Bluetooth: host: l2cap: add alloc_seg callback

This callback allows use-cases where the SDU is much larger than the l2cap
MPS. The stack will then try to allocate using this callback if specified,
and fall-back on using the buffer's pool (previous behavior).

This way one can define two buffer pools, one with a very large buffer
size, and one with a buffer size >= MPS, and the stack will allocate from
that instead.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>

Author: jori-nordic

include/zephyr/bluetooth/l2cap.h

@@ -279,6 +279,19 @@ struct bt_l2cap_chan_ops {
 	 */
 	void (*encrypt_change)(struct bt_l2cap_chan *chan, uint8_t hci_status);
 
+	/** @brief Channel alloc_seg callback
+	 *
+	 *  If this callback is provided the channel will use it to allocate
+	 *  buffers to store segments. This avoids wasting big SDU buffers with
+	 *  potentially much smaller PDUs. If this callback is supplied, it must
+	 *  return a valid buffer.
+	 *
+	 *  @param chan The channel requesting a buffer.
+	 *
+	 *  @return Allocated buffer.
+	 */
+	struct net_buf *(*alloc_seg)(struct bt_l2cap_chan *chan);
+
 	/** @brief Channel alloc_buf callback
 	 *
 	 *  If this callback is provided the channel will use it to allocate

subsys/bluetooth/host/l2cap.c

@@ -1779,13 +1779,20 @@ static void le_disconn_rsp(struct bt_l2cap *l2cap, uint8_t ident,
 	bt_l2cap_chan_del(&chan->chan);
 }
 
-static inline struct net_buf *l2cap_alloc_seg(struct net_buf *buf)
+static inline struct net_buf *l2cap_alloc_seg(struct net_buf *buf, struct bt_l2cap_le_chan *ch)
 {
 	struct net_buf_pool *pool = net_buf_pool_get(buf->pool_id);
 	struct net_buf *seg;
 
-	/* Try to use original pool if possible */
-	seg = net_buf_alloc(pool, K_NO_WAIT);
+	/* Use the dedicated segment callback if registered */
+	if (ch->chan.ops->alloc_seg) {
+		seg = ch->chan.ops->alloc_seg(&ch->chan);
+		__ASSERT_NO_MSG(seg);
+	} else {
+		/* Try to use original pool if possible */
+		seg = net_buf_alloc(pool, K_NO_WAIT);
+	}
+
 	if (seg) {
 		net_buf_reserve(seg, BT_L2CAP_CHAN_SEND_RESERVE);
 		return seg;
@@ -1822,7 +1829,8 @@ static struct net_buf *l2cap_chan_create_seg(struct bt_l2cap_le_chan *ch,
 	}
 
 segment:
-	seg = l2cap_alloc_seg(buf);
+	seg = l2cap_alloc_seg(buf, ch);
+
 	if (!seg) {
 		return NULL;
 	}

tests/bluetooth/bsim_bt/bsim_test_l2cap_stress/prj.conf

@@ -25,12 +25,11 @@ CONFIG_BT_BUF_ACL_TX_COUNT=4
 # L2AP MPS + L2CAP header (4)
 CONFIG_BT_BUF_ACL_RX_SIZE=81
 
-# TODO: find out why we can't use 16 buffers. It should fit.
-
-CONFIG_BT_L2CAP_TX_BUF_COUNT=30
-# CONFIG_BT_L2CAP_TX_BUF_COUNT=100
-
-CONFIG_BT_BUF_ACL_TX_COUNT=4
+# Governs BT_CONN_TX_MAX, and so must be >= than the max number of
+# peers, since we attempt to send one SDU per peer. The test execution
+# is a bit slowed down by having this at the very minimum, but we want
+# to keep it that way as to stress the stack as much as possible.
+CONFIG_BT_L2CAP_TX_BUF_COUNT=6
 
 CONFIG_BT_CTLR_RX_BUFFERS=10
 # The ring buffer now has space for three times as much data

tests/bluetooth/bsim_bt/bsim_test_l2cap_stress/src/main.c

@@ -21,11 +21,15 @@ CREATE_FLAG(flag_l2cap_connected);
 #define INIT_CREDITS	10
 #define SDU_NUM		20
 #define SDU_LEN		1230
+#define NUM_SEGMENTS	30
 
 /* Only one SDU per link will be transmitted at a time */
-/* FIXME: decrease size */
 NET_BUF_POOL_DEFINE(sdu_tx_pool,
-		    100, BT_L2CAP_BUF_SIZE(SDU_LEN),
+		    CONFIG_BT_MAX_CONN, BT_L2CAP_BUF_SIZE(SDU_LEN),
+		    8, NULL);
+
+NET_BUF_POOL_DEFINE(segment_pool,
+		    NUM_SEGMENTS, BT_L2CAP_BUF_SIZE(CONFIG_BT_L2CAP_TX_MTU),
 		    8, NULL);
 
 /* Only one SDU per link will be received at a time */
@@ -66,6 +70,15 @@ int l2cap_chan_send(struct bt_l2cap_chan *chan, uint8_t *data, size_t len)
 	return ret;
 }
 
+struct net_buf *alloc_seg_cb(struct bt_l2cap_chan *chan)
+{
+	struct net_buf *buf = net_buf_alloc(&segment_pool, K_NO_WAIT);
+
+	ASSERT(buf, "Ran out of segment buffers");
+
+	return buf;
+}
+
 struct net_buf *alloc_buf_cb(struct bt_l2cap_chan *chan)
 {
 	return net_buf_alloc(&sdu_rx_pool, K_NO_WAIT);
@@ -137,6 +150,7 @@ static struct bt_l2cap_chan_ops ops = {
 	.connected = l2cap_chan_connected_cb,
 	.disconnected = l2cap_chan_disconnected_cb,
 	.alloc_buf = alloc_buf_cb,
+	.alloc_seg = alloc_seg_cb,
 	.recv = recv_cb,
 	.sent = sent_cb,
 };