/*
 * sched_clock.c: support for extending counters to full 64-bit ns counter
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 */
#include <linux/clocksource.h>
#include <linux/init.h>
#include <linux/jiffies.h>
#include <linux/kernel.h>
#include <linux/moduleparam.h>
#include <linux/sched.h>
#include <linux/syscore_ops.h>
#include <linux/timer.h>
#include <linux/sched_clock.h>

struct clock_data {
        u64 epoch_ns;
        u32 epoch_cyc;
        u32 epoch_cyc_copy;
        unsigned long rate;
        u32 mult;
        u32 shift;
        bool suspended;
};

static void sched_clock_poll(unsigned long wrap_ticks);
static DEFINE_TIMER(sched_clock_timer, sched_clock_poll, 0, 0);
static int irqtime = -1;

core_param(irqtime, irqtime, int, 0400);

static struct clock_data cd = {
        .mult   = NSEC_PER_SEC / HZ,
};

static u32 __read_mostly sched_clock_mask = 0xffffffff;

static u32 notrace jiffy_sched_clock_read(void)
{
        return (u32)(jiffies - INITIAL_JIFFIES);
}

static u32 __read_mostly (*read_sched_clock)(void) = jiffy_sched_clock_read;

static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)
{
        return (cyc * mult) >> shift;
}

static unsigned long long notrace sched_clock_32(void)
{
        u64 epoch_ns;
        u32 epoch_cyc;
        u32 cyc;

        if (cd.suspended)
                return cd.epoch_ns;

        /*
         * Load the epoch_cyc and epoch_ns atomically.  We do this by
         * ensuring that we always write epoch_cyc, epoch_ns and
         * epoch_cyc_copy in strict order, and read them in strict order.
         * If epoch_cyc and epoch_cyc_copy are not equal, then we're in
         * the middle of an update, and we should repeat the load.
         */
        do {
                epoch_cyc = cd.epoch_cyc;
                smp_rmb();
                epoch_ns = cd.epoch_ns;
                smp_rmb();
        } while (epoch_cyc != cd.epoch_cyc_copy);

        cyc = read_sched_clock();
        cyc = (cyc - epoch_cyc) & sched_clock_mask;
        return epoch_ns + cyc_to_ns(cyc, cd.mult, cd.shift);
}

/*
 * Atomically update the sched_clock epoch.
 */
static void notrace update_sched_clock(void)
{
        unsigned long flags;
        u32 cyc;
        u64 ns;

        cyc = read_sched_clock();
        ns = cd.epoch_ns +
                cyc_to_ns((cyc - cd.epoch_cyc) & sched_clock_mask,
                          cd.mult, cd.shift);
        /*
         * Write epoch_cyc and epoch_ns in a way that the update is
         * detectable in cyc_to_fixed_sched_clock().
         */
        raw_local_irq_save(flags);
        cd.epoch_cyc_copy = cyc;
        smp_wmb();
        cd.epoch_ns = ns;
        smp_wmb();
        cd.epoch_cyc = cyc;
        raw_local_irq_restore(flags);
}

static void sched_clock_poll(unsigned long wrap_ticks)
{
        mod_timer(&sched_clock_timer, round_jiffies(jiffies + wrap_ticks));
        update_sched_clock();
}

void __init setup_sched_clock(u32 (*read)(void), int bits, unsigned long rate)
{
        unsigned long r, w;
        u64 res, wrap;
        char r_unit;

        if (cd.rate > rate)
                return;

        BUG_ON(bits > 32);
        WARN_ON(!irqs_disabled());
        read_sched_clock = read;
        sched_clock_mask = (1ULL << bits) - 1;
        cd.rate = rate;

        /* calculate the mult/shift to convert counter ticks to ns. */
        clocks_calc_mult_shift(&cd.mult, &cd.shift, rate, NSEC_PER_SEC, 0);

        r = rate;
        if (r >= 4000000) {
                r /= 1000000;
                r_unit = 'M';
        } else if (r >= 1000) {
                r /= 1000;
                r_unit = 'k';
        } else
                r_unit = ' ';

        /* calculate how many ns until we wrap */
        wrap = cyc_to_ns((1ULL << bits) - 1, cd.mult, cd.shift);
        do_div(wrap, NSEC_PER_MSEC);
        w = wrap;

        /* calculate the ns resolution of this counter */
        res = cyc_to_ns(1ULL, cd.mult, cd.shift);
        pr_info("sched_clock: %u bits at %lu%cHz, resolution %lluns, wraps every %lums\n",
                bits, r, r_unit, res, w);

        /*
         * Start the timer to keep sched_clock() properly updated and
         * sets the initial epoch.
         */
        sched_clock_timer.data = msecs_to_jiffies(w - (w / 10));
        update_sched_clock();

        /*
         * Ensure that sched_clock() starts off at 0ns
         */
        cd.epoch_ns = 0;

        /* Enable IRQ time accounting if we have a fast enough sched_clock */
        if (irqtime > 0 || (irqtime == -1 && rate >= 1000000))
                enable_sched_clock_irqtime();

        pr_debug("Registered %pF as sched_clock source\n", read);
}

unsigned long long __read_mostly (*sched_clock_func)(void) = sched_clock_32;

unsigned long long notrace sched_clock(void)
{
        return sched_clock_func();
}

void __init sched_clock_postinit(void)
{
        /*
         * If no sched_clock function has been provided at that point,
         * make it the final one one.
         */
        if (read_sched_clock == jiffy_sched_clock_read)
                setup_sched_clock(jiffy_sched_clock_read, 32, HZ);

        sched_clock_poll(sched_clock_timer.data);
}

static int sched_clock_suspend(void)
{
        sched_clock_poll(sched_clock_timer.data);
        cd.suspended = true;
        return 0;
}

static void sched_clock_resume(void)
{
        cd.epoch_cyc = read_sched_clock();
        cd.epoch_cyc_copy = cd.epoch_cyc;
        cd.suspended = false;
}

static struct syscore_ops sched_clock_ops = {
        .suspend = sched_clock_suspend,
        .resume = sched_clock_resume,
};

static int __init sched_clock_syscore_init(void)
{
        register_syscore_ops(&sched_clock_ops);
        return 0;
}
device_initcall(sched_clock_syscore_init);