linux-kernel - [PATCH v1] Bluetooth: L2CAP: Introduce minimum limit of rx

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Message-Id: <20250630075656.8970-1-shpakovskiip@gmail.com>
Date: Mon, 30 Jun 2025 10:56:56 +0300
From: Pavel Shpakovskiy <shpakovskiip@...il.com>
To: marcel@...tmann.org,
	johan.hedberg@...il.com,
	luiz.dentz@...il.com,
	davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	horms@...nel.org
Cc: linux-bluetooth@...r.kernel.org,
	netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	kernel@...utedevices.com,
	Pavel Shpakovskiy <shpakovskiip@...il.com>
Subject: [PATCH v1] Bluetooth: L2CAP: Introduce minimum limit of rx_credits value

The commit 96cd8eaa131f 
("Bluetooth: L2CAP: Derive rx credits from MTU and MPS") 
removed the static rx_credits setup to improve BLE packet
communication for high MTU values. However, due to vendor-specific 
issues in the Bluetooth module firmware, using low MTU values 
(especially less than 256 bytes) results in dynamically calculated
rx_credits being too low, causing slow speeds and occasional BLE 
connection failures.

This change aims to improve BLE connection stability and speed 
for low MTU values. It is possible to tune minimum value 
of rx credits with debugfs handle.

Signed-off-by: Pavel Shpakovskiy <shpakovskiip@...il.com>
---
 include/net/bluetooth/l2cap.h |  2 ++
 net/bluetooth/l2cap_core.c    | 17 +++++++++++++++--
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/include/net/bluetooth/l2cap.h b/include/net/bluetooth/l2cap.h
index 4bb0eaedda180..8648d9324a654 100644
--- a/include/net/bluetooth/l2cap.h
+++ b/include/net/bluetooth/l2cap.h
@@ -437,6 +437,8 @@ struct l2cap_conn_param_update_rsp {
 #define L2CAP_CONN_PARAM_ACCEPTED	0x0000
 #define L2CAP_CONN_PARAM_REJECTED	0x0001
 
+#define L2CAP_LE_MIN_CREDITS		10
+
 struct l2cap_le_conn_req {
 	__le16     psm;
 	__le16     scid;
diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index c88f69dde995e..392d7ba0f0737 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -50,6 +50,8 @@ static u32 l2cap_feat_mask = L2CAP_FEAT_FIXED_CHAN | L2CAP_FEAT_UCD;
 static LIST_HEAD(chan_list);
 static DEFINE_RWLOCK(chan_list_lock);
 
+static u16 le_min_credits = L2CAP_LE_MIN_CREDITS;
+
 static struct sk_buff *l2cap_build_cmd(struct l2cap_conn *conn,
 				       u8 code, u8 ident, u16 dlen, void *data);
 static void l2cap_send_cmd(struct l2cap_conn *conn, u8 ident, u8 code, u16 len,
@@ -547,8 +549,17 @@ static __u16 l2cap_le_rx_credits(struct l2cap_chan *chan)
 	/* If we don't know the available space in the receiver buffer, give
 	 * enough credits for a full packet.
 	 */
-	if (chan->rx_avail == -1)
-		return (chan->imtu / chan->mps) + 1;
+	if (chan->rx_avail == -1) {
+		u16 rx_credits = (chan->imtu / chan->mps) + 1;
+
+		if (rx_credits < le_min_credits) {
+			rx_credits = le_min_credits;
+			BT_DBG("chan %p: set rx_credits to minimum value: %u",
+			       chan, chan->rx_credits);
+		}
+
+		return rx_credits;
+	}
 
 	/* If we know how much space is available in the receive buffer, give
 	 * out as many credits as would fill the buffer.
@@ -7661,6 +7672,8 @@ int __init l2cap_init(void)
 	l2cap_debugfs = debugfs_create_file("l2cap", 0444, bt_debugfs,
 					    NULL, &l2cap_debugfs_fops);
 
+	debugfs_create_u16("l2cap_le_min_credits", 0644, bt_debugfs,
+			   &le_min_credits);
 	return 0;
 }
 
-- 
2.34.1