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+/*
+ * Simple 802.11 rate-control algorithm for gPXE.
+ *
+ * Copyright (c) 2009 Joshua Oreman <oremanj@rwcr.net>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+#include <stdlib.h>
+#include <gpxe/net80211.h>
+
+/**
+ * @file
+ *
+ * Simple 802.11 rate-control algorithm
+ */
+
+/** @page rc80211 Rate control philosophy
+ *
+ * We want to maximize our transmission speed, to the extent that we
+ * can do that without dropping undue numbers of packets. We also
+ * don't want to take up very much code space, so our algorithm has to
+ * be pretty simple
+ *
+ * When we receive a packet, we know what rate it was transmitted at,
+ * and whether it had to be retransmitted to get to us.
+ *
+ * When we send a packet, we hear back how many times it had to be
+ * retried to get through, and whether it got through at all.
+ *
+ * Indications of TX success are more reliable than RX success, but RX
+ * information helps us know where to start.
+ *
+ * To handle all of this, we keep for each rate and each direction (TX
+ * and RX separately) some state information for the most recent
+ * packets on that rate and the number of packets for which we have
+ * information. The state is a 32-bit unsigned integer in which two
+ * bits represent a packet: 11 if it went through well, 10 if it went
+ * through with one retry, 01 if it went through with more than one
+ * retry, or 00 if it didn't go through at all. We define the
+ * "goodness" for a particular (rate, direction) combination as the
+ * sum of all the 2-bit fields, times 33, divided by the number of
+ * 2-bit fields containing valid information (16 except when we're
+ * starting out). The number produced is between 0 and 99; we use -1
+ * for rates with less than 4 RX packets or 1 TX, as an indicator that
+ * we do not have enough information to rely on them.
+ *
+ * In deciding which rates are best, we find the weighted average of
+ * TX and RX goodness, where the weighting is by number of packets
+ * with data and TX packets are worth 4 times as much as RX packets.
+ * The weighted average is called "net goodness" and is also a number
+ * between 0 and 99. If 3 consecutive packets fail transmission
+ * outright, we automatically ratchet down the rate; otherwise, we
+ * switch to the best rate whenever the current rate's goodness falls
+ * below some threshold, and try increasing our rate when the goodness
+ * is very high.
+ *
+ * This system is optimized for gPXE's style of usage. Because normal
+ * operation always involves receiving something, we'll make our way
+ * to the best rate pretty quickly. We tend to follow the lead of the
+ * sending AP in choosing rates, but we won't use rates for long that
+ * don't work well for us in transmission. We assume gPXE won't be
+ * running for long enough that rate patterns will change much, so we
+ * don't have to keep time counters or the like. And if this doesn't
+ * work well in practice there are many ways it could be tweaked.
+ *
+ * To avoid staying at 1Mbps for a long time, we don't track any
+ * transmitted packets until we've set our rate based on received
+ * packets.
+ */
+
+/** Two-bit packet status indicator for a packet with no retries */
+#define RC_PKT_OK 0x3
+
+/** Two-bit packet status indicator for a packet with one retry */
+#define RC_PKT_RETRIED_ONCE 0x2
+
+/** Two-bit packet status indicator for a TX packet with multiple retries
+ *
+ * It is not possible to tell whether an RX packet had one or multiple
+ * retries; we rely instead on the fact that failed RX packets won't
+ * get to us at all, so if we receive a lot of RX packets on a certain
+ * rate it must be pretty good.
+ */
+#define RC_PKT_RETRIED_MULTI 0x1
+
+/** Two-bit packet status indicator for a TX packet that was never ACKed
+ *
+ * It is not possible to tell whether an RX packet was setn if it
+ * didn't get through to us, but if we don't see one we won't increase
+ * the goodness for its rate. This asymmetry is part of why TX packets
+ * are weighted much more heavily than RX.
+ */
+#define RC_PKT_FAILED 0x0
+
+/** Number of times to weight TX packets more heavily than RX packets */
+#define RC_TX_FACTOR 4
+
+/** Number of consecutive failed TX packets that cause an automatic rate drop */
+#define RC_TX_EMERG_FAIL 3
+
+/** Minimum net goodness below which we will search for a better rate */
+#define RC_GOODNESS_MIN 85
+
+/** Maximum net goodness above which we will try to increase our rate */
+#define RC_GOODNESS_MAX 95
+
+/** Minimum (num RX + @c RC_TX_FACTOR * num TX) to use a certain rate */
+#define RC_UNCERTAINTY_THRESH 4
+
+/** TX direction */
+#define TX 0
+
+/** RX direction */
+#define RX 1
+
+/** A rate control context */
+struct rc80211_ctx
+{
+ /** Goodness state for each rate, TX and RX */
+ u32 goodness[2][NET80211_MAX_RATES];
+
+ /** Number of packets recorded for each rate */
+ u8 count[2][NET80211_MAX_RATES];
+
+ /** Indication of whether we've set the device rate yet */
+ int started;
+
+ /** Counter of all packets sent and received */
+ int packets;
+};
+
+/**
+ * Initialize rate-control algorithm
+ *
+ * @v dev 802.11 device
+ * @ret ctx Rate-control context, to be stored in @c dev->rctl
+ */
+struct rc80211_ctx * rc80211_init ( struct net80211_device *dev __unused )
+{
+ struct rc80211_ctx *ret = zalloc ( sizeof ( *ret ) );
+ return ret;
+}
+
+/**
+ * Calculate net goodness for a certain rate
+ *
+ * @v ctx Rate-control context
+ * @v rate_idx Index of rate to calculate net goodness for
+ */
+static int rc80211_calc_net_goodness ( struct rc80211_ctx *ctx,
+ int rate_idx )
+{
+ int sum[2], num[2], dir, pkt;
+
+ for ( dir = 0; dir < 2; dir++ ) {
+ u32 good = ctx->goodness[dir][rate_idx];
+
+ num[dir] = ctx->count[dir][rate_idx];
+ sum[dir] = 0;
+
+ for ( pkt = 0; pkt < num[dir]; pkt++ )
+ sum[dir] += ( good >> ( 2 * pkt ) ) & 0x3;
+ }
+
+ if ( ( num[TX] * RC_TX_FACTOR + num[RX] ) < RC_UNCERTAINTY_THRESH )
+ return -1;
+
+ return ( 33 * ( sum[TX] * RC_TX_FACTOR + sum[RX] ) /
+ ( num[TX] * RC_TX_FACTOR + num[RX] ) );
+}
+
+/**
+ * Determine the best rate to switch to and return it
+ *
+ * @v dev 802.11 device
+ * @ret rate_idx Index of the best rate to switch to
+ */
+static int rc80211_pick_best ( struct net80211_device *dev )
+{
+ struct rc80211_ctx *ctx = dev->rctl;
+ int best_net_good = 0, best_rate = -1, i;
+
+ for ( i = 0; i < dev->nr_rates; i++ ) {
+ int net_good = rc80211_calc_net_goodness ( ctx, i );
+
+ if ( net_good > best_net_good ||
+ ( best_net_good > RC_GOODNESS_MIN &&
+ net_good > RC_GOODNESS_MIN ) ) {
+ best_net_good = net_good;
+ best_rate = i;
+ }
+ }
+
+ if ( best_rate >= 0 ) {
+ int old_good = rc80211_calc_net_goodness ( ctx, dev->rate );
+ if ( old_good != best_net_good )
+ DBGC ( ctx, "802.11 RC %p switching from goodness "
+ "%d to %d\n", ctx, old_good, best_net_good );
+
+ ctx->started = 1;
+ return best_rate;
+ }
+
+ return dev->rate;
+}
+
+/**
+ * Set 802.11 device rate
+ *
+ * @v dev 802.11 device
+ * @v rate_idx Index of rate to switch to
+ *
+ * This is a thin wrapper around net80211_set_rate_idx to insert a
+ * debugging message where appropriate.
+ */
+static inline void rc80211_set_rate ( struct net80211_device *dev,
+ int rate_idx )
+{
+ DBGC ( dev->rctl, "802.11 RC %p changing rate %d->%d Mbps\n", dev->rctl,
+ dev->rates[dev->rate] / 10, dev->rates[rate_idx] / 10 );
+
+ net80211_set_rate_idx ( dev, rate_idx );
+}
+
+/**
+ * Check rate-control state and change rate if necessary
+ *
+ * @v dev 802.11 device
+ */
+static void rc80211_maybe_set_new ( struct net80211_device *dev )
+{
+ struct rc80211_ctx *ctx = dev->rctl;
+ int net_good;
+
+ net_good = rc80211_calc_net_goodness ( ctx, dev->rate );
+
+ if ( ! ctx->started ) {
+ rc80211_set_rate ( dev, rc80211_pick_best ( dev ) );
+ return;
+ }
+
+ if ( net_good < 0 ) /* insufficient data */
+ return;
+
+ if ( net_good > RC_GOODNESS_MAX && dev->rate + 1 < dev->nr_rates ) {
+ int higher = rc80211_calc_net_goodness ( ctx, dev->rate + 1 );
+ if ( higher > net_good || higher < 0 )
+ rc80211_set_rate ( dev, dev->rate + 1 );
+ else
+ rc80211_set_rate ( dev, rc80211_pick_best ( dev ) );
+ }
+
+ if ( net_good < RC_GOODNESS_MIN ) {
+ rc80211_set_rate ( dev, rc80211_pick_best ( dev ) );
+ }
+}
+
+/**
+ * Update rate-control state
+ *
+ * @v dev 802.11 device
+ * @v direction One of the direction constants TX or RX
+ * @v rate_idx Index of rate at which packet was sent or received
+ * @v retries Number of times packet was retried before success
+ * @v failed If nonzero, the packet failed to get through
+ */
+static void rc80211_update ( struct net80211_device *dev, int direction,
+ int rate_idx, int retries, int failed )
+{
+ struct rc80211_ctx *ctx = dev->rctl;
+ u32 goodness = ctx->goodness[direction][rate_idx];
+
+ if ( ctx->count[direction][rate_idx] < 16 )
+ ctx->count[direction][rate_idx]++;
+
+ goodness <<= 2;
+ if ( failed )
+ goodness |= RC_PKT_FAILED;
+ else if ( retries > 1 )
+ goodness |= RC_PKT_RETRIED_MULTI;
+ else if ( retries )
+ goodness |= RC_PKT_RETRIED_ONCE;
+ else
+ goodness |= RC_PKT_OK;
+
+ ctx->goodness[direction][rate_idx] = goodness;
+
+ ctx->packets++;
+
+ rc80211_maybe_set_new ( dev );
+}
+
+/**
+ * Update rate-control state for transmitted packet
+ *
+ * @v dev 802.11 device
+ * @v retries Number of times packet was transmitted before success
+ * @v rc Return status code for transmission
+ */
+void rc80211_update_tx ( struct net80211_device *dev, int retries, int rc )
+{
+ struct rc80211_ctx *ctx = dev->rctl;
+
+ if ( ! ctx->started )
+ return;
+
+ rc80211_update ( dev, TX, dev->rate, retries, rc );
+
+ /* Check if the last RC_TX_EMERG_FAIL packets have all failed */
+ if ( ! ( ctx->goodness[TX][dev->rate] &
+ ( ( 1 << ( 2 * RC_TX_EMERG_FAIL ) ) - 1 ) ) ) {
+ if ( dev->rate == 0 )
+ DBGC ( dev->rctl, "802.11 RC %p saw %d consecutive "
+ "failed TX, but cannot lower rate any further\n",
+ dev->rctl, RC_TX_EMERG_FAIL );
+ else {
+ DBGC ( dev->rctl, "802.11 RC %p lowering rate (%d->%d "
+ "Mbps) due to %d consecutive TX failures\n",
+ dev->rctl, dev->rates[dev->rate] / 10,
+ dev->rates[dev->rate - 1] / 10,
+ RC_TX_EMERG_FAIL );
+
+ rc80211_set_rate ( dev, dev->rate - 1 );
+ }
+ }
+}
+
+/**
+ * Update rate-control state for received packet
+ *
+ * @v dev 802.11 device
+ * @v retry Whether the received packet had been retransmitted
+ * @v rate Rate at which packet was received, in 100 kbps units
+ */
+void rc80211_update_rx ( struct net80211_device *dev, int retry, u16 rate )
+{
+ int ridx;
+
+ for ( ridx = 0; ridx < dev->nr_rates && dev->rates[ridx] != rate;
+ ridx++ )
+ ;
+ if ( ridx >= dev->nr_rates )
+ return; /* couldn't find the rate */
+
+ rc80211_update ( dev, RX, ridx, retry, 0 );
+}
+
+/**
+ * Free rate-control context
+ *
+ * @v ctx Rate-control context
+ */
+void rc80211_free ( struct rc80211_ctx *ctx )
+{
+ free ( ctx );
+}