Bluetooth: Host: Add bt_taskq workqueue for quick non-blocking tasks

Add a new workqueue bt_taskq specifically designed for quick
non-blocking work items in the Bluetooth subsystem. This workqueue is
always available and does not depend on any Kconfig option.

Signed-off-by: Aleksander Wasaznik <[email protected]>

Author: alwa-nordic

subsys/bluetooth/host/CMakeLists.txt

@@ -11,6 +11,7 @@ zephyr_library_sources_ifdef(CONFIG_BT_MONITOR          monitor.c)
 zephyr_library_sources_ifdef(CONFIG_BT_SETTINGS         settings.c)
 zephyr_library_sources_ifdef(CONFIG_BT_HOST_CCM         aes_ccm.c)
 zephyr_library_sources_ifdef(CONFIG_BT_LONG_WQ          long_wq.c)
+zephyr_library_sources(bt_taskq.c)
 
 if(CONFIG_BT_HCI_HOST)
   zephyr_library_sources(

subsys/bluetooth/host/Kconfig

@@ -35,6 +35,20 @@ config BT_LONG_WQ_INIT_PRIO
 
 endif # BT_LONG_WQ
 
+config BT_TASKQ_STACK_SIZE_WITH_PROMPT
+	bool "Override BT taskq thread stack size"
+
+config BT_TASKQ_STACK_SIZE
+	int
+	prompt "BT taskq thread stack size" if BT_HCI_TX_STACK_SIZE_WITH_PROMPT
+	default 1024
+
+config BT_TASKQ_THREAD_PRIO
+	int
+	# -1 is the least urgent cooperative priority.
+	# tx_processor() needs a cooperative thread for now.
+	default -1
+
 config BT_HCI_HOST
 	# Hidden option to make the conditions more intuitive
 	bool

subsys/bluetooth/host/bt_taskq.c

@@ -0,0 +1,34 @@
+/* bt_taskq.c - Workqueue for quick non-blocking Bluetooth tasks */
+
+/*
+ * Copyright (c) 2025 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/autoconf.h>
+#include <zephyr/kernel.h>
+#include <zephyr/init.h>
+#include <zephyr/kernel/thread_stack.h>
+
+static K_THREAD_STACK_DEFINE(bt_taskq_stack, CONFIG_BT_TASKQ_STACK_SIZE);
+struct k_work_q bt_taskq;
+
+static int bt_taskq_init(void)
+{
+	const struct k_work_queue_config cfg = {
+		.name = "bt_taskq",
+		/* Enable CONFIG_WORKQUEUE_WORK_TIMEOUT to detect tasks that take too long. */
+		.work_timeout_ms = 10,
+	};
+
+	k_work_queue_start(&bt_taskq, bt_taskq_stack, K_THREAD_STACK_SIZEOF(bt_taskq_stack),
+			   CONFIG_BT_TASKQ_THREAD_PRIO, &cfg);
+
+	return 0;
+}
+
+/* The init priority is set to POST_KERNEL 999, the last level
+ * before APPLICATION.
+ */
+SYS_INIT(bt_taskq_init, POST_KERNEL, 999);

subsys/bluetooth/host/bt_taskq.h

@@ -0,0 +1,40 @@
+/* bt_taskq.h - Workqueue for quick non-blocking Bluetooth tasks */
+
+/*
+ * Copyright (c) 2025 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/kernel.h>
+
+/**
+ * @brief Bluetooth task workqueue
+ *
+ * bt_taskq is a workqueue intended for quick non-blocking work
+ * items ("tasks") in the Bluetooth subsystem. This workqueue
+ * must always exist and is not controlled by any Kconfig
+ * option.
+ *
+ * Blocking means "waiting for something while running". A
+ * task is NOT allowed to block. If a task need to "wait", it
+ * should instead return immediately and schedule itself to run
+ * later.
+ *
+ * Work items submitted to this queue should be:
+ * - Quick to execute (non-blocking).
+ * - Not perform long-running operations.
+ * - Not block.
+ *
+ * @warning Non-blocking violation pitfalls:
+ * - net_buf_unref() on a foreign buffer could have a blocking
+ *   destroy callback
+ * - Any user-defined callback might be blocking
+ * - Avoid any operations that could sleep or block the thread
+ *
+ * Use bt_long_wq for long-running or potentially blocking
+ * operations instead.
+ *
+ * Available in APPLICATION initialization level and later.
+ */
+extern struct k_work_q bt_taskq;

subsys/bluetooth/host/hci_core.c

@@ -282,6 +282,8 @@ void bt_send_one_host_num_completed_packets(uint16_t handle)
 	BT_ASSERT_MSG(err == 0, "Unable to send Host NCP (err %d)", err);
 }
 
+#include "bt_taskq.h"
+
 #if defined(CONFIG_BT_TESTING)
 __weak void bt_testing_trace_event_acl_pool_destroy(struct net_buf *buf)
 {