mac80211: Allow tsf increments via debugfs

Reading and writing back the tsf value via tsf is too slow if one wants
to make small increments to this timer.  With this change you can use
the syntax "+=<some value>" or "-=<some value>" to add or substract a
value from the tsf counter.
Signed-off-by: NJavier Cardona <javier@cozybit.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

net/mac80211/debugfs_netdev.c

@@ -424,6 +424,7 @@ static ssize_t ieee80211_if_parse_tsf(
 	struct ieee80211_local *local = sdata->local;
 	unsigned long long tsf;
 	int ret;
+	int tsf_is_delta = 0;
 
 	if (strncmp(buf, "reset", 5) == 0) {
 		if (local->ops->reset_tsf) {
@@ -431,9 +432,20 @@ static ssize_t ieee80211_if_parse_tsf(
 			wiphy_info(local->hw.wiphy, "debugfs reset TSF\n");
 		}
 	} else {
+		if (buflen > 10 && buf[1] == '=') {
+			if (buf[0] == '+')
+				tsf_is_delta = 1;
+			else if (buf[0] == '-')
+				tsf_is_delta = -1;
+			else
+				return -EINVAL;
+			buf += 2;
+		}
 		ret = kstrtoull(buf, 10, &tsf);
 		if (ret < 0)
 			return -EINVAL;
+		if (tsf_is_delta)
+			tsf = drv_get_tsf(local, sdata) + tsf_is_delta * tsf;
 		if (local->ops->set_tsf) {
 			drv_set_tsf(local, sdata, tsf);
 			wiphy_info(local->hw.wiphy,