/* SPDX-License-Identifier: GPL-2.0 */
#ifndef __NET_SCHED_RED_H
#define __NET_SCHED_RED_H

#include <linux/types.h>
#include <linux/bug.h>
#include <net/pkt_sched.h>
#include <net/inet_ecn.h>
#include <net/dsfield.h>
#include <linux/reciprocal_div.h>

/*      Random Early Detection (RED) algorithm.
        =======================================

        Source: Sally Floyd and Van Jacobson, "Random Early Detection Gateways
        for Congestion Avoidance", 1993, IEEE/ACM Transactions on Networking.

        This file codes a "divisionless" version of RED algorithm
        as written down in Fig.17 of the paper.

        Short description.
        ------------------

        When a new packet arrives we calculate the average queue length:

        avg = (1-W)*avg + W*current_queue_len,

        W is the filter time constant (chosen as 2^(-Wlog)), it controls
        the inertia of the algorithm. To allow larger bursts, W should be
        decreased.

        if (avg > th_max) -> packet marked (dropped).
        if (avg < th_min) -> packet passes.
        if (th_min < avg < th_max) we calculate probability:

        Pb = max_P * (avg - th_min)/(th_max-th_min)

        and mark (drop) packet with this probability.
        Pb changes from 0 (at avg==th_min) to max_P (avg==th_max).
        max_P should be small (not 1), usually 0.01..0.02 is good value.

        max_P is chosen as a number, so that max_P/(th_max-th_min)
        is a negative power of two in order arithmetics to contain
        only shifts.


        Parameters, settable by user:
        -----------------------------

        qth_min         - bytes (should be < qth_max/2)
        qth_max         - bytes (should be at least 2*qth_min and less limit)
        Wlog            - bits (<32) log(1/W).
        Plog            - bits (<32)

        Plog is related to max_P by formula:

        max_P = (qth_max-qth_min)/2^Plog;

        F.e. if qth_max=128K and qth_min=32K, then Plog=22
        corresponds to max_P=0.02

        Scell_log
        Stab

        Lookup table for log((1-W)^(t/t_ave).


        NOTES:

        Upper bound on W.
        -----------------

        If you want to allow bursts of L packets of size S,
        you should choose W:

        L + 1 - th_min/S < (1-(1-W)^L)/W

        th_min/S = 32         th_min/S = 4

        log(W)  L
        -1      33
        -2      35
        -3      39
        -4      46
        -5      57
        -6      75
        -7      101
        -8      135
        -9      190
        etc.
 */

/*
 * Adaptative RED : An Algorithm for Increasing the Robustness of RED's AQM
 * (Sally FLoyd, Ramakrishna Gummadi, and Scott Shenker) August 2001
 *
 * Every 500 ms:
 *  if (avg > target and max_p <= 0.5)
 *   increase max_p : max_p += alpha;
 *  else if (avg < target and max_p >= 0.01)
 *   decrease max_p : max_p *= beta;
 *
 * target :[qth_min + 0.4*(qth_min - qth_max),
 *          qth_min + 0.6*(qth_min - qth_max)].
 * alpha : min(0.01, max_p / 4)
 * beta : 0.9
 * max_P is a Q0.32 fixed point number (with 32 bits mantissa)
 * max_P between 0.01 and 0.5 (1% - 50%) [ Its no longer a negative power of two ]
 */
#define RED_ONE_PERCENT ((u32)DIV_ROUND_CLOSEST(1ULL<<32, 100))

#define MAX_P_MIN (1 * RED_ONE_PERCENT)
#define MAX_P_MAX (50 * RED_ONE_PERCENT)
#define MAX_P_ALPHA(val) min(MAX_P_MIN, val / 4)

#define RED_STAB_SIZE   256
#define RED_STAB_MASK   (RED_STAB_SIZE - 1)

struct red_stats {
        u32             prob_drop;      /* Early probability drops */
        u32             prob_mark;      /* Early probability marks */
        u32             forced_drop;    /* Forced drops, qavg > max_thresh */
        u32             forced_mark;    /* Forced marks, qavg > max_thresh */
        u32             pdrop;          /* Drops due to queue limits */
        u32             other;          /* Drops due to drop() calls */
};

struct red_parms {
        /* Parameters */
        u32             qth_min;        /* Min avg length threshold: Wlog scaled */
        u32             qth_max;        /* Max avg length threshold: Wlog scaled */
        u32             Scell_max;
        u32             max_P;          /* probability, [0 .. 1.0] 32 scaled */
        /* reciprocal_value(max_P / qth_delta) */
        struct reciprocal_value max_P_reciprocal;
        u32             qth_delta;      /* max_th - min_th */
        u32             target_min;     /* min_th + 0.4*(max_th - min_th) */
        u32             target_max;     /* min_th + 0.6*(max_th - min_th) */
        u8              Scell_log;
        u8              Wlog;           /* log(W)               */
        u8              Plog;           /* random number bits   */
        u8              Stab[RED_STAB_SIZE];
};

struct red_vars {
        /* Variables */
        int             qcount;         /* Number of packets since last random
                                           number generation */
        u32             qR;             /* Cached random number */

        unsigned long   qavg;           /* Average queue length: Wlog scaled */
        ktime_t         qidlestart;     /* Start of current idle period */
};

static inline u32 red_maxp(u8 Plog)
{
        return Plog < 32 ? (~0U >> Plog) : ~0U;
}

static inline void red_set_vars(struct red_vars *v)
{
        /* Reset average queue length, the value is strictly bound
         * to the parameters below, reseting hurts a bit but leaving
         * it might result in an unreasonable qavg for a while. --TGR
         */
        v->qavg         = 0;

        v->qcount       = -1;
}

static inline bool red_check_params(u32 qth_min, u32 qth_max, u8 Wlog)
{
        if (fls(qth_min) + Wlog > 32)
                return false;
        if (fls(qth_max) + Wlog > 32)
                return false;
        if (qth_max < qth_min)
                return false;
        return true;
}

static inline void red_set_parms(struct red_parms *p,
                                 u32 qth_min, u32 qth_max, u8 Wlog, u8 Plog,
                                 u8 Scell_log, u8 *stab, u32 max_P)
{
        int delta = qth_max - qth_min;
        u32 max_p_delta;

        p->qth_min      = qth_min << Wlog;
        p->qth_max      = qth_max << Wlog;
        p->Wlog         = Wlog;
        p->Plog         = Plog;
        if (delta <= 0)
                delta = 1;
        p->qth_delta    = delta;
        if (!max_P) {
                max_P = red_maxp(Plog);
                max_P *= delta; /* max_P = (qth_max - qth_min)/2^Plog */
        }
        p->max_P = max_P;
        max_p_delta = max_P / delta;
        max_p_delta = max(max_p_delta, 1U);
        p->max_P_reciprocal  = reciprocal_value(max_p_delta);

        /* RED Adaptative target :
         * [min_th + 0.4*(min_th - max_th),
         *  min_th + 0.6*(min_th - max_th)].
         */
        delta /= 5;
        p->target_min = qth_min + 2*delta;
        p->target_max = qth_min + 3*delta;

        p->Scell_log    = Scell_log;
        p->Scell_max    = (255 << Scell_log);

        if (stab)
                memcpy(p->Stab, stab, sizeof(p->Stab));
}

static inline int red_is_idling(const struct red_vars *v)
{
        return v->qidlestart != 0;
}

static inline void red_start_of_idle_period(struct red_vars *v)
{
        v->qidlestart = ktime_get();
}

static inline void red_end_of_idle_period(struct red_vars *v)
{
        v->qidlestart = 0;
}

static inline void red_restart(struct red_vars *v)
{
        red_end_of_idle_period(v);
        v->qavg = 0;
        v->qcount = -1;
}

static inline unsigned long red_calc_qavg_from_idle_time(const struct red_parms *p,
                                                         const struct red_vars *v)
{
        s64 delta = ktime_us_delta(ktime_get(), v->qidlestart);
        long us_idle = min_t(s64, delta, p->Scell_max);
        int  shift;

        /*
         * The problem: ideally, average length queue recalcultion should
         * be done over constant clock intervals. This is too expensive, so
         * that the calculation is driven by outgoing packets.
         * When the queue is idle we have to model this clock by hand.
         *
         * SF+VJ proposed to "generate":
         *
         *      m = idletime / (average_pkt_size / bandwidth)
         *
         * dummy packets as a burst after idle time, i.e.
         *
         *      v->qavg *= (1-W)^m
         *
         * This is an apparently overcomplicated solution (f.e. we have to
         * precompute a table to make this calculation in reasonable time)
         * I believe that a simpler model may be used here,
         * but it is field for experiments.
         */

        shift = p->Stab[(us_idle >> p->Scell_log) & RED_STAB_MASK];

        if (shift)
                return v->qavg >> shift;
        else {
                /* Approximate initial part of exponent with linear function:
                 *
                 *      (1-W)^m ~= 1-mW + ...
                 *
                 * Seems, it is the best solution to
                 * problem of too coarse exponent tabulation.
                 */
                us_idle = (v->qavg * (u64)us_idle) >> p->Scell_log;

                if (us_idle < (v->qavg >> 1))
                        return v->qavg - us_idle;
                else
                        return v->qavg >> 1;
        }
}

static inline unsigned long red_calc_qavg_no_idle_time(const struct red_parms *p,
                                                       const struct red_vars *v,
                                                       unsigned int backlog)
{
        /*
         * NOTE: v->qavg is fixed point number with point at Wlog.
         * The formula below is equvalent to floating point
         * version:
         *
         *      qavg = qavg*(1-W) + backlog*W;
         *
         * --ANK (980924)
         */
        return v->qavg + (backlog - (v->qavg >> p->Wlog));
}

static inline unsigned long red_calc_qavg(const struct red_parms *p,
                                          const struct red_vars *v,
                                          unsigned int backlog)
{
        if (!red_is_idling(v))
                return red_calc_qavg_no_idle_time(p, v, backlog);
        else
                return red_calc_qavg_from_idle_time(p, v);
}


static inline u32 red_random(const struct red_parms *p)
{
        return reciprocal_divide(prandom_u32(), p->max_P_reciprocal);
}

static inline int red_mark_probability(const struct red_parms *p,
                                       const struct red_vars *v,
                                       unsigned long qavg)
{
        /* The formula used below causes questions.

           OK. qR is random number in the interval
                (0..1/max_P)*(qth_max-qth_min)
           i.e. 0..(2^Plog). If we used floating point
           arithmetics, it would be: (2^Plog)*rnd_num,
           where rnd_num is less 1.

           Taking into account, that qavg have fixed
           point at Wlog, two lines
           below have the following floating point equivalent:

           max_P*(qavg - qth_min)/(qth_max-qth_min) < rnd/qcount

           Any questions? --ANK (980924)
         */
        return !(((qavg - p->qth_min) >> p->Wlog) * v->qcount < v->qR);
}

enum {
        RED_BELOW_MIN_THRESH,
        RED_BETWEEN_TRESH,
        RED_ABOVE_MAX_TRESH,
};

static inline int red_cmp_thresh(const struct red_parms *p, unsigned long qavg)
{
        if (qavg < p->qth_min)
                return RED_BELOW_MIN_THRESH;
        else if (qavg >= p->qth_max)
                return RED_ABOVE_MAX_TRESH;
        else
                return RED_BETWEEN_TRESH;
}

enum {
        RED_DONT_MARK,
        RED_PROB_MARK,
        RED_HARD_MARK,
};

static inline int red_action(const struct red_parms *p,
                             struct red_vars *v,
                             unsigned long qavg)
{
        switch (red_cmp_thresh(p, qavg)) {
                case RED_BELOW_MIN_THRESH:
                        v->qcount = -1;
                        return RED_DONT_MARK;

                case RED_BETWEEN_TRESH:
                        if (++v->qcount) {
                                if (red_mark_probability(p, v, qavg)) {
                                        v->qcount = 0;
                                        v->qR = red_random(p);
                                        return RED_PROB_MARK;
                                }
                        } else
                                v->qR = red_random(p);

                        return RED_DONT_MARK;

                case RED_ABOVE_MAX_TRESH:
                        v->qcount = -1;
                        return RED_HARD_MARK;
        }

        BUG();
        return RED_DONT_MARK;
}

static inline void red_adaptative_algo(struct red_parms *p, struct red_vars *v)
{
        unsigned long qavg;
        u32 max_p_delta;

        qavg = v->qavg;
        if (red_is_idling(v))
                qavg = red_calc_qavg_from_idle_time(p, v);

        /* v->qavg is fixed point number with point at Wlog */
        qavg >>= p->Wlog;

        if (qavg > p->target_max && p->max_P <= MAX_P_MAX)
                p->max_P += MAX_P_ALPHA(p->max_P); /* maxp = maxp + alpha */
        else if (qavg < p->target_min && p->max_P >= MAX_P_MIN)
                p->max_P = (p->max_P/10)*9; /* maxp = maxp * Beta */

        max_p_delta = DIV_ROUND_CLOSEST(p->max_P, p->qth_delta);
        max_p_delta = max(max_p_delta, 1U);
        p->max_P_reciprocal = reciprocal_value(max_p_delta);
}
#endif