wifi: mac80211: fix monitor channel with chanctx emulation

After the channel context emulation, there were reports that
changing the monitor channel no longer works. This is because
those drivers don't have WANT_MONITOR_VIF, so the setting the
channel always exits out quickly.

Fix this by always allocating the virtual monitor sdata, and
simply not telling the driver about it unless it wanted to.
This way, we have an interface/sdata to bind the chanctx to,
and the emulation can work correctly.

Cc: stable@vger.kernel.org
Fixes: 0a44dfc070 ("wifi: mac80211: simplify non-chanctx drivers")
Reported-and-tested-by: Savyasaachi Vanga <savyasaachiv@gmail.com>
Closes: https://lore.kernel.org/r/chwoymvpzwtbmzryrlitpwmta5j6mtndocxsyqvdyikqu63lon@gfds653hkknl
Link: https://msgid.link/20240612122351.b12d4a109dde.I1831a44417faaab92bea1071209abbe4efbe3fba@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/driver-ops.c

@@ -311,6 +311,18 @@ int drv_assign_vif_chanctx(struct ieee80211_local *local,
 	might_sleep();
 	lockdep_assert_wiphy(local->hw.wiphy);
 
+	/*
+	 * We should perhaps push emulate chanctx down and only
+	 * make it call ->config() when the chanctx is actually
+	 * assigned here (and unassigned below), but that's yet
+	 * another change to all drivers to add assign/unassign
+	 * emulation callbacks. Maybe later.
+	 */
+	if (sdata->vif.type == NL80211_IFTYPE_MONITOR &&
+	    local->emulate_chanctx &&
+	    !ieee80211_hw_check(&local->hw, WANT_MONITOR_VIF))
+		return 0;
+
 	if (!check_sdata_in_driver(sdata))
 		return -EIO;
 
@@ -338,6 +350,11 @@ void drv_unassign_vif_chanctx(struct ieee80211_local *local,
 	might_sleep();
 	lockdep_assert_wiphy(local->hw.wiphy);
 
+	if (sdata->vif.type == NL80211_IFTYPE_MONITOR &&
+	    local->emulate_chanctx &&
+	    !ieee80211_hw_check(&local->hw, WANT_MONITOR_VIF))
+		return;
+
 	if (!check_sdata_in_driver(sdata))
 		return;
 

net/mac80211/iface.c

@@ -1122,9 +1122,6 @@ int ieee80211_add_virtual_monitor(struct ieee80211_local *local)
 	struct ieee80211_sub_if_data *sdata;
 	int ret;
 
-	if (!ieee80211_hw_check(&local->hw, WANT_MONITOR_VIF))
-		return 0;
-
 	ASSERT_RTNL();
 	lockdep_assert_wiphy(local->hw.wiphy);
 
@@ -1146,12 +1143,14 @@ int ieee80211_add_virtual_monitor(struct ieee80211_local *local)
 
 	ieee80211_set_default_queues(sdata);
 
+	if (ieee80211_hw_check(&local->hw, WANT_MONITOR_VIF)) {
 		ret = drv_add_interface(local, sdata);
 		if (WARN_ON(ret)) {
 			/* ok .. stupid driver, it asked for this! */
 			kfree(sdata);
 			return ret;
 		}
+	}
 
 	set_bit(SDATA_STATE_RUNNING, &sdata->state);
 
@@ -1188,9 +1187,6 @@ void ieee80211_del_virtual_monitor(struct ieee80211_local *local)
 {
 	struct ieee80211_sub_if_data *sdata;
 
-	if (!ieee80211_hw_check(&local->hw, WANT_MONITOR_VIF))
-		return;
-
 	ASSERT_RTNL();
 	lockdep_assert_wiphy(local->hw.wiphy);
 
@@ -1210,6 +1206,7 @@ void ieee80211_del_virtual_monitor(struct ieee80211_local *local)
 
 	ieee80211_link_release_channel(&sdata->deflink);
 
+	if (ieee80211_hw_check(&local->hw, WANT_MONITOR_VIF))
 		drv_remove_interface(local, sdata);
 
 	kfree(sdata);

net/mac80211/util.c

@@ -1843,7 +1843,7 @@ int ieee80211_reconfig(struct ieee80211_local *local)
 
 	/* add interfaces */
 	sdata = wiphy_dereference(local->hw.wiphy, local->monitor_sdata);
-	if (sdata) {
+	if (sdata && ieee80211_hw_check(&local->hw, WANT_MONITOR_VIF)) {
 		/* in HW restart it exists already */
 		WARN_ON(local->resuming);
 		res = drv_add_interface(local, sdata);