Bluetooth: host: Add lower bound for CONFIG_BT_BUF_ACL_RX_COUNT

See comment above assert for more.

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

doc/releases/migration-guide-3.7.rst

@@ -806,6 +806,12 @@ Bluetooth Host
 
   (:github:`75065`)
 
+* :kconfig:option:`CONFIG_BT_BUF_ACL_RX_COUNT` now needs to be larger than
+  :kconfig:option:`CONFIG_BT_MAX_CONN`. This was always the case due to the design of the HCI
+  interface. It is now being enforced through a build-time assertion.
+
+  (:github:`75592`)
+
 Bluetooth Crypto
 ================
 

samples/bluetooth/central_multilink/prj.conf

@@ -4,6 +4,7 @@ CONFIG_BT_AUTO_PHY_UPDATE=n
 CONFIG_BT_PRIVACY=y
 
 CONFIG_BT_MAX_CONN=62
+CONFIG_BT_BUF_ACL_RX_COUNT=63
 
 # CONFIG_BT_GATT_CLIENT=y
 

samples/bluetooth/hci_uart_async/prj.conf

@@ -10,6 +10,7 @@ CONFIG_BT_HCI_RAW_H4_ENABLE=y
 
 # Controller configuration. Modify these for your application's needs.
 CONFIG_BT_MAX_CONN=16
+CONFIG_BT_BUF_ACL_RX_COUNT=17
 CONFIG_BT_BUF_ACL_RX_SIZE=255
 CONFIG_BT_BUF_CMD_TX_SIZE=255
 CONFIG_BT_BUF_EVT_DISCARDABLE_SIZE=255

samples/bluetooth/peripheral_identity/prj.conf

@@ -8,6 +8,7 @@ CONFIG_BT_GAP_AUTO_UPDATE_CONN_PARAMS=n
 
 CONFIG_BT_MAX_CONN=62
 CONFIG_BT_ID_MAX=62
+CONFIG_BT_BUF_ACL_RX_COUNT=63
 
 # CONFIG_BT_SMP=y
 # CONFIG_BT_MAX_PAIRED=62

subsys/bluetooth/common/Kconfig

@@ -80,7 +80,7 @@ config BT_BUF_ACL_RX_SIZE
 config BT_BUF_ACL_RX_COUNT
 	int "Number of incoming ACL data buffers"
 	default 6
-	range 1 64
+	range 2 256
 	help
 	  Number or incoming ACL data buffers sent from the Controller to the
 	  Host.
@@ -94,6 +94,10 @@ config BT_BUF_ACL_RX_COUNT
 	  When Controller to Host flow control is not enabled the Controller
 	  can assume that the Host has infinite amount of buffers.
 
+	  For both configurations, there is an additional requirement that is
+	  enforced by a build-time check: BT_BUF_ACL_RX_COUNT needs to be at
+	  least one greater than BT_MAX_CONN.
+
 config BT_BUF_EVT_RX_SIZE
 	int "Maximum supported HCI Event buffer length"
 	default 255 if (BT_EXT_ADV && BT_OBSERVER) || BT_PER_ADV_SYNC || BT_DF_CONNECTION_CTE_RX || BT_CLASSIC

subsys/bluetooth/common/dummy.c

@@ -13,6 +13,7 @@
 
 #include <zephyr/kernel.h>
 #include <zephyr/bluetooth/bluetooth.h>
+#include <zephyr/bluetooth/buf.h>
 
 /*
  * The unpacked structs below are used inside __packed structures that reflect
@@ -59,3 +60,27 @@ BUILD_ASSERT(!IS_ENABLED(CONFIG_LOG_MODE_IMMEDIATE), "Immediate logging "
 	     "on selected backend(s) not "
 	     "supported with the software Link Layer");
 #endif
+
+#if defined(CONFIG_BT_CONN) && defined(CONFIG_BT_HCI_HOST)
+/* The host needs more ACL buffers than maximum ACL links. This is because of
+ * the way we re-assemble ACL packets into L2CAP PDUs.
+ *
+ * We keep around the first buffer (that comes from the driver) to do
+ * re-assembly into, and if all links are re-assembling, there will be no buffer
+ * available for the HCI driver to allocate from.
+ *
+ * Fixing it properly involves a re-design of the HCI driver interface.
+ */
+#if defined(CONFIG_BT_HCI_ACL_FLOW_CONTROL)
+BUILD_ASSERT(CONFIG_BT_BUF_ACL_RX_COUNT > CONFIG_BT_MAX_CONN);
+
+#else /* !CONFIG_BT_HCI_ACL_FLOW_CONTROL */
+
+/* BT_BUF_RX_COUNT is defined in include/zephyr/bluetooth/buf.h */
+BUILD_ASSERT(BT_BUF_RX_COUNT > CONFIG_BT_MAX_CONN,
+	     "BT_BUF_RX_COUNT needs to be greater than CONFIG_BT_MAX_CONN. "
+	     "In order to do that, increase CONFIG_BT_BUF_ACL_RX_COUNT.");
+
+#endif /* CONFIG_BT_HCI_ACL_FLOW_CONTROL */
+
+#endif /* CONFIG_BT_CONN */

tests/bluetooth/tester/boards/frdm_rw612.conf

@@ -1,4 +1,5 @@
 CONFIG_BT_MAX_CONN=16
+CONFIG_BT_BUF_ACL_RX_COUNT=17
 
 # debug options
 # CONFIG_UART_CONSOLE=y

tests/bluetooth/tester/boards/rd_rw612_bga.conf

@@ -1,4 +1,5 @@
 CONFIG_BT_MAX_CONN=16
+CONFIG_BT_BUF_ACL_RX_COUNT=17
 
 # debug options
 # CONFIG_UART_CONSOLE=y

tests/bsim/bluetooth/host/l2cap/split/tester/prj.conf

@@ -3,7 +3,7 @@ CONFIG_ASSERT=y
 
 CONFIG_BT=y
 CONFIG_BT_HCI_RAW=y
-CONFIG_BT_MAX_CONN=16
+CONFIG_BT_MAX_CONN=1
 
 CONFIG_BT_BUF_CMD_TX_COUNT=10
 

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

@@ -42,6 +42,7 @@ CONFIG_BT_CTLR_DATA_LENGTH_MAX=27
 CONFIG_BT_CTLR_RX_BUFFERS=10
 
 CONFIG_BT_MAX_CONN=10
+CONFIG_BT_BUF_ACL_RX_COUNT=11
 
 CONFIG_LOG=y
 CONFIG_ASSERT=y

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

@@ -34,6 +34,7 @@ CONFIG_BT_CTLR_DATA_LENGTH_MAX=81
 CONFIG_BT_CTLR_RX_BUFFERS=10
 
 CONFIG_BT_MAX_CONN=10
+CONFIG_BT_BUF_ACL_RX_COUNT=11
 
 CONFIG_LOG=y
 CONFIG_ASSERT=y

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

@@ -42,6 +42,7 @@ CONFIG_BT_CTLR_DATA_LENGTH_MAX=27
 CONFIG_BT_CTLR_RX_BUFFERS=10
 
 CONFIG_BT_MAX_CONN=10
+CONFIG_BT_BUF_ACL_RX_COUNT=11
 
 CONFIG_LOG=y
 CONFIG_ASSERT=y

tests/bsim/bluetooth/host/misc/conn_stress/central/prj.conf

@@ -2,6 +2,7 @@ CONFIG_BT=y
 CONFIG_LOG=y
 CONFIG_BT_CENTRAL=y
 CONFIG_BT_MAX_CONN=12
+CONFIG_BT_BUF_ACL_RX_COUNT=13
 CONFIG_BT_MAX_PAIRED=12
 CONFIG_BT_SMP=y
 CONFIG_BT_PRIVACY=y

tests/bsim/bluetooth/host/misc/hfc_multilink/dut/prj.conf

@@ -26,11 +26,10 @@ CONFIG_BT_AUTO_PHY_UPDATE=n
 CONFIG_BT_AUTO_DATA_LEN_UPDATE=n
 CONFIG_BT_GAP_AUTO_UPDATE_CONN_PARAMS=n
 
-# As many host buffers as connection slots
-# The whole test hinges on bufs <= links, which is a valid
-# configuration at the time of writing this test.
 CONFIG_BT_MAX_CONN=3
-CONFIG_BT_BUF_ACL_RX_COUNT=3
+CONFIG_BT_BUF_ACL_RX_COUNT=4
-# It passes with 4
+
-# CONFIG_BT_BUF_ACL_RX_COUNT=4
+# This test will fail when CONFIG_BT_MAX_CONN == CONFIG_BT_BUF_ACL_RX_COUNT
+# CONFIG_BT_BUF_ACL_RX_COUNT=3
+
 CONFIG_BT_HCI_ACL_FLOW_CONTROL=y

tests/bsim/bluetooth/host/misc/hfc_multilink/dut/src/dut.c

@@ -23,8 +23,8 @@ LOG_MODULE_REGISTER(dut, CONFIG_APP_LOG_LEVEL);
 
 #define NUM_TESTERS CONFIG_BT_MAX_CONN
 
-/* This test will fail when CONFIG_BT_MAX_CONN == CONFIG_BT_BUF_ACL_RX_COUNT */
+/* Build with the minimum possible amount of RX buffers */
-BUILD_ASSERT(CONFIG_BT_BUF_ACL_RX_COUNT == CONFIG_BT_MAX_CONN);
+BUILD_ASSERT(CONFIG_BT_BUF_ACL_RX_COUNT == (CONFIG_BT_MAX_CONN + 1));
 
 struct tester {
 	size_t sdu_count;
@@ -151,7 +151,7 @@ void entrypoint_dut(void)
 	 * Test purpose:
 	 *
 	 * Verifies that we are able to do L2CAP recombination on multiple links
-	 * when we only have as many buffers as links.
+	 * when we have the smallest possible amount of ACL buffers.
 	 *
 	 * Devices:
 	 * - `dut`: receives L2CAP PDUs from testers

tests/bsim/bluetooth/ll/multiple_id/prj.conf

@@ -12,6 +12,7 @@ CONFIG_BT_AUTO_DATA_LEN_UPDATE=y
 
 CONFIG_BT_MAX_CONN=250
 CONFIG_BT_ID_MAX=250
+CONFIG_BT_BUF_ACL_RX_COUNT=251
 
 # L2CAP, ATT and SMP usage cause data transmission deadlock due to shortage
 # of buffers when transactions crossover amongst the connections in the same