Received-SPF: pass (google.com: domain of linux-bluetooth-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Message-ID: <d43dffdc43a40782ec6d5d6c24b1638005992a8f.camel@iki.fi>
Subject: [PATCH] Bluetooth: btusb: check SCO MTU before enabling USB ALT 3
 for WBS
From:   Pauli Virtanen <pav@iki.fi>
To:     linux-bluetooth <linux-bluetooth@vger.kernel.org>
Cc:     Hilda Wu <hildawu@realtek.com>
Date:   Fri, 16 Jul 2021 20:58:17 +0300
Content-Type: text/plain; charset="UTF-8"
User-Agent: Evolution 3.40.3 (3.40.3-1.fc34) 
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Precedence: bulk

Some USB BT adapters don't satisfy the MTU requirement mentioned in
commit e848dbd364ac ("Bluetooth: btusb: Add support USB ALT 3 for WBS")
and have ALT 3 setting that produces no/garbled audio. Check that the MTU
condition is satisfied, and fall back to ALT 1 if not.

Tested with USB adapters (mtu<72, produce sound only with ALT1)
BCM20702A1 0b05:17cb, CSR8510A10 0a12:0001, and (mtu>=72, ALT3)
RTL8761BU 0bda:8771, Intel AX200 8087:0029 (after disabling ALT6).

Signed-off-by: Pauli Virtanen <pav@iki.fi>
---
 drivers/bluetooth/btusb.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/bluetooth/btusb.c b/drivers/bluetooth/btusb.c
index a9855a2dd561..3ee66e415c4d 100644
--- a/drivers/bluetooth/btusb.c
+++ b/drivers/bluetooth/btusb.c
@@ -1763,9 +1763,11 @@ static void btusb_work(struct work_struct *work)
 			/* Because mSBC frames do not need to be aligned to the
 			 * SCO packet boundary. If support the Alt 3, use the
 			 * Alt 3 for HCI payload >= 60 Bytes let air packet
-			 * data satisfy 60 bytes.
+			 * data satisfy 60 bytes. USB Alt 3 support also needs
+			 * HFP transparent MTU >= 72 Bytes.
 			 */
-			if (new_alts == 1 && btusb_find_altsetting(data, 3))
+			if (new_alts == 1 && hdev->sco_mtu >= 72 &&
+			    btusb_find_altsetting(data, 3))
 				new_alts = 3;
 		}
 
-- 
2.31.1