Bluetooth: Controller: build assert if comand buffer is too small

Host Number of Completed Packets command does not follow normal flow
control of HCI commands and the Controller side HCI drivers that
allocates HCI command buffers with K_NO_WAIT can end up running out of
command buffers.

Host will generate up to acl_pkts number of Host Number of Completed
Packets command plus a number of normal HCI commands.

Normal HCI commands follow the HCI command flow control using
Num_HCI_Command_Packets return in HCI command complete and status.

When Controller to Host data flow control is supported, this commit
ensures that BT_BUF_CMD_TX_COUNT is greater than or equal to
(BT_BUF_ACL_RX_COUNT + Ncmd), where Ncmd is the supported maximum
Num_HCI_Command_Packets.

Note: The SDC controller (currently) does not support
Num_HCI_Command_Packets > 1, which means Ncmd is always 1.

Signed-off-by: Kyra Lengfeld <[email protected]>

doc/nrf/releases_and_maturity/known_issues.rst

@@ -189,6 +189,16 @@ KRKNWK-14299: NRPA MAC address cannot be set in Zephyr
 Bluetooth LE
 ============
 
+.. rst-class:: v2-9-0-nRF54H20-1 v2-9-0 v2-8-0 v2-7-0 v2-6-2 v2-6-1 v2-6-0
+
+DRGN-24352: Missing disconnection events when HCI Controller to host flow control is enabled
+  :zephyr:code-sample:`bluetooth_hci_ipc` and :ref:`ipc_radio application` do not support stream flow control and drop bytes.
+  With the deferred disconnection complete generation added in the |NCS| v2.6.0, there are cases where the :kconfig:option:`CONFIG_BT_HCI_ACL_FLOW_CONTROL` Kconfig option has value ``y``, and the value of :kconfig:option:`CONFIG_BT_BUF_CMD_TX_COUNT` is smaller than (:kconfig:option:`CONFIG_BT_BUF_ACL_RX_COUNT` + 1).
+  This might result in dropped HCI command packets.
+  The symptom visible is a missing disconnect event, and subsequent failure in connection, if a peripheral device falls out of range or is powered off.
+
+  **Workaround:** Users should ensure in their project that the value of :kconfig:option:`CONFIG_BT_BUF_ACL_RX_COUNT` is smaller thant the one for :kconfig:option:`CONFIG_BT_BUF_CMD_TX_COUNT`.
+
 .. rst-class:: v2-9-0-nRF54H20-rc1 v2-9-0 v2-8-0 v2-7-0 v2-6-2 v2-6-1 v2-6-0
 
 NCSDK-31095: Issues with the :kconfig:option:`CONFIG_SEGGER_SYSVIEW` Kconfig option

subsys/bluetooth/controller/hci_driver.c

@@ -68,6 +68,23 @@ BUILD_ASSERT(!IS_ENABLED(CONFIG_BT_CENTRAL) ||
 BUILD_ASSERT(!IS_ENABLED(CONFIG_BT_PERIPHERAL) ||
 			 (CONFIG_BT_CTLR_SDC_PERIPHERAL_COUNT > 0));
 
+#if defined(CONFIG_BT_HCI_ACL_FLOW_CONTROL)
+  /*
+   * The Host will generate up to acl_pkts number of Host Number of Completed Packets command plus a
+   * number of normal HCI commands, as such we need to ensure the tx command buffer count is big
+   * enough to not block incoming ACKs from the host.
+   *
+   * When Controller to Host data flow control is supported, ensure that BT_BUF_CMD_TX_COUNT is
+   * greater than or equal to (BT_BUF_ACL_RX_COUNT + Ncmd), where Ncmd is the supported maximum
+   * Num_HCI_Command_Packets.
+   *
+   * The SDC controller (currently) does not support Num_HCI_Command_Packets > 1, which means Ncmd
+   * is always 1.
+   */
+BUILD_ASSERT(CONFIG_BT_BUF_ACL_RX_COUNT < CONFIG_BT_BUF_CMD_TX_COUNT,
+	     "Too low HCI command buffers compared to ACL Rx buffers.");
+#endif
+
 #if defined(CONFIG_BT_BROADCASTER)
 	#if defined(CONFIG_BT_CTLR_ADV_EXT)
 		#define SDC_ADV_SET_COUNT CONFIG_BT_CTLR_ADV_SET