Some MT6639 Bluetooth USB interfaces (e.g. IMC Networks 13d3:3588 on
ASUS ROG STRIX X870E-E and ProArt X870E-Creator boards) expose only a
single alternate setting (alt 0) on the ISO interface. The driver
unconditionally requests alt setting 1, which fails with EINVAL on
these devices, causing a ~20 second initialization delay and no LE
audio support.

Check the number of available alternate settings before selecting one.
If only alt 0 exists, use it; otherwise request alt 1 as before.

Link: https://github.com/jetm/mediatek-mt7927-dkms/pull/39
Signed-off-by: Javier Tia <redacted>
Reported-by: Ryan Gilbert <redacted>
Tested-by: Ryan Gilbert <redacted>
---
 drivers/bluetooth/btmtk.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/bluetooth/btmtk.c b/drivers/bluetooth/btmtk.c
index 3f12f2ab6369..95b09b1ea78f 100644
--- a/drivers/bluetooth/btmtk.c
+++ b/drivers/bluetooth/btmtk.c

@@ -1029,7 +1029,8 @@ static int __set_mtk_intr_interface(struct hci_dev *hdev)
 	if (!btmtk_data->isopkt_intf)
 		return -ENODEV;
 
-	err = usb_set_interface(btmtk_data->udev, MTK_ISO_IFNUM, 1);
+	err = usb_set_interface(btmtk_data->udev, MTK_ISO_IFNUM,
+			       (intf->num_altsetting > 1) ? 1 : 0);
 	if (err < 0) {
 		bt_dev_err(hdev, "setting interface failed (%d)", -err);
 		return err;

-- 
2.53.0