mac80211: Implement mesh synchronization framework

This patch adds MBSS extensible synchronization framework (Sec.
13.13.2 of IEEE Std. 802.11-2012).

The framework is implemented via an ops table which defines the
following functions:

    rx_bcn_presp() - this is called every time a mesh beacon is
received.
    adjust_tbtt() - this is called immediately before a beacon is about
to be transmitted.

The default neighbor offset synchronization defined in the standard is
implemented.  We also provide template functions for vendor specific
methods.

When neighbor offset synchronization is active (which is the default)
mesh neighbors in the same MBSS will track timing offsets to each other
and compensate clock drift.

In our tests we observed that this mesh synchronization implementation
successfully corrected drifts between stations of ~2PPM while
introducing a jitter of ~20us.

It is also possible to test this framework on mac80211_hwsim simulated
phys to see how it behaves under different topologies, over poor links,
etc.

Signed-off-by: Marco Porsch <marco.porsch@s2005.tu-chemnitz.de>
Signed-off-by: Pavel Zubarev <pavel.zubarev@gmail.com>
Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

1 files changed, 296 insertions, 0 deletions

diff --git a/net/mac80211/mesh_sync.c b/net/mac80211/mesh_sync.c
new file mode 100644
index 00000000000..f78b0139856
--- /dev/null
+++ b/net/mac80211/mesh_sync.c
@@ -0,0 +1,296 @@
+/*
+ * Copyright 2011-2012, Pavel Zubarev <pavel.zubarev@gmail.com>
+ * Copyright 2011-2012, Marco Porsch <marco.porsch@s2005.tu-chemnitz.de>
+ * Copyright 2011-2012, cozybit Inc.
+ *
+ * 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 "ieee80211_i.h"
+#include "mesh.h"
+#include "driver-ops.h"
+
+#ifdef CONFIG_MAC80211_VERBOSE_MESH_SYNC_DEBUG
+#define msync_dbg(fmt, args...) \
+	printk(KERN_DEBUG "Mesh sync (%s): " fmt "\n", sdata->name, ##args)
+#else
+#define msync_dbg(fmt, args...)   do { (void)(0); } while (0)
+#endif
+
+/* This is not in the standard.  It represents a tolerable tbtt drift below
+ * which we do no TSF adjustment.
+ */
+#define TBTT_MINIMUM_ADJUSTMENT 10
+
+struct sync_method {
+	u8 method;
+	struct ieee80211_mesh_sync_ops ops;
+};
+
+/**
+ * mesh_peer_tbtt_adjusting - check if an mp is currently adjusting its TBTT
+ *
+ * @ie: information elements of a management frame from the mesh peer
+ */
+static bool mesh_peer_tbtt_adjusting(struct ieee802_11_elems *ie)
+{
+	return (ie->mesh_config->meshconf_cap &
+	    MESHCONF_CAPAB_TBTT_ADJUSTING) != 0;
+}
+
+void mesh_sync_adjust_tbtt(struct ieee80211_sub_if_data *sdata)
+{
+	struct ieee80211_local *local = sdata->local;
+	struct ieee80211_if_mesh *ifmsh = &sdata->u.mesh;
+	/* sdata->vif.bss_conf.beacon_int in 1024us units, 0.04% */
+	u64 beacon_int_fraction = sdata->vif.bss_conf.beacon_int * 1024 / 2500;
+	u64 tsf;
+	u64 tsfdelta;
+
+	spin_lock_bh(&ifmsh->sync_offset_lock);
+
+	if (ifmsh->sync_offset_clockdrift_max < beacon_int_fraction) {
+		msync_dbg("TBTT : max clockdrift=%lld; adjusting",
+			(long long) ifmsh->sync_offset_clockdrift_max);
+		tsfdelta = -ifmsh->sync_offset_clockdrift_max;
+		ifmsh->sync_offset_clockdrift_max = 0;
+	} else {
+		msync_dbg("TBTT : max clockdrift=%lld; adjusting by %llu",
+			(long long) ifmsh->sync_offset_clockdrift_max,
+			(unsigned long long) beacon_int_fraction);
+		tsfdelta = -beacon_int_fraction;
+		ifmsh->sync_offset_clockdrift_max -= beacon_int_fraction;
+	}
+
+	tsf = drv_get_tsf(local, sdata);
+	if (tsf != -1ULL)
+		drv_set_tsf(local, sdata, tsf + tsfdelta);
+	spin_unlock_bh(&ifmsh->sync_offset_lock);
+}
+
+static void mesh_sync_offset_rx_bcn_presp(struct ieee80211_sub_if_data *sdata,
+				   u16 stype,
+				   struct ieee80211_mgmt *mgmt,
+				   struct ieee802_11_elems *elems,
+				   struct ieee80211_rx_status *rx_status)
+{
+	struct ieee80211_if_mesh *ifmsh = &sdata->u.mesh;
+	struct ieee80211_local *local = sdata->local;
+	struct sta_info *sta;
+	u64 t_t, t_r;
+
+	WARN_ON(ifmsh->mesh_sp_id != IEEE80211_SYNC_METHOD_NEIGHBOR_OFFSET);
+
+	/* standard mentions only beacons */
+	if (stype != IEEE80211_STYPE_BEACON)
+		return;
+
+	/* The current tsf is a first approximation for the timestamp
+	 * for the received beacon.  Further down we try to get a
+	 * better value from the rx_status->mactime field if
+	 * available. Also we have to call drv_get_tsf() before
+	 * entering the rcu-read section.*/
+	t_r = drv_get_tsf(local, sdata);
+
+	rcu_read_lock();
+	sta = sta_info_get(sdata, mgmt->sa);
+	if (!sta)
+		goto no_sync;
+
+	/* check offset sync conditions (13.13.2.2.1)
+	 *
+	 * TODO also sync to
+	 * dot11MeshNbrOffsetMaxNeighbor non-peer non-MBSS neighbors
+	 */
+
+	if (elems->mesh_config && mesh_peer_tbtt_adjusting(elems)) {
+		clear_sta_flag(sta, WLAN_STA_TOFFSET_KNOWN);
+		msync_dbg("STA %pM : is adjusting TBTT", sta->sta.addr);
+		goto no_sync;
+	}
+
+	if (rx_status->flag & RX_FLAG_MACTIME_MPDU && rx_status->mactime) {
+		/*
+		 * The mactime is defined as the time the first data symbol
+		 * of the frame hits the PHY, and the timestamp of the beacon
+		 * is defined as "the time that the data symbol containing the
+		 * first bit of the timestamp is transmitted to the PHY plus
+		 * the transmitting STA's delays through its local PHY from the
+		 * MAC-PHY interface to its interface with the WM" (802.11
+		 * 11.1.2)
+		 *
+		 * T_r, in 13.13.2.2.2, is just defined as "the frame reception
+		 * time" but we unless we interpret that time to be the same
+		 * time of the beacon timestamp, the offset calculation will be
+		 * off.  Below we adjust t_r to be "the time at which the first
+		 * symbol of the timestamp element in the beacon is received".
+		 * This correction depends on the rate.
+		 *
+		 * Based on similar code in ibss.c
+		 */
+		int rate;
+
+		if (rx_status->flag & RX_FLAG_HT) {
+			/* TODO:
+			 * In principle there could be HT-beacons (Dual Beacon
+			 * HT Operation options), but for now ignore them and
+			 * just use the primary (i.e. non-HT) beacons for
+			 * synchronization.
+			 * */
+			goto no_sync;
+		} else
+			rate = local->hw.wiphy->bands[rx_status->band]->
+				bitrates[rx_status->rate_idx].bitrate;
+
+		/* 24 bytes of header * 8 bits/byte *
+		 * 10*(100 Kbps)/Mbps / rate (100 Kbps)*/
+		t_r = rx_status->mactime + (24 * 8 * 10 / rate);
+	}
+
+	/* Timing offset calculation (see 13.13.2.2.2) */
+	t_t = le64_to_cpu(mgmt->u.beacon.timestamp);
+	sta->t_offset = t_t - t_r;
+
+	if (test_sta_flag(sta, WLAN_STA_TOFFSET_KNOWN)) {
+		s64 t_clockdrift = sta->t_offset_setpoint
+				   - sta->t_offset;
+
+		msync_dbg("STA %pM : sta->t_offset=%lld,"
+			  " sta->t_offset_setpoint=%lld,"
+			  " t_clockdrift=%lld",
+			  sta->sta.addr,
+			  (long long) sta->t_offset,
+			  (long long)
+			  sta->t_offset_setpoint,
+			  (long long) t_clockdrift);
+		rcu_read_unlock();
+
+		spin_lock_bh(&ifmsh->sync_offset_lock);
+		if (t_clockdrift >
+		    ifmsh->sync_offset_clockdrift_max)
+			ifmsh->sync_offset_clockdrift_max
+				= t_clockdrift;
+		spin_unlock_bh(&ifmsh->sync_offset_lock);
+
+	} else {
+		sta->t_offset_setpoint = sta->t_offset;
+		set_sta_flag(sta, WLAN_STA_TOFFSET_KNOWN);
+		msync_dbg("STA %pM : offset was invalid, "
+			  " sta->t_offset=%lld",
+			  sta->sta.addr,
+			  (long long) sta->t_offset);
+		rcu_read_unlock();
+	}
+	return;
+
+no_sync:
+	rcu_read_unlock();
+}
+
+static void mesh_sync_offset_adjust_tbtt(struct ieee80211_sub_if_data *sdata)
+{
+	struct ieee80211_if_mesh *ifmsh = &sdata->u.mesh;
+
+	WARN_ON(ifmsh->mesh_sp_id
+		!= IEEE80211_SYNC_METHOD_NEIGHBOR_OFFSET);
+	BUG_ON(!rcu_read_lock_held());
+
+	spin_lock_bh(&ifmsh->sync_offset_lock);
+
+	if (ifmsh->sync_offset_clockdrift_max >
+		TBTT_MINIMUM_ADJUSTMENT) {
+		/* Since ajusting the tsf here would
+		 * require a possibly blocking call
+		 * to the driver tsf setter, we punt
+		 * the tsf adjustment to the mesh tasklet
+		 */
+		msync_dbg("TBTT : kicking off TBTT "
+			  "adjustment with "
+			  "clockdrift_max=%lld",
+		  ifmsh->sync_offset_clockdrift_max);
+		set_bit(MESH_WORK_DRIFT_ADJUST,
+			&ifmsh->wrkq_flags);
+	} else {
+		msync_dbg("TBTT : max clockdrift=%lld; "
+			  "too small to adjust",
+			  (long long)
+		       ifmsh->sync_offset_clockdrift_max);
+		ifmsh->sync_offset_clockdrift_max = 0;
+	}
+	spin_unlock_bh(&ifmsh->sync_offset_lock);
+}
+
+static const u8 *mesh_get_vendor_oui(struct ieee80211_sub_if_data *sdata)
+{
+	struct ieee80211_if_mesh *ifmsh = &sdata->u.mesh;
+	u8 offset;
+
+	if (!ifmsh->ie || !ifmsh->ie_len)
+		return NULL;
+
+	offset = ieee80211_ie_split_vendor(ifmsh->ie,
+					ifmsh->ie_len, 0);
+
+	if (!offset)
+		return NULL;
+
+	return ifmsh->ie + offset + 2;
+}
+
+static void mesh_sync_vendor_rx_bcn_presp(struct ieee80211_sub_if_data *sdata,
+				   u16 stype,
+				   struct ieee80211_mgmt *mgmt,
+				   struct ieee802_11_elems *elems,
+				   struct ieee80211_rx_status *rx_status)
+{
+	const u8 *oui;
+
+	WARN_ON(sdata->u.mesh.mesh_sp_id != IEEE80211_SYNC_METHOD_VENDOR);
+	msync_dbg("called mesh_sync_vendor_rx_bcn_presp");
+	oui = mesh_get_vendor_oui(sdata);
+	/*  here you would implement the vendor offset tracking for this oui */
+}
+
+static void mesh_sync_vendor_adjust_tbtt(struct ieee80211_sub_if_data *sdata)
+{
+	const u8 *oui;
+
+	WARN_ON(sdata->u.mesh.mesh_sp_id != IEEE80211_SYNC_METHOD_VENDOR);
+	msync_dbg("called mesh_sync_vendor_adjust_tbtt");
+	oui = mesh_get_vendor_oui(sdata);
+	/*  here you would implement the vendor tsf adjustment for this oui */
+}
+
+/* global variable */
+static struct sync_method sync_methods[] = {
+	{
+		.method = IEEE80211_SYNC_METHOD_NEIGHBOR_OFFSET,
+		.ops = {
+			.rx_bcn_presp = &mesh_sync_offset_rx_bcn_presp,
+			.adjust_tbtt = &mesh_sync_offset_adjust_tbtt,
+		}
+	},
+	{
+		.method = IEEE80211_SYNC_METHOD_VENDOR,
+		.ops = {
+			.rx_bcn_presp = &mesh_sync_vendor_rx_bcn_presp,
+			.adjust_tbtt = &mesh_sync_vendor_adjust_tbtt,
+		}
+	},
+};
+
+struct ieee80211_mesh_sync_ops *ieee80211_mesh_sync_ops_get(u8 method)
+{
+	struct ieee80211_mesh_sync_ops *ops = NULL;
+	u8 i;
+
+	for (i = 0 ; i < ARRAY_SIZE(sync_methods); ++i) {
+		if (sync_methods[i].method == method) {
+			ops = &sync_methods[i].ops;
+			break;
+		}
+	}
+	return ops;
+}