[RFC PATCH v2 1/1] Bluetooth: btusb: test larger LE packets on TP-Link UB500

[Kyle Pepper <cochcoder@proton.me>]

The tested TP-Link UB500 (2357:0604, RTL8761BU firmware 0xdfc6d922)
reports an LE ACL length of 27 bytes and eight packets. It accepts
167-byte LE HCI ACL packets during ASHA playback. A size-only test
streams clearly with the eight reported LE credits retained, so the
previous shared-buffer workaround does not establish shared pools.

Use post_init to test a host LE ACL length of 167 for this USB ID,
without changing LE packet/credit counts or BR/EDR fields. Refuse
unexpected initialization values rather than applying this diagnostic
to a different reported buffer configuration.

This is an experimental RFC: 167 is a demonstrated working length,
not a measured maximum. The appropriate general limit and firmware
scope still need confirmation. Mixed BR/EDR and LE capacity and
exceptional-congestion behavior have not been established.

The ASHA 7.2.9 trial completes all 3269 HCI packets, including 3243
whole audio SDUs, with a maximum of three outstanding packets. The
human listening verdict is CLEAR. PipeWire and its actual 4608-byte
socket send buffer are unchanged. Increasing only that socket buffer
to 8192 with HCI length 27 previously crackled and underfed the stream.

Prepared with AI assistance for source, evidence analysis and text;
hardware playback and listening were performed by the submitter.

Assisted-by: LLM
Signed-off-by: Kyle Pepper <cochcoder@proton.me>
---
 drivers/bluetooth/btusb_main.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/drivers/bluetooth/btusb_main.c b/drivers/bluetooth/btusb_main.c
index 92a78a7e6..2b955ed4b 100644
--- a/drivers/bluetooth/btusb_main.c
+++ b/drivers/bluetooth/btusb_main.c
@@ -2972,6 +2972,18 @@ static int btusb_setup_realtek(struct hci_dev *hdev)
 	return ret;
 }
 
+/* Private diagnostic: test a known accepted packet size, not a pool claim. */
+static int btusb_ub500_test_post_init(struct hci_dev *hdev)
+{
+	if (hdev->le_mtu != 27 || hdev->le_pkts != 8)
+		return -EINVAL;
+
+	bt_dev_info(hdev, "UB500 TEST ONLY: LE ACL MTU 27 -> 167; LE packets %u unchanged",
+		    hdev->le_pkts);
+	hdev->le_mtu = 167;
+	return 0;
+}
+
 static int btusb_recv_event_realtek(struct hci_dev *hdev, struct sk_buff *skb)
 {
 	if (skb->len >= HCI_EVENT_HDR_SIZE + 1 &&
@@ -4715,6 +4727,9 @@ static int btusb_probe(struct usb_interface *intf,
 	    (id->driver_info & BTUSB_REALTEK)) {
 		btrtl_set_driver_name(hdev, btusb_driver.name);
 		hdev->setup = btusb_setup_realtek;
+		if (le16_to_cpu(data->udev->descriptor.idVendor) == 0x2357 &&
+		    le16_to_cpu(data->udev->descriptor.idProduct) == 0x0604)
+			hdev->post_init = btusb_ub500_test_post_init;
 		hdev->shutdown = btrtl_shutdown_realtek;
 		hdev->reset = btusb_rtl_reset;
 		hdev->hw_error = btusb_rtl_hw_error;
-- 
2.56.0