Re: [PATCH v2] Bluetooth: btusb: check conditions before enabling USB ALT 3 for WBS

---

Changes in v2:
- Explain magic number 72 in a comment; didn't add the table for them,
 because it's not used elsewhere and we need just one number from it.
- Add flag for ALT3 support, restoring the behavior
 for non-Realtek devices the same as before e848dbd364ac, due to
 the problems reported on an Intel adapter. Don't have the device
 myself.

drivers/bluetooth/btusb.c | 22 ++++++++++++++--------
1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/drivers/bluetooth/btusb.c b/drivers/bluetooth/btusb.c
index 6d23308119d1..5cec719f6cba 100644
--- a/drivers/bluetooth/btusb.c
+++ b/drivers/bluetooth/btusb.c

@@ -516,6 +516,7 @@ static const struct dmi_system_id btusb_needs_reset_resume_table[] = {

#define BTUSB_HW_RESET_ACTIVE	12
#define BTUSB_TX_WAIT_VND_EVT	13
#define BTUSB_WAKEUP_DISABLE	14
+#define BTUSB_ALT3_OK_FOR_WBS	15

struct btusb_data {
	struct hci_dev       *hdev;

@@ -1748,16 +1749,20 @@ static void btusb_work(struct work_struct *work)

			/* Bluetooth USB spec recommends alt 6 (63 bytes), but
			 * many adapters do not support it.  Alt 1 appears to
			 * work for all adapters that do not have alt 6, and
-			 * which work with WBS at all.
+			 * which work with WBS at all.  Some devices prefer
+			 * alt 3 (HCI payload >= 60 Bytes let air packet
+			 * data satisfy 60 bytes), requiring
+			 * MTU >= 3 (packets) * 25 (size) - 3 (headers) = 72
+			 * see also Core spec 5, vol 4, B 2.1.1 & Table 2.1.
			 */
-			new_alts = btusb_find_altsetting(data, 6) ? 6 : 1;
-			/* 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.
-			 */
-			if (new_alts == 1 && btusb_find_altsetting(data, 3))
+			if (btusb_find_altsetting(data, 6))
+				new_alts = 6;
+			else if (test_bit(BTUSB_ALT3_OK_FOR_WBS, &data->flags) &&
+				 hdev->sco_mtu >= 72 &&
+				 btusb_find_altsetting(data, 3))

				new_alts = 3;
+			else
+				new_alts = 1;
		}

		if (btusb_switch_alt_setting(hdev, new_alts) < 0)

@@ -4733,6 +4738,7 @@ static int btusb_probe(struct usb_interface *intf,

		 * (DEVICE_REMOTE_WAKEUP)
		 */
		set_bit(BTUSB_WAKEUP_DISABLE, &data->flags);
+		set_bit(BTUSB_ALT3_OK_FOR_WBS, &data->flags);
	}