/*
 * Copyright (c) 2020 The DragonFly Project.  All rights reserved.
 *
 * This code is derived from software contributed to The DragonFly Project
 * by Matthew Dillon <dillon@backplane.com>
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 * 3. Neither the name of The DragonFly Project nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific, prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */
/*
 * Brute-force implementation for linux RCU functions.  Just use a delay
 * line and per-cpu callouts.
 */
#include <sys/types.h>
#include <sys/errno.h>
#include <sys/kernel.h>
#include <sys/spinlock.h>
#include <sys/spinlock2.h>
#include <linux/gfp.h>
#include <linux/slab.h>
#include <linux/rcupdate.h>

#include <machine/atomic.h>

typedef struct rcu_elm {
        enum { RCU_NULL, RCU_CALL, RCU_FREE } type;
        int     ticks;
        void    (*func)(struct rcu_head *arg);
        void    *ptr;
} rcu_elm_t;

typedef struct rcu_pcpu {
        rcu_elm_t *elms;
        int     size;
        int     mask;
        int     s;
        int     e;
        int     running;
        struct callout timer_callout;
} rcu_pcpu_t;

static rcu_pcpu_t *rcupcpu;

/*
 * Timer callout (pcpu)
 */
static void
rcu_timer(void *arg)
{
        rcu_pcpu_t *rcu = arg;
        rcu_elm_t *elm;
        int delta;

        crit_enter();
        while (rcu->s != rcu->e) {
                elm = &rcu->elms[rcu->s & rcu->mask];
                delta = ticks - elm->ticks;     /* 2s compl underflow */
                if (delta < hz)
                        break;

                switch(elm->type) {
                case RCU_NULL:
                        break;
                case RCU_CALL:
                        elm->func(elm->ptr);
                        break;
                case RCU_FREE:
                        kfree(elm->ptr);
                        break;
                }
                elm->type = RCU_NULL;
                ++rcu->s;
        }
        if (rcu->s == rcu->e) {
                rcu->running = 0;
        } else {
                callout_reset_bycpu(&rcu->timer_callout, hz / 10, rcu_timer,
                                    rcu, mycpuid);
        }
        crit_exit();
}


/*
 * Ping timer callout (pcpu).
 *
 * Must be in critical section.
 */
static void
rcu_ping(rcu_pcpu_t *rcu)
{
        if (rcu->running == 0) {
                rcu->running = 1;
                callout_reset_bycpu(&rcu->timer_callout, hz / 10, rcu_timer,
                                    rcu, mycpuid);
        }
}

/*
 * Expand the rcu array for the current cpu
 */
static void
rcu_expand(rcu_pcpu_t *rcu)
{
        rcu_elm_t *oelms;
        rcu_elm_t *nelms;
        int count;
        int nsize;
        int nmask;
        int n;

        count = rcu->e - rcu->s;        /* note: 2s complement underflow */
        while (unlikely(count == rcu->size)) {
                nsize = count ? count * 2 : 16;
                nelms = kzalloc(nsize * sizeof(*nelms), GFP_KERNEL);
                kprintf("drm: expand RCU cpu %d to %d\n", mycpuid, nsize);
                if (likely(count == rcu->size)) {
                        nmask = nsize - 1;
                        oelms = rcu->elms;
                        n = rcu->s;
                        while (n != rcu->e) {
                                nelms[n & nmask] = oelms[n & rcu->mask];
                                ++n;
                        }
                        rcu->elms = nelms;
                        rcu->size = nsize;
                        rcu->mask = nmask;
                        nelms = oelms;
                }
                if (likely(nelms != NULL))
                        kfree(nelms);
                count = rcu->e - rcu->s;
        }
        KKASSERT(count >= 0 && count < rcu->size);
}

void
__kfree_rcu(void *ptr)
{
        rcu_pcpu_t *rcu;
        rcu_elm_t *elm;

        if (unlikely(rcupcpu == NULL)) {
                kfree(ptr);
                return;
        }
        rcu = &rcupcpu[mycpuid];

        crit_enter();
        rcu_expand(rcu);
        elm = &rcu->elms[rcu->e & rcu->mask];
        ++rcu->e;

        elm->type = RCU_FREE;
        elm->ticks = ticks;
        elm->ptr = ptr;

        rcu_ping(rcu);
        crit_exit();
}

void
call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *))
{
        rcu_pcpu_t *rcu;
        rcu_elm_t *elm;

        if (unlikely(rcupcpu == NULL)) {
                func(head);
                return;
        }
        rcu = &rcupcpu[mycpuid];

        crit_enter();
        rcu_expand(rcu);
        elm = &rcu->elms[rcu->e & rcu->mask];
        ++rcu->e;

        elm->type = RCU_CALL;
        elm->ticks = ticks;
        elm->func = func;
        elm->ptr = head;

        rcu_ping(rcu);
        crit_exit();
}

static int
init_rcu(void *dummy __unused)
{
        int i;

        rcupcpu = kzalloc(ncpus * sizeof(*rcupcpu), GFP_KERNEL);
        for (i = 0; i < ncpus; ++i) {
                callout_init_mp(&rcupcpu[i].timer_callout);
        }
        return 0;
}

SYSINIT(linux_rcu_init, SI_SUB_DRIVERS, SI_ORDER_MIDDLE, init_rcu, NULL);