Bluetooth: L2CAP: stop stealing buffers from SDU pool

It seems like a nice idea at first, but leads to hard-to-debug
situations for the application.

The previous behavior can be implemented by the app by defining
`alloc_seg` and allocating from the same pool as `buf`.

Signed-off-by: Jonathan Rico <[email protected]>

Author: jori-nordic

doc/releases/migration-guide-3.6.rst

@@ -241,6 +241,10 @@ Bluetooth
   Any pointer to a UUID must be prefixed with `const`, otherwise there will be a compilation warning.
   For example change ``struct bt_uuid *uuid = BT_UUID_DECLARE_16(xx)`` to
   ``const struct bt_uuid *uuid = BT_UUID_DECLARE_16(xx)``. (:github:`66136`)
+* The :c:func:`bt_l2cap_chan_send` API no longer allocates buffers from the same pool as its `buf`
+  parameter when segmenting SDUs into PDUs. In order to reproduce the previous behavior, the
+  application should register the `alloc_seg` channel callback and allocate from the same pool as
+  `buf`.
 
 * Mesh
 

include/zephyr/bluetooth/l2cap.h

@@ -587,9 +587,10 @@ int bt_l2cap_chan_disconnect(struct bt_l2cap_chan *chan);
  *  If the application is reserving the bytes it should use the
  *  BT_L2CAP_BUF_SIZE() helper to correctly size the buffers for the for the
  *  outgoing buffer pool.
- *  When segmenting an L2CAP SDU into L2CAP PDUs the stack will first attempt
- *  to allocate buffers from the original buffer pool of the L2CAP SDU before
- *  using the stacks own buffer pool.
+ *  When segmenting an L2CAP SDU into L2CAP PDUs the stack will first attempt to
+ *  allocate buffers from the channel's `alloc_seg` callback and will fallback
+ *  on the stack's global buffer pool (sized
+ *  @kconfig{CONFIG_BT_L2CAP_TX_BUF_COUNT}).
  *
  *  @note Buffer ownership is transferred to the stack in case of success, in
  *  case of an error the caller retains the ownership of the buffer.

subsys/bluetooth/host/l2cap.c

@@ -1818,25 +1818,24 @@ static void le_disconn_rsp(struct bt_l2cap *l2cap, uint8_t ident,
 
 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;
+	struct net_buf *seg = NULL;
 
-	/* Use the dedicated segment callback if registered */
+	/* Use the user-defined allocator */
 	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);
+	}
+
+	/* Fallback to using global connection tx pool */
+	if (!seg) {
+		seg = bt_l2cap_create_pdu_timeout(NULL, 0, K_NO_WAIT);
 	}
 
 	if (seg) {
 		net_buf_reserve(seg, BT_L2CAP_CHAN_SEND_RESERVE);
-		return seg;
 	}
 
-	/* Fallback to using global connection tx pool */
-	return bt_l2cap_create_pdu_timeout(NULL, 0, K_NO_WAIT);
+	return seg;
 }
 
 static struct net_buf *l2cap_chan_create_seg(struct bt_l2cap_le_chan *ch,