bluetooth: host: Deprecated BT_CONN_TX_MAX

After #72090,
`conn_tx_alloc` no longer blocks, and each buffer always has a
corresponding `bt_conn_tx` object. This eliminates the need to configure
the number of `bt_conn_tx` objects via `CONFIG_BT_CONN_TX_MAX`, since
every buffer now carries its own context even when no callback is used.

This commit deprecates `CONFIG_BT_CONN_TX_MAX` as it is no longer
necessary. Instead, `CONFIG_BT_BUF_ACL_TX_COUNT` is used to allocate
`bt_conn_tx` objects for outgoing ACL data. ZLL already uses
`CONFIG_BT_BUF_ACL_TX_COUNT` to configure the number of outgoing ACL
packets. With this change, modifying the packet count will automatically
adjust the number of corresponding contexts, preventing both context
starvatoin and underutilization.

This approach also aligns with ISO, where the number of `bt_conn_tx`
objects for outgoing ISOdata matches `CONFIG_BT_ISO_TX_BUF_COUNT`.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>

Author: PavelVPV

doc/releases/release-notes-4.2.rst

@@ -103,6 +103,10 @@ Deprecated APIs and options
   deprecated, because support for anonymous authentication had been removed from the
   hawkBit server in version 0.8.0.
 
+* The :kconfig:option:`CONFIG_BT_CONN_TX_MAX` Kconfig option has been deprecated. The number of
+  pending TX buffers is now aligned with the :kconfig:option:`CONFIG_BT_BUF_ACL_TX_COUNT` Kconfig
+  option.
+
 New APIs and options
 ====================
 

include/zephyr/bluetooth/gatt.h

@@ -1383,8 +1383,6 @@ struct bt_gatt_notify_params {
  *  The callback is run from System Workqueue context.
  *  When called from the System Workqueue context this API will not wait for
  *  resources for the callback but instead return an error.
- *  The number of pending callbacks can be increased with the
- *  @kconfig{CONFIG_BT_CONN_TX_MAX} option.
  *
  *  Alternatively it is possible to notify by UUID by setting it on the
  *  parameters, when using this method the attribute if provided is used as the
@@ -2078,8 +2076,6 @@ int bt_gatt_write(struct bt_conn *conn, struct bt_gatt_write_params *params);
  *  The callback is run from System Workqueue context.
  *  When called from the System Workqueue context this API will not wait for
  *  resources for the callback but instead return an error.
- *  The number of pending callbacks can be increased with the
- *  @kconfig{CONFIG_BT_CONN_TX_MAX} option.
  *
  *  @param conn Connection object.
  *  @param handle Attribute handle.

subsys/bluetooth/host/Kconfig

@@ -329,7 +329,7 @@ config BT_CONN_FRAG_COUNT
 if BT_CONN
 
 config BT_CONN_TX_MAX
-	int "Maximum number of pending TX buffers with a callback"
+	int "Maximum number of pending TX buffers with a callback [DEPRECATED]"
 	default BT_BUF_ACL_TX_COUNT
 	range BT_BUF_ACL_TX_COUNT $(UINT8_MAX)
 	help

subsys/bluetooth/host/classic/Kconfig

@@ -140,8 +140,8 @@ config BT_RFCOMM_L2CAP_MTU
 
 config BT_RFCOMM_TX_MAX
 	int "Maximum number of pending TX buffers for RFCOMM"
-	default BT_CONN_TX_MAX
-	range BT_CONN_TX_MAX $(UINT8_MAX)
+	default BT_BUF_ACL_TX_COUNT
+	range BT_BUF_ACL_TX_COUNT $(UINT8_MAX)
 	help
 	  Maximum number of pending TX buffers that have an associated
 	  sending buf. Normally this can be left to the default value, which

subsys/bluetooth/host/conn.c

@@ -120,7 +120,7 @@ sys_slist_t bt_auth_info_cbs = SYS_SLIST_STATIC_INIT(&bt_auth_info_cbs);
 
 static sys_slist_t conn_cbs = SYS_SLIST_STATIC_INIT(&conn_cbs);
 
-static struct bt_conn_tx conn_tx[CONFIG_BT_CONN_TX_MAX];
+static struct bt_conn_tx conn_tx[CONFIG_BT_BUF_ACL_TX_COUNT];
 
 #if defined(CONFIG_BT_CLASSIC)
 static struct bt_conn sco_conns[CONFIG_BT_MAX_SCO_CONN];

tests/bluetooth/host/conn/prj.conf

@@ -12,7 +12,6 @@ CONFIG_BT_PER_ADV_SYNC=y
 
 CONFIG_BT_MAX_CONN=1
 CONFIG_BT_L2CAP_TX_MTU=23
-CONFIG_BT_CONN_TX_MAX=3
 CONFIG_BT_CONN_PARAM_UPDATE_TIMEOUT=5000
 CONFIG_BT_L2CAP_TX_BUF_COUNT=3
 CONFIG_BT_ID_MAX=1

tests/bsim/bluetooth/host/l2cap/stress/prj.conf

@@ -32,7 +32,7 @@ CONFIG_BT_BUF_ACL_TX_COUNT=4
 # L2AP MPS + L2CAP header (4)
 CONFIG_BT_BUF_ACL_RX_SIZE=81
 
-# Governs BT_CONN_TX_MAX, and so must be >= than the max number of
+# Governs CONFIG_BT_BUF_ACL_TX_COUNT, 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.

tests/bsim/bluetooth/host/misc/acl_tx_frag/prj.conf

@@ -26,10 +26,6 @@ CONFIG_BT_GAP_AUTO_UPDATE_CONN_PARAMS=n
 
 CONFIG_BT_MAX_CONN=1
 
-# We don't want to be constrained on the number of TX
-# contexts, rather on the number of LL TX buffers.
-CONFIG_BT_CONN_TX_MAX=10
-
 # Outgoing ATT buffers
 CONFIG_BT_ATT_TX_COUNT=1