// SPDX-License-Identifier: GPL-2.0-only
/* Copyright (C) 2013 Cisco Systems, Inc, 2013.
 *
 * Author: Vijay Subramanian <vijaynsu@cisco.com>
 * Author: Mythili Prabhu <mysuryan@cisco.com>
 *
 * ECN support is added by Naeem Khademi <naeemk@ifi.uio.no>
 * University of Oslo, Norway.
 *
 * References:
 * RFC 8033: https://tools.ietf.org/html/rfc8033
 */

#include <linux/module.h>
#include <linux/slab.h>
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/errno.h>
#include <linux/skbuff.h>
#include <net/pkt_sched.h>
#include <net/inet_ecn.h>
#include <net/pie.h>

/* private data for the Qdisc */
struct pie_sched_data {
        struct pie_vars vars;
        struct pie_params params;
        struct pie_stats stats;
        struct timer_list adapt_timer;
        struct Qdisc *sch;
};

bool pie_drop_early(struct Qdisc *sch, struct pie_params *params,
                    struct pie_vars *vars, u32 backlog, u32 packet_size)
{
        u64 rnd;
        u64 local_prob = vars->prob;
        u32 mtu = psched_mtu(qdisc_dev(sch));

        /* If there is still burst allowance left skip random early drop */
        if (vars->burst_time > 0)
                return false;

        /* If current delay is less than half of target, and
         * if drop prob is low already, disable early_drop
         */
        if ((vars->qdelay < params->target / 2) &&
            (vars->prob < MAX_PROB / 5))
                return false;

        /* If we have fewer than 2 mtu-sized packets, disable pie_drop_early,
         * similar to min_th in RED
         */
        if (backlog < 2 * mtu)
                return false;

        /* If bytemode is turned on, use packet size to compute new
         * probablity. Smaller packets will have lower drop prob in this case
         */
        if (params->bytemode && packet_size <= mtu)
                local_prob = (u64)packet_size * div_u64(local_prob, mtu);
        else
                local_prob = vars->prob;

        if (local_prob == 0)
                vars->accu_prob = 0;
        else
                vars->accu_prob += local_prob;

        if (vars->accu_prob < (MAX_PROB / 100) * 85)
                return false;
        if (vars->accu_prob >= (MAX_PROB / 2) * 17)
                return true;

        prandom_bytes(&rnd, 8);
        if ((rnd >> BITS_PER_BYTE) < local_prob) {
                vars->accu_prob = 0;
                return true;
        }

        return false;
}
EXPORT_SYMBOL_GPL(pie_drop_early);

static int pie_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
                             struct sk_buff **to_free)
{
        struct pie_sched_data *q = qdisc_priv(sch);
        bool enqueue = false;

        if (unlikely(qdisc_qlen(sch) >= sch->limit)) {
                q->stats.overlimit++;
                goto out;
        }

        if (!pie_drop_early(sch, &q->params, &q->vars, sch->qstats.backlog,
                            skb->len)) {
                enqueue = true;
        } else if (q->params.ecn && (q->vars.prob <= MAX_PROB / 10) &&
                   INET_ECN_set_ce(skb)) {
                /* If packet is ecn capable, mark it if drop probability
                 * is lower than 10%, else drop it.
                 */
                q->stats.ecn_mark++;
                enqueue = true;
        }

        /* we can enqueue the packet */
        if (enqueue) {
                /* Set enqueue time only when dq_rate_estimator is disabled. */
                if (!q->params.dq_rate_estimator)
                        pie_set_enqueue_time(skb);

                q->stats.packets_in++;
                if (qdisc_qlen(sch) > q->stats.maxq)
                        q->stats.maxq = qdisc_qlen(sch);

                return qdisc_enqueue_tail(skb, sch);
        }

out:
        q->stats.dropped++;
        q->vars.accu_prob = 0;
        return qdisc_drop(skb, sch, to_free);
}

static const struct nla_policy pie_policy[TCA_PIE_MAX + 1] = {
        [TCA_PIE_TARGET]                = {.type = NLA_U32},
        [TCA_PIE_LIMIT]                 = {.type = NLA_U32},
        [TCA_PIE_TUPDATE]               = {.type = NLA_U32},
        [TCA_PIE_ALPHA]                 = {.type = NLA_U32},
        [TCA_PIE_BETA]                  = {.type = NLA_U32},
        [TCA_PIE_ECN]                   = {.type = NLA_U32},
        [TCA_PIE_BYTEMODE]              = {.type = NLA_U32},
        [TCA_PIE_DQ_RATE_ESTIMATOR]     = {.type = NLA_U32},
};

static int pie_change(struct Qdisc *sch, struct nlattr *opt,
                      struct netlink_ext_ack *extack)
{
        struct pie_sched_data *q = qdisc_priv(sch);
        struct nlattr *tb[TCA_PIE_MAX + 1];
        unsigned int qlen, dropped = 0;
        int err;

        if (!opt)
                return -EINVAL;

        err = nla_parse_nested_deprecated(tb, TCA_PIE_MAX, opt, pie_policy,
                                          NULL);
        if (err < 0)
                return err;

        sch_tree_lock(sch);

        /* convert from microseconds to pschedtime */
        if (tb[TCA_PIE_TARGET]) {
                /* target is in us */
                u32 target = nla_get_u32(tb[TCA_PIE_TARGET]);

                /* convert to pschedtime */
                q->params.target = PSCHED_NS2TICKS((u64)target * NSEC_PER_USEC);
        }

        /* tupdate is in jiffies */
        if (tb[TCA_PIE_TUPDATE])
                q->params.tupdate =
                        usecs_to_jiffies(nla_get_u32(tb[TCA_PIE_TUPDATE]));

        if (tb[TCA_PIE_LIMIT]) {
                u32 limit = nla_get_u32(tb[TCA_PIE_LIMIT]);

                q->params.limit = limit;
                sch->limit = limit;
        }

        if (tb[TCA_PIE_ALPHA])
                q->params.alpha = nla_get_u32(tb[TCA_PIE_ALPHA]);

        if (tb[TCA_PIE_BETA])
                q->params.beta = nla_get_u32(tb[TCA_PIE_BETA]);

        if (tb[TCA_PIE_ECN])
                q->params.ecn = nla_get_u32(tb[TCA_PIE_ECN]);

        if (tb[TCA_PIE_BYTEMODE])
                q->params.bytemode = nla_get_u32(tb[TCA_PIE_BYTEMODE]);

        if (tb[TCA_PIE_DQ_RATE_ESTIMATOR])
                q->params.dq_rate_estimator =
                                nla_get_u32(tb[TCA_PIE_DQ_RATE_ESTIMATOR]);

        /* Drop excess packets if new limit is lower */
        qlen = sch->q.qlen;
        while (sch->q.qlen > sch->limit) {
                struct sk_buff *skb = __qdisc_dequeue_head(&sch->q);

                dropped += qdisc_pkt_len(skb);
                qdisc_qstats_backlog_dec(sch, skb);
                rtnl_qdisc_drop(skb, sch);
        }
        qdisc_tree_reduce_backlog(sch, qlen - sch->q.qlen, dropped);

        sch_tree_unlock(sch);
        return 0;
}

void pie_process_dequeue(struct sk_buff *skb, struct pie_params *params,
                         struct pie_vars *vars, u32 backlog)
{
        psched_time_t now = psched_get_time();
        u32 dtime = 0;

        /* If dq_rate_estimator is disabled, calculate qdelay using the
         * packet timestamp.
         */
        if (!params->dq_rate_estimator) {
                vars->qdelay = now - pie_get_enqueue_time(skb);

                if (vars->dq_tstamp != DTIME_INVALID)
                        dtime = now - vars->dq_tstamp;

                vars->dq_tstamp = now;

                if (backlog == 0)
                        vars->qdelay = 0;

                if (dtime == 0)
                        return;

                goto burst_allowance_reduction;
        }

        /* If current queue is about 10 packets or more and dq_count is unset
         * we have enough packets to calculate the drain rate. Save
         * current time as dq_tstamp and start measurement cycle.
         */
        if (backlog >= QUEUE_THRESHOLD && vars->dq_count == DQCOUNT_INVALID) {
                vars->dq_tstamp = psched_get_time();
                vars->dq_count = 0;
        }

        /* Calculate the average drain rate from this value. If queue length
         * has receded to a small value viz., <= QUEUE_THRESHOLD bytes, reset
         * the dq_count to -1 as we don't have enough packets to calculate the
         * drain rate anymore. The following if block is entered only when we
         * have a substantial queue built up (QUEUE_THRESHOLD bytes or more)
         * and we calculate the drain rate for the threshold here.  dq_count is
         * in bytes, time difference in psched_time, hence rate is in
         * bytes/psched_time.
         */
        if (vars->dq_count != DQCOUNT_INVALID) {
                vars->dq_count += skb->len;

                if (vars->dq_count >= QUEUE_THRESHOLD) {
                        u32 count = vars->dq_count << PIE_SCALE;

                        dtime = now - vars->dq_tstamp;

                        if (dtime == 0)
                                return;

                        count = count / dtime;

                        if (vars->avg_dq_rate == 0)
                                vars->avg_dq_rate = count;
                        else
                                vars->avg_dq_rate =
                                    (vars->avg_dq_rate -
                                     (vars->avg_dq_rate >> 3)) + (count >> 3);

                        /* If the queue has receded below the threshold, we hold
                         * on to the last drain rate calculated, else we reset
                         * dq_count to 0 to re-enter the if block when the next
                         * packet is dequeued
                         */
                        if (backlog < QUEUE_THRESHOLD) {
                                vars->dq_count = DQCOUNT_INVALID;
                        } else {
                                vars->dq_count = 0;
                                vars->dq_tstamp = psched_get_time();
                        }

                        goto burst_allowance_reduction;
                }
        }

        return;

burst_allowance_reduction:
        if (vars->burst_time > 0) {
                if (vars->burst_time > dtime)
                        vars->burst_time -= dtime;
                else
                        vars->burst_time = 0;
        }
}
EXPORT_SYMBOL_GPL(pie_process_dequeue);

void pie_calculate_probability(struct pie_params *params, struct pie_vars *vars,
                               u32 backlog)
{
        psched_time_t qdelay = 0;       /* in pschedtime */
        psched_time_t qdelay_old = 0;   /* in pschedtime */
        s64 delta = 0;          /* determines the change in probability */
        u64 oldprob;
        u64 alpha, beta;
        u32 power;
        bool update_prob = true;

        if (params->dq_rate_estimator) {
                qdelay_old = vars->qdelay;
                vars->qdelay_old = vars->qdelay;

                if (vars->avg_dq_rate > 0)
                        qdelay = (backlog << PIE_SCALE) / vars->avg_dq_rate;
                else
                        qdelay = 0;
        } else {
                qdelay = vars->qdelay;
                qdelay_old = vars->qdelay_old;
        }

        /* If qdelay is zero and backlog is not, it means backlog is very small,
         * so we do not update probabilty in this round.
         */
        if (qdelay == 0 && backlog != 0)
                update_prob = false;

        /* In the algorithm, alpha and beta are between 0 and 2 with typical
         * value for alpha as 0.125. In this implementation, we use values 0-32
         * passed from user space to represent this. Also, alpha and beta have
         * unit of HZ and need to be scaled before they can used to update
         * probability. alpha/beta are updated locally below by scaling down
         * by 16 to come to 0-2 range.
         */
        alpha = ((u64)params->alpha * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 4;
        beta = ((u64)params->beta * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 4;

        /* We scale alpha and beta differently depending on how heavy the
         * congestion is. Please see RFC 8033 for details.
         */
        if (vars->prob < MAX_PROB / 10) {
                alpha >>= 1;
                beta >>= 1;

                power = 100;
                while (vars->prob < div_u64(MAX_PROB, power) &&
                       power <= 1000000) {
                        alpha >>= 2;
                        beta >>= 2;
                        power *= 10;
                }
        }

        /* alpha and beta should be between 0 and 32, in multiples of 1/16 */
        delta += alpha * (qdelay - params->target);
        delta += beta * (qdelay - qdelay_old);

        oldprob = vars->prob;

        /* to ensure we increase probability in steps of no more than 2% */
        if (delta > (s64)(MAX_PROB / (100 / 2)) &&
            vars->prob >= MAX_PROB / 10)
                delta = (MAX_PROB / 100) * 2;

        /* Non-linear drop:
         * Tune drop probability to increase quickly for high delays(>= 250ms)
         * 250ms is derived through experiments and provides error protection
         */

        if (qdelay > (PSCHED_NS2TICKS(250 * NSEC_PER_MSEC)))
                delta += MAX_PROB / (100 / 2);

        vars->prob += delta;

        if (delta > 0) {
                /* prevent overflow */
                if (vars->prob < oldprob) {
                        vars->prob = MAX_PROB;
                        /* Prevent normalization error. If probability is at
                         * maximum value already, we normalize it here, and
                         * skip the check to do a non-linear drop in the next
                         * section.
                         */
                        update_prob = false;
                }
        } else {
                /* prevent underflow */
                if (vars->prob > oldprob)
                        vars->prob = 0;
        }

        /* Non-linear drop in probability: Reduce drop probability quickly if
         * delay is 0 for 2 consecutive Tupdate periods.
         */

        if (qdelay == 0 && qdelay_old == 0 && update_prob)
                /* Reduce drop probability to 98.4% */
                vars->prob -= vars->prob / 64;

        vars->qdelay = qdelay;
        vars->backlog_old = backlog;

        /* We restart the measurement cycle if the following conditions are met
         * 1. If the delay has been low for 2 consecutive Tupdate periods
         * 2. Calculated drop probability is zero
         * 3. If average dq_rate_estimator is enabled, we have at least one
         *    estimate for the avg_dq_rate ie., is a non-zero value
         */
        if ((vars->qdelay < params->target / 2) &&
            (vars->qdelay_old < params->target / 2) &&
            vars->prob == 0 &&
            (!params->dq_rate_estimator || vars->avg_dq_rate > 0)) {
                pie_vars_init(vars);
        }

        if (!params->dq_rate_estimator)
                vars->qdelay_old = qdelay;
}
EXPORT_SYMBOL_GPL(pie_calculate_probability);

static void pie_timer(struct timer_list *t)
{
        struct pie_sched_data *q = from_timer(q, t, adapt_timer);
        struct Qdisc *sch = q->sch;
        spinlock_t *root_lock = qdisc_lock(qdisc_root_sleeping(sch));

        spin_lock(root_lock);
        pie_calculate_probability(&q->params, &q->vars, sch->qstats.backlog);

        /* reset the timer to fire after 'tupdate'. tupdate is in jiffies. */
        if (q->params.tupdate)
                mod_timer(&q->adapt_timer, jiffies + q->params.tupdate);
        spin_unlock(root_lock);
}

static int pie_init(struct Qdisc *sch, struct nlattr *opt,
                    struct netlink_ext_ack *extack)
{
        struct pie_sched_data *q = qdisc_priv(sch);

        pie_params_init(&q->params);
        pie_vars_init(&q->vars);
        sch->limit = q->params.limit;

        q->sch = sch;
        timer_setup(&q->adapt_timer, pie_timer, 0);

        if (opt) {
                int err = pie_change(sch, opt, extack);

                if (err)
                        return err;
        }

        mod_timer(&q->adapt_timer, jiffies + HZ / 2);
        return 0;
}

static int pie_dump(struct Qdisc *sch, struct sk_buff *skb)
{
        struct pie_sched_data *q = qdisc_priv(sch);
        struct nlattr *opts;

        opts = nla_nest_start_noflag(skb, TCA_OPTIONS);
        if (!opts)
                goto nla_put_failure;

        /* convert target from pschedtime to us */
        if (nla_put_u32(skb, TCA_PIE_TARGET,
                        ((u32)PSCHED_TICKS2NS(q->params.target)) /
                        NSEC_PER_USEC) ||
            nla_put_u32(skb, TCA_PIE_LIMIT, sch->limit) ||
            nla_put_u32(skb, TCA_PIE_TUPDATE,
                        jiffies_to_usecs(q->params.tupdate)) ||
            nla_put_u32(skb, TCA_PIE_ALPHA, q->params.alpha) ||
            nla_put_u32(skb, TCA_PIE_BETA, q->params.beta) ||
            nla_put_u32(skb, TCA_PIE_ECN, q->params.ecn) ||
            nla_put_u32(skb, TCA_PIE_BYTEMODE, q->params.bytemode) ||
            nla_put_u32(skb, TCA_PIE_DQ_RATE_ESTIMATOR,
                        q->params.dq_rate_estimator))
                goto nla_put_failure;

        return nla_nest_end(skb, opts);

nla_put_failure:
        nla_nest_cancel(skb, opts);
        return -1;
}

static int pie_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
{
        struct pie_sched_data *q = qdisc_priv(sch);
        struct tc_pie_xstats st = {
                .prob           = q->vars.prob << BITS_PER_BYTE,
                .delay          = ((u32)PSCHED_TICKS2NS(q->vars.qdelay)) /
                                   NSEC_PER_USEC,
                .packets_in     = q->stats.packets_in,
                .overlimit      = q->stats.overlimit,
                .maxq           = q->stats.maxq,
                .dropped        = q->stats.dropped,
                .ecn_mark       = q->stats.ecn_mark,
        };

        /* avg_dq_rate is only valid if dq_rate_estimator is enabled */
        st.dq_rate_estimating = q->params.dq_rate_estimator;

        /* unscale and return dq_rate in bytes per sec */
        if (q->params.dq_rate_estimator)
                st.avg_dq_rate = q->vars.avg_dq_rate *
                                 (PSCHED_TICKS_PER_SEC) >> PIE_SCALE;

        return gnet_stats_copy_app(d, &st, sizeof(st));
}

static struct sk_buff *pie_qdisc_dequeue(struct Qdisc *sch)
{
        struct pie_sched_data *q = qdisc_priv(sch);
        struct sk_buff *skb = qdisc_dequeue_head(sch);

        if (!skb)
                return NULL;

        pie_process_dequeue(skb, &q->params, &q->vars, sch->qstats.backlog);
        return skb;
}

static void pie_reset(struct Qdisc *sch)
{
        struct pie_sched_data *q = qdisc_priv(sch);

        qdisc_reset_queue(sch);
        pie_vars_init(&q->vars);
}

static void pie_destroy(struct Qdisc *sch)
{
        struct pie_sched_data *q = qdisc_priv(sch);

        q->params.tupdate = 0;
        del_timer_sync(&q->adapt_timer);
}

static struct Qdisc_ops pie_qdisc_ops __read_mostly = {
        .id             = "pie",
        .priv_size      = sizeof(struct pie_sched_data),
        .enqueue        = pie_qdisc_enqueue,
        .dequeue        = pie_qdisc_dequeue,
        .peek           = qdisc_peek_dequeued,
        .init           = pie_init,
        .destroy        = pie_destroy,
        .reset          = pie_reset,
        .change         = pie_change,
        .dump           = pie_dump,
        .dump_stats     = pie_dump_stats,
        .owner          = THIS_MODULE,
};

static int __init pie_module_init(void)
{
        return register_qdisc(&pie_qdisc_ops);
}

static void __exit pie_module_exit(void)
{
        unregister_qdisc(&pie_qdisc_ops);
}

module_init(pie_module_init);
module_exit(pie_module_exit);

MODULE_DESCRIPTION("Proportional Integral controller Enhanced (PIE) scheduler");
MODULE_AUTHOR("Vijay Subramanian");
MODULE_AUTHOR("Mythili Prabhu");
MODULE_LICENSE("GPL");