rc80211_minstrel_ht.c

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/*
 * Copyright (C) 2010 Felix Fietkau <nbd@openwrt.org>
 *
 * 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/netdevice.h>
#include <linux/types.h>
#include <linux/skbuff.h>
#include <linux/debugfs.h>
#include <linux/random.h>
#include <linux/ieee80211.h>
#include <net/mac80211.h>
#include "rate.h"
#include "rc80211_minstrel.h"
#include "rc80211_minstrel_ht.h"

#define AVG_PKT_SIZE    1200
#define SAMPLE_COLUMNS  10
#define EWMA_LEVEL              75

/* Number of bits for an average sized packet */
#define MCS_NBITS (AVG_PKT_SIZE << 3)

/* Number of symbols for a packet with (bps) bits per symbol */
#define MCS_NSYMS(bps) ((MCS_NBITS + (bps) - 1) / (bps))

/* Transmission time for a packet containing (syms) symbols */
#define MCS_SYMBOL_TIME(sgi, syms)                                      \
        (sgi ?                                                          \
          ((syms) * 18 + 4) / 5 :       /* syms * 3.6 us */             \
          (syms) << 2                   /* syms * 4 us */               \
        )

/* Transmit duration for the raw data part of an average sized packet */
#define MCS_DURATION(streams, sgi, bps) MCS_SYMBOL_TIME(sgi, MCS_NSYMS((streams) * (bps)))

/*
 * Define group sort order: HT40 -> SGI -> #streams
 */
#define GROUP_IDX(_streams, _sgi, _ht40)        \
        MINSTREL_MAX_STREAMS * 2 * _ht40 +      \
        MINSTREL_MAX_STREAMS * _sgi +           \
        _streams - 1

/* MCS rate information for an MCS group */
#define MCS_GROUP(_streams, _sgi, _ht40)                                \
        [GROUP_IDX(_streams, _sgi, _ht40)] = {                          \
        .streams = _streams,                                            \
        .flags =                                                        \
                (_sgi ? IEEE80211_TX_RC_SHORT_GI : 0) |                 \
                (_ht40 ? IEEE80211_TX_RC_40_MHZ_WIDTH : 0),             \
        .duration = {                                                   \
                MCS_DURATION(_streams, _sgi, _ht40 ? 54 : 26),          \
                MCS_DURATION(_streams, _sgi, _ht40 ? 108 : 52),         \
                MCS_DURATION(_streams, _sgi, _ht40 ? 162 : 78),         \
                MCS_DURATION(_streams, _sgi, _ht40 ? 216 : 104),        \
                MCS_DURATION(_streams, _sgi, _ht40 ? 324 : 156),        \
                MCS_DURATION(_streams, _sgi, _ht40 ? 432 : 208),        \
                MCS_DURATION(_streams, _sgi, _ht40 ? 486 : 234),        \
                MCS_DURATION(_streams, _sgi, _ht40 ? 540 : 260)         \
        }                                                               \
}

/*
 * To enable sufficiently targeted rate sampling, MCS rates are divided into
 * groups, based on the number of streams and flags (HT40, SGI) that they
 * use.
 *
 * Sortorder has to be fixed for GROUP_IDX macro to be applicable:
 * HT40 -> SGI -> #streams
 */
const struct mcs_group minstrel_mcs_groups[] = {
        MCS_GROUP(1, 0, 0),
        MCS_GROUP(2, 0, 0),
#if MINSTREL_MAX_STREAMS >= 3
        MCS_GROUP(3, 0, 0),
#endif

        MCS_GROUP(1, 1, 0),
        MCS_GROUP(2, 1, 0),
#if MINSTREL_MAX_STREAMS >= 3
        MCS_GROUP(3, 1, 0),
#endif

        MCS_GROUP(1, 0, 1),
        MCS_GROUP(2, 0, 1),
#if MINSTREL_MAX_STREAMS >= 3
        MCS_GROUP(3, 0, 1),
#endif

        MCS_GROUP(1, 1, 1),
        MCS_GROUP(2, 1, 1),
#if MINSTREL_MAX_STREAMS >= 3
        MCS_GROUP(3, 1, 1),
#endif
};

static u8 sample_table[SAMPLE_COLUMNS][MCS_GROUP_RATES];

/*
 * Perform EWMA (Exponentially Weighted Moving Average) calculation
 */
static int
minstrel_ewma(int old, int new, int weight)
{
        return (new * (100 - weight) + old * weight) / 100;
}

/*
 * Look up an MCS group index based on mac80211 rate information
 */
static int
minstrel_ht_get_group_idx(struct ieee80211_tx_rate *rate)
{
        return GROUP_IDX((rate->idx / MCS_GROUP_RATES) + 1,
                         !!(rate->flags & IEEE80211_TX_RC_SHORT_GI),
                         !!(rate->flags & IEEE80211_TX_RC_40_MHZ_WIDTH));
}

static inline struct minstrel_rate_stats *
minstrel_get_ratestats(struct minstrel_ht_sta *mi, int index)
{
        return &mi->groups[index / MCS_GROUP_RATES].rates[index % MCS_GROUP_RATES];
}


/*
 * Recalculate success probabilities and counters for a rate using EWMA
 */
static void
minstrel_calc_rate_ewma(struct minstrel_rate_stats *mr)
{
        if (unlikely(mr->attempts > 0)) {
                mr->sample_skipped = 0;
                mr->cur_prob = MINSTREL_FRAC(mr->success, mr->attempts);
                if (!mr->att_hist)
                        mr->probability = mr->cur_prob;
                else
                        mr->probability = minstrel_ewma(mr->probability,
                                mr->cur_prob, EWMA_LEVEL);
                mr->att_hist += mr->attempts;
                mr->succ_hist += mr->success;
        } else {
                mr->sample_skipped++;
        }
        mr->last_success = mr->success;
        mr->last_attempts = mr->attempts;
        mr->success = 0;
        mr->attempts = 0;
}

/*
 * Calculate throughput based on the average A-MPDU length, taking into account
 * the expected number of retransmissions and their expected length
 */
static void
minstrel_ht_calc_tp(struct minstrel_ht_sta *mi, int group, int rate)
{
        struct minstrel_rate_stats *mr;
        unsigned int usecs;

        mr = &mi->groups[group].rates[rate];

        if (mr->probability < MINSTREL_FRAC(1, 10)) {
                mr->cur_tp = 0;
                return;
        }

        usecs = mi->overhead / MINSTREL_TRUNC(mi->avg_ampdu_len);
        usecs += minstrel_mcs_groups[group].duration[rate];
        mr->cur_tp = MINSTREL_TRUNC((1000000 / usecs) * mr->probability);
}

/*
 * Update rate statistics and select new primary rates
 *
 * Rules for rate selection:
 *  - max_prob_rate must use only one stream, as a tradeoff between delivery
 *    probability and throughput during strong fluctuations
 *  - as long as the max prob rate has a probability of more than 3/4, pick
 *    higher throughput rates, even if the probablity is a bit lower
 */
static void
minstrel_ht_update_stats(struct minstrel_priv *mp, struct minstrel_ht_sta *mi)
{
        struct minstrel_mcs_group_data *mg;
        struct minstrel_rate_stats *mr;
        int cur_prob, cur_prob_tp, cur_tp, cur_tp2;
        int group, i, index;

        if (mi->ampdu_packets > 0) {
                mi->avg_ampdu_len = minstrel_ewma(mi->avg_ampdu_len,
                        MINSTREL_FRAC(mi->ampdu_len, mi->ampdu_packets), EWMA_LEVEL);
                mi->ampdu_len = 0;
                mi->ampdu_packets = 0;
        }

        mi->sample_slow = 0;
        mi->sample_count = 0;
        mi->max_tp_rate = 0;
        mi->max_tp_rate2 = 0;
        mi->max_prob_rate = 0;

        for (group = 0; group < ARRAY_SIZE(minstrel_mcs_groups); group++) {
                cur_prob = 0;
                cur_prob_tp = 0;
                cur_tp = 0;
                cur_tp2 = 0;

                mg = &mi->groups[group];
                if (!mg->supported)
                        continue;

                mg->max_tp_rate = 0;
                mg->max_tp_rate2 = 0;
                mg->max_prob_rate = 0;
                mi->sample_count++;

                for (i = 0; i < MCS_GROUP_RATES; i++) {
                        if (!(mg->supported & BIT(i)))
                                continue;

                        mr = &mg->rates[i];
                        mr->retry_updated = false;
                        index = MCS_GROUP_RATES * group + i;
                        minstrel_calc_rate_ewma(mr);
                        minstrel_ht_calc_tp(mi, group, i);

                        if (!mr->cur_tp)
                                continue;

                        /* ignore the lowest rate of each single-stream group */
                        if (!i && minstrel_mcs_groups[group].streams == 1)
                                continue;

                        if ((mr->cur_tp > cur_prob_tp && mr->probability >
                             MINSTREL_FRAC(3, 4)) || mr->probability > cur_prob) {
                                mg->max_prob_rate = index;
                                cur_prob = mr->probability;
                                cur_prob_tp = mr->cur_tp;
                        }

                        if (mr->cur_tp > cur_tp) {
                                swap(index, mg->max_tp_rate);
                                cur_tp = mr->cur_tp;
                                mr = minstrel_get_ratestats(mi, index);
                        }

                        if (index >= mg->max_tp_rate)
                                continue;

                        if (mr->cur_tp > cur_tp2) {
                                mg->max_tp_rate2 = index;
                                cur_tp2 = mr->cur_tp;
                        }
                }
        }

        /* try to sample up to half of the available rates during each interval */
        mi->sample_count *= 4;

        cur_prob = 0;
        cur_prob_tp = 0;
        cur_tp = 0;
        cur_tp2 = 0;
        for (group = 0; group < ARRAY_SIZE(minstrel_mcs_groups); group++) {
                mg = &mi->groups[group];
                if (!mg->supported)
                        continue;

                mr = minstrel_get_ratestats(mi, mg->max_prob_rate);
                if (cur_prob_tp < mr->cur_tp &&
                    minstrel_mcs_groups[group].streams == 1) {
                        mi->max_prob_rate = mg->max_prob_rate;
                        cur_prob = mr->cur_prob;
                        cur_prob_tp = mr->cur_tp;
                }

                mr = minstrel_get_ratestats(mi, mg->max_tp_rate);
                if (cur_tp < mr->cur_tp) {
                        mi->max_tp_rate2 = mi->max_tp_rate;
                        cur_tp2 = cur_tp;
                        mi->max_tp_rate = mg->max_tp_rate;
                        cur_tp = mr->cur_tp;
                }

                mr = minstrel_get_ratestats(mi, mg->max_tp_rate2);
                if (cur_tp2 < mr->cur_tp) {
                        mi->max_tp_rate2 = mg->max_tp_rate2;
                        cur_tp2 = mr->cur_tp;
                }
        }

        mi->stats_update = jiffies;
}

static bool
minstrel_ht_txstat_valid(struct ieee80211_tx_rate *rate)
{
        if (rate->idx < 0)
                return false;

        if (!rate->count)
                return false;

        return !!(rate->flags & IEEE80211_TX_RC_MCS);
}

static void
minstrel_next_sample_idx(struct minstrel_ht_sta *mi)
{
        struct minstrel_mcs_group_data *mg;

        for (;;) {
                mi->sample_group++;
                mi->sample_group %= ARRAY_SIZE(minstrel_mcs_groups);
                mg = &mi->groups[mi->sample_group];

                if (!mg->supported)
                        continue;

                if (++mg->index >= MCS_GROUP_RATES) {
                        mg->index = 0;
                        if (++mg->column >= ARRAY_SIZE(sample_table))
                                mg->column = 0;
                }
                break;
        }
}

static void
minstrel_downgrade_rate(struct minstrel_ht_sta *mi, unsigned int *idx,
                        bool primary)
{
        int group, orig_group;

        orig_group = group = *idx / MCS_GROUP_RATES;
        while (group > 0) {
                group--;

                if (!mi->groups[group].supported)
                        continue;

                if (minstrel_mcs_groups[group].streams >
                    minstrel_mcs_groups[orig_group].streams)
                        continue;

                if (primary)
                        *idx = mi->groups[group].max_tp_rate;
                else
                        *idx = mi->groups[group].max_tp_rate2;
                break;
        }
}

static void
minstrel_aggr_check(struct ieee80211_sta *pubsta, struct sk_buff *skb)
{
        struct ieee80211_hdr *hdr = (struct ieee80211_hdr *) skb->data;
        struct sta_info *sta = container_of(pubsta, struct sta_info, sta);
        u16 tid;

        if (unlikely(!ieee80211_is_data_qos(hdr->frame_control)))
                return;

        if (unlikely(skb->protocol == cpu_to_be16(ETH_P_PAE)))
                return;

        tid = *ieee80211_get_qos_ctl(hdr) & IEEE80211_QOS_CTL_TID_MASK;
        if (likely(sta->ampdu_mlme.tid_tx[tid]))
                return;

        if (skb_get_queue_mapping(skb) == IEEE80211_AC_VO)
                return;

        ieee80211_start_tx_ba_session(pubsta, tid, 5000);
}

static void
minstrel_ht_tx_status(void *priv, struct ieee80211_supported_band *sband,
                      struct ieee80211_sta *sta, void *priv_sta,
                      struct sk_buff *skb)
{
        struct minstrel_ht_sta_priv *msp = priv_sta;
        struct minstrel_ht_sta *mi = &msp->ht;
        struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
        struct ieee80211_tx_rate *ar = info->status.rates;
        struct minstrel_rate_stats *rate, *rate2;
        struct minstrel_priv *mp = priv;
        bool last = false;
        int group;
        int i = 0;

        if (!msp->is_ht)
                return mac80211_minstrel.tx_status(priv, sband, sta, &msp->legacy, skb);

        /* This packet was aggregated but doesn't carry status info */
        if ((info->flags & IEEE80211_TX_CTL_AMPDU) &&
            !(info->flags & IEEE80211_TX_STAT_AMPDU))
                return;

        if (!(info->flags & IEEE80211_TX_STAT_AMPDU)) {
                info->status.ampdu_ack_len =
                        (info->flags & IEEE80211_TX_STAT_ACK ? 1 : 0);
                info->status.ampdu_len = 1;
        }

        mi->ampdu_packets++;
        mi->ampdu_len += info->status.ampdu_len;

        if (!mi->sample_wait && !mi->sample_tries && mi->sample_count > 0) {
                mi->sample_wait = 16 + 2 * MINSTREL_TRUNC(mi->avg_ampdu_len);
                mi->sample_tries = 2;
                mi->sample_count--;
        }

        if (info->flags & IEEE80211_TX_CTL_RATE_CTRL_PROBE)
                mi->sample_packets += info->status.ampdu_len;

        for (i = 0; !last; i++) {
                last = (i == IEEE80211_TX_MAX_RATES - 1) ||
                       !minstrel_ht_txstat_valid(&ar[i + 1]);

                if (!minstrel_ht_txstat_valid(&ar[i]))
                        break;

                group = minstrel_ht_get_group_idx(&ar[i]);
                rate = &mi->groups[group].rates[ar[i].idx % 8];

                if (last)
                        rate->success += info->status.ampdu_ack_len;

                rate->attempts += ar[i].count * info->status.ampdu_len;
        }

        /*
         * check for sudden death of spatial multiplexing,
         * downgrade to a lower number of streams if necessary.
         */
        rate = minstrel_get_ratestats(mi, mi->max_tp_rate);
        if (rate->attempts > 30 &&
            MINSTREL_FRAC(rate->success, rate->attempts) <
            MINSTREL_FRAC(20, 100))
                minstrel_downgrade_rate(mi, &mi->max_tp_rate, true);

        rate2 = minstrel_get_ratestats(mi, mi->max_tp_rate2);
        if (rate2->attempts > 30 &&
            MINSTREL_FRAC(rate2->success, rate2->attempts) <
            MINSTREL_FRAC(20, 100))
                minstrel_downgrade_rate(mi, &mi->max_tp_rate2, false);

        if (time_after(jiffies, mi->stats_update + (mp->update_interval / 2 * HZ) / 1000)) {
                minstrel_ht_update_stats(mp, mi);
                if (!(info->flags & IEEE80211_TX_CTL_AMPDU))
                        minstrel_aggr_check(sta, skb);
        }
}

static void
minstrel_calc_retransmit(struct minstrel_priv *mp, struct minstrel_ht_sta *mi,
                         int index)
{
        struct minstrel_rate_stats *mr;
        const struct mcs_group *group;
        unsigned int tx_time, tx_time_rtscts, tx_time_data;
        unsigned int cw = mp->cw_min;
        unsigned int ctime = 0;
        unsigned int t_slot = 9; /* FIXME */
        unsigned int ampdu_len = MINSTREL_TRUNC(mi->avg_ampdu_len);

        mr = minstrel_get_ratestats(mi, index);
        if (mr->probability < MINSTREL_FRAC(1, 10)) {
                mr->retry_count = 1;
                mr->retry_count_rtscts = 1;
                return;
        }

        mr->retry_count = 2;
        mr->retry_count_rtscts = 2;
        mr->retry_updated = true;

        group = &minstrel_mcs_groups[index / MCS_GROUP_RATES];
        tx_time_data = group->duration[index % MCS_GROUP_RATES] * ampdu_len;

        /* Contention time for first 2 tries */
        ctime = (t_slot * cw) >> 1;
        cw = min((cw << 1) | 1, mp->cw_max);
        ctime += (t_slot * cw) >> 1;
        cw = min((cw << 1) | 1, mp->cw_max);

        /* Total TX time for data and Contention after first 2 tries */
        tx_time = ctime + 2 * (mi->overhead + tx_time_data);
        tx_time_rtscts = ctime + 2 * (mi->overhead_rtscts + tx_time_data);

        /* See how many more tries we can fit inside segment size */
        do {
                /* Contention time for this try */
                ctime = (t_slot * cw) >> 1;
                cw = min((cw << 1) | 1, mp->cw_max);

                /* Total TX time after this try */
                tx_time += ctime + mi->overhead + tx_time_data;
                tx_time_rtscts += ctime + mi->overhead_rtscts + tx_time_data;

                if (tx_time_rtscts < mp->segment_size)
                        mr->retry_count_rtscts++;
        } while ((tx_time < mp->segment_size) &&
                 (++mr->retry_count < mp->max_retry));
}


static void
minstrel_ht_set_rate(struct minstrel_priv *mp, struct minstrel_ht_sta *mi,
                     struct ieee80211_tx_rate *rate, int index,
                     bool sample, bool rtscts)
{
        const struct mcs_group *group = &minstrel_mcs_groups[index / MCS_GROUP_RATES];
        struct minstrel_rate_stats *mr;

        mr = minstrel_get_ratestats(mi, index);
        if (!mr->retry_updated)
                minstrel_calc_retransmit(mp, mi, index);

        if (sample)
                rate->count = 1;
        else if (mr->probability < MINSTREL_FRAC(20, 100))
                rate->count = 2;
        else if (rtscts)
                rate->count = mr->retry_count_rtscts;
        else
                rate->count = mr->retry_count;

        rate->flags = IEEE80211_TX_RC_MCS | group->flags;
        if (rtscts)
                rate->flags |= IEEE80211_TX_RC_USE_RTS_CTS;
        rate->idx = index % MCS_GROUP_RATES + (group->streams - 1) * MCS_GROUP_RATES;
}

static inline int
minstrel_get_duration(int index)
{
        const struct mcs_group *group = &minstrel_mcs_groups[index / MCS_GROUP_RATES];
        return group->duration[index % MCS_GROUP_RATES];
}

static int
minstrel_get_sample_rate(struct minstrel_priv *mp, struct minstrel_ht_sta *mi)
{
        struct minstrel_rate_stats *mr;
        struct minstrel_mcs_group_data *mg;
        int sample_idx = 0;

        if (mi->sample_wait > 0) {
                mi->sample_wait--;
                return -1;
        }

        if (!mi->sample_tries)
                return -1;

        mi->sample_tries--;
        mg = &mi->groups[mi->sample_group];
        sample_idx = sample_table[mg->column][mg->index];
        mr = &mg->rates[sample_idx];
        sample_idx += mi->sample_group * MCS_GROUP_RATES;
        minstrel_next_sample_idx(mi);

        /*
         * Sampling might add some overhead (RTS, no aggregation)
         * to the frame. Hence, don't use sampling for the currently
         * used max TP rate.
         */
        if (sample_idx == mi->max_tp_rate)
                return -1;
        /*
         * When not using MRR, do not sample if the probability is already
         * higher than 95% to avoid wasting airtime
         */
        if (!mp->has_mrr && (mr->probability > MINSTREL_FRAC(95, 100)))
                return -1;

        /*
         * Make sure that lower rates get sampled only occasionally,
         * if the link is working perfectly.
         */
        if (minstrel_get_duration(sample_idx) >
            minstrel_get_duration(mi->max_tp_rate)) {
                if (mr->sample_skipped < 20)
                        return -1;

                if (mi->sample_slow++ > 2)
                        return -1;
        }

        return sample_idx;
}

static void
minstrel_ht_get_rate(void *priv, struct ieee80211_sta *sta, void *priv_sta,
                     struct ieee80211_tx_rate_control *txrc)
{
        struct ieee80211_tx_info *info = IEEE80211_SKB_CB(txrc->skb);
        struct ieee80211_tx_rate *ar = info->status.rates;
        struct minstrel_ht_sta_priv *msp = priv_sta;
        struct minstrel_ht_sta *mi = &msp->ht;
        struct minstrel_priv *mp = priv;
        int sample_idx;
        bool sample = false;

        if (rate_control_send_low(sta, priv_sta, txrc))
                return;

        if (!msp->is_ht)
                return mac80211_minstrel.get_rate(priv, sta, &msp->legacy, txrc);

        info->flags |= mi->tx_flags;

        /* Don't use EAPOL frames for sampling on non-mrr hw */
        if (mp->hw->max_rates == 1 &&
            txrc->skb->protocol == cpu_to_be16(ETH_P_PAE))
                sample_idx = -1;
        else
                sample_idx = minstrel_get_sample_rate(mp, mi);

#ifdef CONFIG_MAC80211_DEBUGFS
        /* use fixed index if set */
        if (mp->fixed_rate_idx != -1)
                sample_idx = mp->fixed_rate_idx;
#endif

        if (sample_idx >= 0) {
                sample = true;
                minstrel_ht_set_rate(mp, mi, &ar[0], sample_idx,
                        true, false);
                info->flags |= IEEE80211_TX_CTL_RATE_CTRL_PROBE;
        } else {
                minstrel_ht_set_rate(mp, mi, &ar[0], mi->max_tp_rate,
                        false, false);
        }

        if (mp->hw->max_rates >= 3) {
                /*
                 * At least 3 tx rates supported, use
                 * sample_rate -> max_tp_rate -> max_prob_rate for sampling and
                 * max_tp_rate -> max_tp_rate2 -> max_prob_rate by default.
                 */
                if (sample_idx >= 0)
                        minstrel_ht_set_rate(mp, mi, &ar[1], mi->max_tp_rate,
                                false, false);
                else
                        minstrel_ht_set_rate(mp, mi, &ar[1], mi->max_tp_rate2,
                                false, true);

                minstrel_ht_set_rate(mp, mi, &ar[2], mi->max_prob_rate,
                                     false, !sample);

                ar[3].count = 0;
                ar[3].idx = -1;
        } else if (mp->hw->max_rates == 2) {
                /*
                 * Only 2 tx rates supported, use
                 * sample_rate -> max_prob_rate for sampling and
                 * max_tp_rate -> max_prob_rate by default.
                 */
                minstrel_ht_set_rate(mp, mi, &ar[1], mi->max_prob_rate,
                                     false, !sample);

                ar[2].count = 0;
                ar[2].idx = -1;
        } else {
                /* Not using MRR, only use the first rate */
                ar[1].count = 0;
                ar[1].idx = -1;
        }

        mi->total_packets++;

        /* wraparound */
        if (mi->total_packets == ~0) {
                mi->total_packets = 0;
                mi->sample_packets = 0;
        }
}

static void
minstrel_ht_update_caps(void *priv, struct ieee80211_supported_band *sband,
                        struct ieee80211_sta *sta, void *priv_sta)
{
        struct minstrel_priv *mp = priv;
        struct minstrel_ht_sta_priv *msp = priv_sta;
        struct minstrel_ht_sta *mi = &msp->ht;
        struct ieee80211_mcs_info *mcs = &sta->ht_cap.mcs;
        u16 sta_cap = sta->ht_cap.cap;
        int n_supported = 0;
        int ack_dur;
        int stbc;
        int i;
        unsigned int smps;

        /* fall back to the old minstrel for legacy stations */
        if (!sta->ht_cap.ht_supported)
                goto use_legacy;

        BUILD_BUG_ON(ARRAY_SIZE(minstrel_mcs_groups) !=
                MINSTREL_MAX_STREAMS * MINSTREL_STREAM_GROUPS);

        msp->is_ht = true;
        memset(mi, 0, sizeof(*mi));
        mi->stats_update = jiffies;

        ack_dur = ieee80211_frame_duration(sband->band, 10, 60, 1, 1);
        mi->overhead = ieee80211_frame_duration(sband->band, 0, 60, 1, 1) + ack_dur;
        mi->overhead_rtscts = mi->overhead + 2 * ack_dur;

        mi->avg_ampdu_len = MINSTREL_FRAC(1, 1);

        /* When using MRR, sample more on the first attempt, without delay */
        if (mp->has_mrr) {
                mi->sample_count = 16;
                mi->sample_wait = 0;
        } else {
                mi->sample_count = 8;
                mi->sample_wait = 8;
        }
        mi->sample_tries = 4;

        stbc = (sta_cap & IEEE80211_HT_CAP_RX_STBC) >>
                IEEE80211_HT_CAP_RX_STBC_SHIFT;
        mi->tx_flags |= stbc << IEEE80211_TX_CTL_STBC_SHIFT;

        if (sta_cap & IEEE80211_HT_CAP_LDPC_CODING)
                mi->tx_flags |= IEEE80211_TX_CTL_LDPC;

        smps = (sta_cap & IEEE80211_HT_CAP_SM_PS) >>
                IEEE80211_HT_CAP_SM_PS_SHIFT;

        for (i = 0; i < ARRAY_SIZE(mi->groups); i++) {
                u16 req = 0;

                mi->groups[i].supported = 0;
                if (minstrel_mcs_groups[i].flags & IEEE80211_TX_RC_SHORT_GI) {
                        if (minstrel_mcs_groups[i].flags & IEEE80211_TX_RC_40_MHZ_WIDTH)
                                req |= IEEE80211_HT_CAP_SGI_40;
                        else
                                req |= IEEE80211_HT_CAP_SGI_20;
                }

                if (minstrel_mcs_groups[i].flags & IEEE80211_TX_RC_40_MHZ_WIDTH)
                        req |= IEEE80211_HT_CAP_SUP_WIDTH_20_40;

                if ((sta_cap & req) != req)
                        continue;

                /* Mark MCS > 7 as unsupported if STA is in static SMPS mode */
                if (smps == WLAN_HT_CAP_SM_PS_STATIC &&
                    minstrel_mcs_groups[i].streams > 1)
                        continue;

                mi->groups[i].supported =
                        mcs->rx_mask[minstrel_mcs_groups[i].streams - 1];

                if (mi->groups[i].supported)
                        n_supported++;
        }

        if (!n_supported)
                goto use_legacy;

        return;

use_legacy:
        msp->is_ht = false;
        memset(&msp->legacy, 0, sizeof(msp->legacy));
        msp->legacy.r = msp->ratelist;
        msp->legacy.sample_table = msp->sample_table;
        return mac80211_minstrel.rate_init(priv, sband, sta, &msp->legacy);
}

static void
minstrel_ht_rate_init(void *priv, struct ieee80211_supported_band *sband,
                      struct ieee80211_sta *sta, void *priv_sta)
{
        minstrel_ht_update_caps(priv, sband, sta, priv_sta);
}

static void
minstrel_ht_rate_update(void *priv, struct ieee80211_supported_band *sband,
                        struct ieee80211_sta *sta, void *priv_sta,
                        u32 changed)
{
        minstrel_ht_update_caps(priv, sband, sta, priv_sta);
}

static void *
minstrel_ht_alloc_sta(void *priv, struct ieee80211_sta *sta, gfp_t gfp)
{
        struct ieee80211_supported_band *sband;
        struct minstrel_ht_sta_priv *msp;
        struct minstrel_priv *mp = priv;
        struct ieee80211_hw *hw = mp->hw;
        int max_rates = 0;
        int i;

        for (i = 0; i < IEEE80211_NUM_BANDS; i++) {
                sband = hw->wiphy->bands[i];
                if (sband && sband->n_bitrates > max_rates)
                        max_rates = sband->n_bitrates;
        }

        msp = kzalloc(sizeof(*msp), gfp);
        if (!msp)
                return NULL;

        msp->ratelist = kzalloc(sizeof(struct minstrel_rate) * max_rates, gfp);
        if (!msp->ratelist)
                goto error;

        msp->sample_table = kmalloc(SAMPLE_COLUMNS * max_rates, gfp);
        if (!msp->sample_table)
                goto error1;

        return msp;

error1:
        kfree(msp->ratelist);
error:
        kfree(msp);
        return NULL;
}

static void
minstrel_ht_free_sta(void *priv, struct ieee80211_sta *sta, void *priv_sta)
{
        struct minstrel_ht_sta_priv *msp = priv_sta;

        kfree(msp->sample_table);
        kfree(msp->ratelist);
        kfree(msp);
}

static void *
minstrel_ht_alloc(struct ieee80211_hw *hw, struct dentry *debugfsdir)
{
        return mac80211_minstrel.alloc(hw, debugfsdir);
}

static void
minstrel_ht_free(void *priv)
{
        mac80211_minstrel.free(priv);
}

static struct rate_control_ops mac80211_minstrel_ht = {
        .name = "minstrel_ht",
        .tx_status = minstrel_ht_tx_status,
        .get_rate = minstrel_ht_get_rate,
        .rate_init = minstrel_ht_rate_init,
        .rate_update = minstrel_ht_rate_update,
        .alloc_sta = minstrel_ht_alloc_sta,
        .free_sta = minstrel_ht_free_sta,
        .alloc = minstrel_ht_alloc,
        .free = minstrel_ht_free,
#ifdef CONFIG_MAC80211_DEBUGFS
        .add_sta_debugfs = minstrel_ht_add_sta_debugfs,
        .remove_sta_debugfs = minstrel_ht_remove_sta_debugfs,
#endif
};


static void
init_sample_table(void)
{
        int col, i, new_idx;
        u8 rnd[MCS_GROUP_RATES];

        memset(sample_table, 0xff, sizeof(sample_table));
        for (col = 0; col < SAMPLE_COLUMNS; col++) {
                for (i = 0; i < MCS_GROUP_RATES; i++) {
                        get_random_bytes(rnd, sizeof(rnd));
                        new_idx = (i + rnd[i]) % MCS_GROUP_RATES;

                        while (sample_table[col][new_idx] != 0xff)
                                new_idx = (new_idx + 1) % MCS_GROUP_RATES;

                        sample_table[col][new_idx] = i;
                }
        }
}

int __init
rc80211_minstrel_ht_init(void)
{
        init_sample_table();
        return ieee80211_rate_control_register(&mac80211_minstrel_ht);
}

void
rc80211_minstrel_ht_exit(void)
{
        ieee80211_rate_control_unregister(&mac80211_minstrel_ht);
}