// SPDX-License-Identifier: GPL-2.0
/*
 * Smp timebase synchronization for ppc.
 *
 * Copyright (C) 2003 Samuel Rydh (samuel@ibrium.se)
 *
 */

#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/smp.h>
#include <linux/unistd.h>
#include <linux/slab.h>
#include <linux/atomic.h>
#include <asm/smp.h>
#include <asm/time.h>

#define NUM_ITER                300

enum {
        kExit=0, kSetAndTest, kTest
};

static struct {
        volatile u64            tb;
        volatile u64            mark;
        volatile int            cmd;
        volatile int            handshake;
        int                     filler[2];

        volatile int            ack;
        int                     filler2[7];

        volatile int            race_result;
} *tbsync;

static volatile int             running;

static void enter_contest(u64 mark, long add)
{
        while (get_tb() < mark)
                tbsync->race_result = add;
}

void smp_generic_take_timebase(void)
{
        int cmd;
        u64 tb;
        unsigned long flags;

        local_irq_save(flags);
        while (!running)
                barrier();
        rmb();

        for (;;) {
                tbsync->ack = 1;
                while (!tbsync->handshake)
                        barrier();
                rmb();

                cmd = tbsync->cmd;
                tb = tbsync->tb;
                mb();
                tbsync->ack = 0;
                if (cmd == kExit)
                        break;

                while (tbsync->handshake)
                        barrier();
                if (cmd == kSetAndTest)
                        set_tb(tb >> 32, tb & 0xfffffffful);
                enter_contest(tbsync->mark, -1);
        }
        local_irq_restore(flags);
}

static int start_contest(int cmd, long offset, int num)
{
        int i, score=0;
        u64 tb;
        u64 mark;

        tbsync->cmd = cmd;

        local_irq_disable();
        for (i = -3; i < num; ) {
                tb = get_tb() + 400;
                tbsync->tb = tb + offset;
                tbsync->mark = mark = tb + 400;

                wmb();

                tbsync->handshake = 1;
                while (tbsync->ack)
                        barrier();

                while (get_tb() <= tb)
                        barrier();
                tbsync->handshake = 0;
                enter_contest(mark, 1);

                while (!tbsync->ack)
                        barrier();

                if (i++ > 0)
                        score += tbsync->race_result;
        }
        local_irq_enable();
        return score;
}

void smp_generic_give_timebase(void)
{
        int i, score, score2, old, min=0, max=5000, offset=1000;

        pr_debug("Software timebase sync\n");

        /* if this fails then this kernel won't work anyway... */
        tbsync = kzalloc_obj(*tbsync);
        mb();
        running = 1;

        while (!tbsync->ack)
                barrier();

        pr_debug("Got ack\n");

        /* binary search */
        for (old = -1; old != offset ; offset = (min+max) / 2) {
                score = start_contest(kSetAndTest, offset, NUM_ITER);

                pr_debug("score %d, offset %d\n", score, offset );

                if( score > 0 )
                        max = offset;
                else
                        min = offset;
                old = offset;
        }
        score = start_contest(kSetAndTest, min, NUM_ITER);
        score2 = start_contest(kSetAndTest, max, NUM_ITER);

        pr_debug("Min %d (score %d), Max %d (score %d)\n",
                 min, score, max, score2);
        score = abs(score);
        score2 = abs(score2);
        offset = (score < score2) ? min : max;

        /* guard against inaccurate mttb */
        for (i = 0; i < 10; i++) {
                start_contest(kSetAndTest, offset, NUM_ITER/10);

                if ((score2 = start_contest(kTest, offset, NUM_ITER)) < 0)
                        score2 = -score2;
                if (score2 <= score || score2 < 20)
                        break;
        }
        pr_debug("Final offset: %d (%d/%d)\n", offset, score2, NUM_ITER );

        /* exiting */
        tbsync->cmd = kExit;
        wmb();
        tbsync->handshake = 1;
        while (tbsync->ack)
                barrier();
        tbsync->handshake = 0;
        kfree(tbsync);
        tbsync = NULL;
        running = 0;
}