FEAT_PERIODIC | CLOCK_EVT_FEAT_ONESHOT,
        .set_mode       = apbt_set_mode,
        .set_next_event = apbt_next_event,
        .shift          = APBT_SHIFT,
        .irq            = 0,
        .rating         = APBT_CLOCKEVENT_RATING,
};

/*
 * start count down from 0xffff_ffff. this is done by toggling the enable bit
 * then load initial load count to ~0.
 */
static void apbt_start_counter(int n)
{
        unsigned long ctrl = apbt_readl(n, APBTMR_N_CONTROL);

        ctrl &= ~APBTMR_CONTROL_ENABLE;
        apbt_writel(n, ctrl, APBTMR_N_CONTROL);
        apbt_writel(n, ~0, APBTMR_N_LOAD_COUNT);
        /* enable, mask interrupt */
        ctrl &= ~APBTMR_CONTROL_MODE_PERIODIC;
        ctrl |= (APBTMR_CONTROL_ENABLE | APBTMR_CONTROL_INT);
        apbt_writel(n, ctrl, APBTMR_N_CONTROL);
        /* read it once to get cached counter value initialized */
        apbt_read_clocksource(&clocksource_apbt);
}

static irqreturn_t apbt_interrupt_handler(int irq, void *data)
{
        struct apbt_dev *dev = (struct apbt_dev *)data;
        struct clock_event_device *aevt = &dev->evt;

        if (!aevt->event_handler) {
                printk(KERN_INFO "Spurious APBT timer interrupt on %d\n",
                       dev->num);
                return IRQ_NONE;
        }
        aevt->event_handler(aevt);
        return IRQ_HANDLED;
}

static void apbt_restart_clocksource(struct clocksource *cs)
{
        apbt_start_counter(phy_cs_timer_id);
}

static void apbt_enable_int(int n)
{
        unsigned long ctrl = apbt_readl(n, APBTMR_N_CONTROL);
        /* clear pending intr */
        apbt_readl(n, APBTMR_N_EOI);
        ctrl &= ~APBTMR_CONTROL_INT;
        apbt_writel(n, ctrl, APBTMR_N_CONTROL);
}

static void apbt_disable_int(int n)
{
        unsigned long ctrl = apbt_readl(n, APBTMR_N_CONTROL);

        ctrl |= APBTMR_CONTROL_INT;
        apbt_writel(n, ctrl, APBTMR_N_CONTROL);
}


static int __init apbt_clockevent_register(void)
{
        struct sfi_timer_table_entry *mtmr;
        struct apbt_dev *adev = &__get_cpu_var(cpu_apbt_dev);

        mtmr = sfi_get_mtmr(APBT_CLOCKEVENT0_NUM);
        if (mtmr == NULL) {
                printk(KERN_ERR "Failed to get MTMR %d from SFI\n",
                       APBT_CLOCKEVENT0_NUM);
                return -ENODEV;
        }

        /*
         * We need to calculate the scaled math multiplication factor for
         * nanosecond to apbt tick conversion.
         * mult = (nsec/cycle)*2^APBT_SHIFT
         */
        apbt_clockevent.mult = div_sc((unsigned long) mtmr->freq_hz
                                      , NSEC_PER_SEC, APBT_SHIFT);

        /* Calculate the min / max delta */
        apbt_clockevent.max_delta_ns = clockevent_delta2ns(0x7FFFFFFF,
                                                           &apbt_clockevent);
        apbt_clockevent.min_delta_ns = clockevent_delta2ns(
                APBT_MIN_DELTA_USEC*apbt_freq,
                &apbt_clockevent);
        /*
         * Start apbt with the boot cpu mask and make it
         * global if not used for per cpu timer.
         */
        apbt_clockevent.cpumask = cpumask_of(smp_processor_id());
        adev->num = smp_processor_id();
        memcpy(&adev->evt, &apbt_clockevent, sizeof(struct clock_event_device));

        if (mrst_timer_options == MRST_TIMER_LAPIC_APBT) {
                adev->evt.rating = APBT_CLOCKEVENT_RATING - 100;
                global_clock_event = &adev->evt;
                printk(KERN_DEBUG "%s clockevent registered as global\n",
                       global_clock_event->name);
        }

        if (request_irq(apbt_clockevent.irq, apbt_interrupt_handler,
                        IRQF_TIMER | IRQF_DISABLED | IRQF_NOBALANCING,
                        apbt_clockevent.name, adev)) {
                printk(KERN_ERR "Failed request IRQ for APBT%d\n",
                       apbt_clockevent.irq);
        }

        clockevents_register_device(&adev->evt);
        /* Start APBT 0 interrupts */
        apbt_enable_int(APBT_CLOCKEVENT0_NUM);

        sfi_free_mtmr(mtmr);
        return 0;
}

#ifdef CONFIG_SMP

static void apbt_setup_irq(struct apbt_dev *adev)
{
        /* timer0 irq has been setup early */
        if (adev->irq == 0)
                return;

        irq_modify_status(adev->irq, 0, IRQ_MOVE_PCNTXT);
        irq_set_affinity(adev->irq, cpumask_of(adev->cpu));
        /* APB timer irqs are set up as mp_irqs, timer is edge type */
        __set_irq_handler(adev->irq, handle_edge_irq, 0, "edge");

        if (system_state == SYSTEM_BOOTING) {
                if (request_irq(adev->irq, apbt_interrupt_handler,
                                        IRQF_TIMER | IRQF_DISABLED |
                                        IRQF_NOBALANCING,
                                        adev->name, adev)) {
                        printk(KERN_ERR "Failed request IRQ for APBT%d\n",
                               adev->num);
                }
        } else
                enable_irq(adev->irq);
}

/* Should be called with per cpu */
void apbt_setup_secondary_clock(void)
{
        struct apbt_dev *adev;
        struct clock_event_device *aevt;
        int cpu;

        /* Don't register boot CPU clockevent */
        cpu = smp_processor_id();
        if (!cpu)
                return;
        /*
         * We need to calculate the scaled math multiplication factor for
         * nanosecond to apbt tick conversion.
         * mult = (nsec/cycle)*2^APBT_SHIFT
         */
        printk(KERN_INFO "Init per CPU clockevent %d\n", cpu);
        adev = &per_cpu(cpu_apbt_dev, cpu);
        aevt = &adev->evt;

        memcpy(aevt, &apbt_clockevent, sizeof(*aevt));
        aevt->cpumask = cpumask_of(cpu);
        aevt->name = adev->name;
        aevt->mode = CLOCK_EVT_MODE_UNUSED;

        printk(KERN_INFO "Registering CPU %d clockevent device %s, mask %08x\n",
               cpu, aevt->name, *(u32 *)aevt->cpumask);

        apbt_setup_irq(adev);

        clockevents_register_device(aevt);

        apbt_enable_int(cpu);

        return;
}

/*
 * this notify handler process CPU hotplug events. in case of S0i3, nonboot
 * cpus are disabled/enabled frequently, for performance reasons, we keep the
 * per cpu timer irq registered so that we do need to do free_irq/request_irq.
 *
 * TODO: it might be more reliable to directly disable percpu clockevent device
 * without the notifier chain. currently, cpu 0 may get interrupts from other
 * cpu timers during the offline process due to the ordering of notification.
 * the extra interrupt is harmless.
 */
static int apbt_cpuhp_notify(struct notifier_block *n,
                             unsigned long action, void *hcpu)
{
        unsigned long cpu = (unsigned long)hcpu;
        struct apbt_dev *adev = &per_cpu(cpu_apbt_dev, cpu);

        switch (action & 0xf) {
        case CPU_DEAD:
                disable_irq(adev->irq);
                apbt_disable_int(cpu);
                if (system_state == SYSTEM_RUNNING) {
                        pr_debug("skipping APBT CPU %lu offline\n", cpu);
                } else if (adev) {
                        pr_debug("APBT clockevent for cpu %lu offline\n", cpu);
                        free_irq(adev->irq, adev);
                }
                break;
        default:
                pr_debug("APBT notified %lu, no action\n", action);
        }
        return NOTIFY_OK;
}

static __init int apbt_late_init(void)
{
        if (mrst_timer_options == MRST_TIMER_LAPIC_APBT ||
                !apb_timer_block_enabled)
                return 0;
        /* This notifier should be called after workqueue is ready */
        hotcpu_notifier(apbt_cpuhp_notify, -20);
        return 0;
}
fs_initcall(apbt_late_init);
#else

void apbt_setup_secondary_clock(void) {}

#endif /* CONFIG_SMP */

static void apbt_set_mode(enum clock_event_mode mode,
                          struct clock_event_device *evt)
{
        unsigned long ctrl;
        uint64_t delta;
        int timer_num;
        struct apbt_dev *adev = EVT_TO_APBT_DEV(evt);

        BUG_ON(!apbt_virt_address);

        timer_num = adev->num;
        pr_debug("%s CPU %d timer %d mode=%d\n",
                 __func__, first_cpu(*evt->cpumask), timer_num, mode);

        switch (mode) {
        case CLOCK_EVT_MODE_PERIODIC:
                delta = ((uint64_t)(NSEC_PER_SEC/HZ)) * apbt_clockevent.mult;
                delta >>= apbt_clockevent.shift;
                ctrl = apbt_readl(timer_num, APBTMR_N_CONTROL);
                ctrl |= APBTMR_CONTROL_MODE_PERIODIC;
                apbt_writel(timer_num, ctrl, APBTMR_N_CONTROL);
                /*
                 * DW APB p. 46, have to disable timer before load counter,
                 * may cause sync problem.
                 */
                ctrl &= ~APBTMR_CONTROL_ENABLE;
                apbt_writel(timer_num, ctrl, APBTMR_N_CONTROL);
                udelay(1);
                pr_debug("Setting clock period %d for HZ %d\n", (int)delta, HZ);
                apbt_writel(timer_num, delta, APBTMR_N_LOAD_COUNT);
                ctrl |= APBTMR_CONTROL_ENABLE;
                apbt_writel(timer_num, ctrl, APBTMR_N_CONTROL);
                break;
                /* APB timer does not have one-shot mode, use free running mode */
        case CLOCK_EVT_MODE_ONESHOT:
                ctrl = apbt_readl(timer_num, APBTMR_N_CONTROL);
                /*
                 * set free running mode, this mode will let timer reload max
                 * timeout which will give time (3min on 25MHz clock) to rearm
                 * the next event, therefore emulate the one-shot mode.
                 */
                ctrl &= ~APBTMR_CONTROL_ENABLE;
                ctrl &= ~APBTMR_CONTROL_MODE_PERIODIC;

                apbt_writel(timer_num, ctrl, APBTMR_N_CONTROL);
                /* write again to set free running mode */
                apbt_writel(timer_num, ctrl, APBTMR_N_CONTROL);

                /*
                 * DW APB p. 46, load counter with all 1s before starting free
                 * running mode.
                 */
                apbt_writel(timer_num, ~0, APBTMR_N_LOAD_COUNT);
                ctrl &= ~APBTMR_CONTROL_INT;
                ctrl |= APBTMR_CONTROL_ENABLE;
                apbt_writel(timer_num, ctrl, APBTMR_N_CONTROL);
                break;

        case CLOCK_EVT_MODE_UNUSED:
        case CLOCK_EVT_MODE_SHUTDOWN:
                apbt_disable_int(timer_num);
                ctrl = apbt_readl(timer_num, APBTMR_N_CONTROL);
                ctrl &= ~APBTMR_CONTROL_ENABLE;
                apbt_writel(timer_num, ctrl, APBTMR_N_CONTROL);
                break;

        case CLOCK_EVT_MODE_RESUME:
                apbt_enable_int(timer_num);
                break;
        }
}

static int apbt_next_event(unsigned long delta,
                           struct clock_event_device *evt)
{
        unsigned long ctrl;
        int timer_num;

        struct apbt_dev *adev = EVT_TO_APBT_DEV(evt);

        timer_num = adev->num;
        /* Disable timer */
        ctrl = apbt_readl(timer_num, APBTMR_N_CONTROL);
        ctrl &= ~APBTMR_CONTROL_ENABLE;
        apbt_writel(timer_num, ctrl, APBTMR_N_CONTROL);
        /* write new count */
        apbt_writel(timer_num, delta, APBTMR_N_LOAD_COUNT);
        ctrl |= APBTMR_CONTROL_ENABLE;
        apbt_writel(timer_num, ctrl, APBTMR_N_CONTROL);
        return 0;
}

static cycle_t apbt_read_clocksource(struct clocksource *cs)
{
        unsigned long current_count;

        current_count = apbt_readl(phy_cs_timer_id, APBTMR_N_CURRENT_VALUE);
        return (cycle_t)~current_count;
}

static int apbt_clocksource_register(void)
{
        u64 start, now;
        cycle_t t1;

        /* Start the counter, use timer 2 as source, timer 0/1 for event */
        apbt_start_counter(phy_cs_timer_id);

        /* Verify whether apbt counter works */
        t1 = apbt_read_clocksource(&clocksource_apbt);
        rdtscll(start);

        /*
         * We don't know the TSC frequency yet, but waiting for
         * 200000 TSC cycles is safe:
         * 4 GHz == 50us
         * 1 GHz == 200us
         */
        do {
                rep_nop();
                rdtscll(now);
        } while ((now - start) < 200000UL);

        /* APBT is the only always on clocksource, it has to work! */
        if (t1 == apbt_read_clocksource(&clocksource_apbt))
                panic("APBT counter not counting. APBT disabled\n");

        /*
         * initialize and register APBT clocksource
         * convert that to ns/clock cycle
         * mult = (ns/c) * 2^APBT_SHIFT
         */
        clocksource_apbt.mult = div_sc(MSEC_PER_SEC,
                                       (unsigned long) apbt_freq, APBT_SHIFT);
        clocksource_register(&clocksource_apbt);

        return 0;
}

/*
 * Early setup the APBT timer, only use timer 0 for booting then switch to
 * per CPU timer if possible.
 * returns 1 if per cpu apbt is setup
 * returns 0 if no per cpu apbt is chosen
 * panic if set up failed, this is the only platform timer on Moorestown.
 */
void __init apbt_time_init(void)
{
#ifdef CONFIG_SMP
        int i;
        struct sfi_timer_table_entry *p_mtmr;
        unsigned int percpu_timer;
        struct apbt_dev *adev;
#endif

        if (apb_timer_block_enabled)
                return;
        apbt_set_mapping();
        if (apbt_virt_address) {
                pr_debug("Found APBT version 0x%lx\n",\
                         apbt_readl_reg(APBTMRS_COMP_VERSION));
        } else
                goto out_noapbt;
        /*
         * Read the frequency and check for a sane value, for ESL model
         * we extend the possible clock range to allow time scaling.
         */

        if (apbt_freq < APBT_MIN_FREQ || apbt_freq > APBT_MAX_FREQ) {
                pr_debug("APBT has invalid freq 0x%llx\n", apbt_freq);
                goto out_noapbt;
        }
        if (apbt_clocksource_register()) {
                pr_debug("APBT has failed to register clocksource\n");
                goto out_noapbt;
        }
        if (!apbt_clockevent_register())
                apb_timer_block_enabled = 1;
        else {
                pr_debug("APBT has failed to register clockevent\n");
                goto out_noapbt;
        }
#ifdef CONFIG_SMP
        /* kernel cmdline disable apb timer, so we will use lapic timers */
        if (mrst_timer_options == MRST_TIMER_LAPIC_APBT) {
                printk(KERN_INFO "apbt: disabled per cpu timer\n");
                return;
        }
        pr_debug("%s: %d CPUs online\n", __func__, num_online_cpus());
        if (num_possible_cpus() <= sfi_mtimer_num) {
                percpu_timer = 1;
                apbt_num_timers_used = num_possible_cpus();
        } else {
                percpu_timer = 0;
                apbt_num_timers_used = 1;
                adev = &per_cpu(cpu_apbt_dev, 0);
                adev->flags &= ~APBT_DEV_USED;
        }
        pr_debug("%s: %d APB timers used\n", __func__, apbt_num_timers_used);

        /* here we set up per CPU timer data structure */
        apbt_devs = kzalloc(sizeof(struct apbt_dev) * apbt_num_timers_used,
                            GFP_KERNEL);
        if (!apbt_devs) {
                printk(KERN_ERR "Failed to allocate APB timer devices\n");
                return;
        }
        for (i = 0; i < apbt_num_timers_used; i++) {
                adev = &per_cpu(cpu_apbt_dev, i);
                adev->num = i;
                adev->cpu = i;
                p_mtmr = sfi_get_mtmr(i);
                if (p_mtmr) {
                        adev->tick = p_mtmr->freq_hz;
                        adev->irq = p_mtmr->irq;
                } else
                        printk(KERN_ERR "Failed to get timer for cpu %d\n", i);
                adev->count = 0;
                sprintf(adev->name, "apbt%d", i);
        }
#endif

        return;

out_noapbt:
        apbt_clear_mapping();
        apb_timer_block_enabled = 0;
        panic("failed to enable APB timer\n");
}

static inline void apbt_disable(int n)
{
        if (is_apbt_capable()) {
                unsigned long ctrl =  apbt_readl(n, APBTMR_N_CONTROL);
                ctrl &= ~APBTMR_CONTROL_ENABLE;
                apbt_writel(n, ctrl, APBTMR_N_CONTROL);
        }
}

/* called before apb_timer_enable, use early map */
unsigned long apbt_quick_calibrate()
{
        int i, scale;
        u64 old, new;
        cycle_t t1, t2;
        unsigned long khz = 0;
        u32 loop, shift;

        apbt_set_mapping();
        apbt_start_counter(phy_cs_timer_id);

        /* check if the timer can count down, otherwise return */
        old = apbt_read_clocksource(&clocksource_apbt);
        i = 10000;
        while (--i) {
                if (old != apbt_read_clocksource(&clocksource_apbt))
                        break;
        }
        if (!i)
                goto failed;

        /* count 16 ms */
        loop = (apbt_freq * 1000) << 4;

        /* restart the timer to ensure it won't get to 0 in the calibration */
        apbt_start_counter(phy_cs_timer_id);

        old = apbt_read_clocksource(&clocksource_apbt);
        old += loop;

        t1 = __native_read_tsc();

        do {
                new = apbt_read_clocksource(&clocksource_apbt);
        } while (new < old);

        t2 = __native_read_tsc();

        shift = 5;
        if (unlikely(loop >> shift == 0)) {
                printk(KERN_INFO
                       "APBT TSC calibration failed, not enough resolution\n");
                return 0;
        }
        scale = (int)div_u64((t2 - t1), loop >> shift);
        khz = (scale * apbt_freq * 1000) >> shift;
        printk(KERN_INFO "TSC freq calculated by APB timer is %lu khz\n", khz);
        return khz;
failed:
        return 0;
}