[PATCH] wifi: mt76: mt7925: restore the legacy BSS after MLO teardown

[Aaron Ma <aaron.ma@canonical.com>]

Removing the last link of an MLD interface leaves the firmware with the
MLD BSS and DEV entry: the removal loop skips the deflink, and the
hweight16(mvif->valid_links) fallback reads the stale pre-update
valid_links, which is still non-zero. The entry keeps the per-link MAC
address, so every subsequent legacy association fails authentication
until the interface is removed or the module is reloaded.

Remove the deflink BSS and add it back as a legacy BSS on the transition
to zero links.

Fixes: 69acd6d910b0 ("wifi: mt76: mt7925: add mt7925_change_vif_links")
Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
---
 drivers/net/wireless/mediatek/mt76/mt7925/main.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/drivers/net/wireless/mediatek/mt76/mt7925/main.c b/drivers/net/wireless/mediatek/mt76/mt7925/main.c
index e7e6d20848d27..bb93008d3d75e 100644
--- a/drivers/net/wireless/mediatek/mt76/mt7925/main.c
+++ b/drivers/net/wireless/mediatek/mt76/mt7925/main.c
@@ -2036,6 +2036,21 @@ mt7925_change_vif_links(struct ieee80211_hw *hw, struct ieee80211_vif *vif,
 
 	mvif->valid_links = new_links;
 
+	/* The firmware would keep the MLD BSS and DEV entry: the removal
+	 * loop skips the deflink and the hweight16() fallback above reads
+	 * the stale pre-update valid_links. Restore the legacy BSS.
+	 */
+	if (old_links && !new_links) {
+		mt792x_mac_link_bss_remove(dev, &mvif->bss_conf,
+					   &mvif->sta.deflink);
+		err = mt7925_mac_link_bss_add(dev, &vif->bss_conf,
+					      &mvif->sta.deflink);
+		if (err < 0) {
+			mt792x_mutex_release(dev);
+			return err;
+		}
+	}
+
 	mt792x_mutex_release(dev);
 
 	return 0;
-- 
2.53.0