mac80211: implement beacon filtering in software

Regardless of whether the hardware implements beacon filtering,
there's no need to process all beacons in software all the time
throughout the stack (mac80211 does a lot, then cfg80211, then
in the future possibly userspace).

This patch implements the "best possible" beacon filtering in
mac80211. "Best possible" means that it can look for changes in
all requested information elements, and distinguish vendor IEs
by their OUI.

In the future, we will add nl80211 API for userspace to request
information elements and vendor IE OUIs to watch -- drivers can
then implement the best they can do while software implements
it fully.

It is unclear whether or not this actually saves CPU time, but
the data is all in the cache already so it should be fairly
cheap. The additional _testing_, however, has great benefit;
Without this, and on hardware that doesn't implement beacon
filtering, wrong assumptions about, for example, scan result
updates could quickly creep into code.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

1 files changed, 40 insertions, 12 deletions

diff --git a/net/mac80211/mlme.c b/net/mac80211/mlme.c
index c39a214e7ad0..7a8d4c494246 100644
--- a/net/mac80211/mlme.c
+++ b/net/mac80211/mlme.c
@@ -18,6 +18,7 @@
 #include <linux/etherdevice.h>
 #include <linux/rtnetlink.h>
 #include <linux/pm_qos_params.h>
+#include <linux/crc32.h>
 #include <net/mac80211.h>
 #include <asm/unaligned.h>
 
@@ -1752,46 +1753,73 @@ static void ieee80211_rx_mgmt_probe_resp(struct ieee80211_sub_if_data *sdata,
 		ifmgd->flags &= ~IEEE80211_STA_PROBEREQ_POLL;
 }
 
+/*
+ * This is the canonical list of information elements we care about,
+ * the filter code also gives us all changes to the Microsoft OUI
+ * (00:50:F2) vendor IE which is used for WMM which we need to track.
+ *
+ * We implement beacon filtering in software since that means we can
+ * avoid processing the frame here and in cfg80211, and userspace
+ * will not be able to tell whether the hardware supports it or not.
+ *
+ * XXX: This list needs to be dynamic -- userspace needs to be able to
+ *	add items it requires. It also needs to be able to tell us to
+ *	look out for other vendor IEs.
+ */
+static const u64 care_about_ies =
+	BIT(WLAN_EID_COUNTRY) |
+	BIT(WLAN_EID_ERP_INFO) |
+	BIT(WLAN_EID_CHANNEL_SWITCH) |
+	BIT(WLAN_EID_PWR_CONSTRAINT) |
+	BIT(WLAN_EID_HT_CAPABILITY) |
+	BIT(WLAN_EID_HT_INFORMATION);
+
 static void ieee80211_rx_mgmt_beacon(struct ieee80211_sub_if_data *sdata,
 				     struct ieee80211_mgmt *mgmt,
 				     size_t len,
 				     struct ieee80211_rx_status *rx_status)
 {
-	struct ieee80211_if_managed *ifmgd;
+	struct ieee80211_if_managed *ifmgd = &sdata->u.mgd;
 	size_t baselen;
 	struct ieee802_11_elems elems;
 	struct ieee80211_local *local = sdata->local;
 	u32 changed = 0;
-	bool erp_valid, directed_tim;
+	bool erp_valid, directed_tim = false;
 	u8 erp_value = 0;
+	u32 ncrc;
 
 	/* Process beacon from the current BSS */
 	baselen = (u8 *) mgmt->u.beacon.variable - (u8 *) mgmt;
 	if (baselen > len)
 		return;
 
-	ieee802_11_parse_elems(mgmt->u.beacon.variable, len - baselen, &elems);
-
-	ieee80211_rx_bss_info(sdata, mgmt, len, rx_status, &elems, true);
-
-	if (sdata->vif.type != NL80211_IFTYPE_STATION)
+	if (rx_status->freq != local->hw.conf.channel->center_freq)
 		return;
 
-	ifmgd = &sdata->u.mgd;
-
 	if (!(ifmgd->flags & IEEE80211_STA_ASSOCIATED) ||
 	    memcmp(ifmgd->bssid, mgmt->bssid, ETH_ALEN) != 0)
 		return;
 
-	if (rx_status->freq != local->hw.conf.channel->center_freq)
+	ncrc = crc32_be(0, (void *)&mgmt->u.beacon.beacon_int, 4);
+	ncrc = ieee802_11_parse_elems_crc(mgmt->u.beacon.variable,
+					  len - baselen, &elems,
+					  care_about_ies, ncrc);
+
+	if (local->hw.flags & IEEE80211_HW_PS_NULLFUNC_STACK)
+		directed_tim = ieee80211_check_tim(&elems, ifmgd->aid);
+
+	ncrc = crc32_be(ncrc, (void *)&directed_tim, sizeof(directed_tim));
+
+	if (ncrc == ifmgd->beacon_crc)
 		return;
+	ifmgd->beacon_crc = ncrc;
+
+	ieee80211_rx_bss_info(sdata, mgmt, len, rx_status, &elems, true);
 
 	ieee80211_sta_wmm_params(local, ifmgd, elems.wmm_param,
 				 elems.wmm_param_len);
 
 	if (local->hw.flags & IEEE80211_HW_PS_NULLFUNC_STACK) {
-		directed_tim = ieee80211_check_tim(&elems, ifmgd->aid);
-
 		if (directed_tim) {
 			if (local->hw.conf.dynamic_ps_timeout > 0) {
 				local->hw.conf.flags &= ~IEEE80211_CONF_PS;