Bluetooth: Enforce cooperative priorities in Bluetooth threads

The Bluetooth subsystem assumes execution of its system threads in
cooperative priority, including the system workqueue and the thread
that interact with the controller (i.e. calling bt_send). This commit
adds a compile-time check for the system workqueue priority and
documentation for the bt_send API call.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

include/drivers/bluetooth/hci_driver.h

@@ -134,6 +134,8 @@ struct bt_hci_driver {
 	 * Send an HCI command or ACL data to the controller. The exact
 	 * type of the data can be checked with the help of bt_buf_get_type().
 	 *
+	 * @note This function must only be called from a cooperative thread.
+	 *
 	 * @param buf Buffer containing data to be sent to the controller.
 	 *
 	 * @return 0 on success or negative error number on failure.

subsys/bluetooth/common/Makefile

@@ -1,4 +1,5 @@
 ccflags-y += -I$(srctree)/subsys/bluetooth
 
+obj-y += dummy.o
 obj-$(CONFIG_BLUETOOTH_DEBUG) += log.o
 obj-$(CONFIG_BLUETOOTH_RPA) += rpa.o

subsys/bluetooth/common/dummy.c

@@ -0,0 +1,19 @@
+/**
+ * @file dummy.c
+ * Static compilation checks.
+ */
+
+/*
+ * Copyright (c) 2017 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr.h>
+
+/* The Bluetooth subsystem requires the system workqueue to execute at a
+ * cooperative priority to function correctly. If this build assert triggers
+ * verify your configuration to ensure that cooperative threads are enabled
+ * and that the system workqueue priority is negative (cooperative).
+ */
+BUILD_ASSERT(CONFIG_SYSTEM_WORKQUEUE_PRIORITY < 0);