fix(splits): Send pos notify from dedicated thread

* Avoid deadlocks by using a deadicated workqueue for sending
  position state notifications from peripherals.

app/Kconfig

@@ -102,7 +102,7 @@ config ZMK_SPLIT
 
 if ZMK_SPLIT
 
-config ZMK_SPLIT_BLE
+menuconfig ZMK_SPLIT_BLE
 	bool "Split keyboard support via BLE transport"
 	depends on ZMK_BLE
 	default y
@@ -125,6 +125,18 @@ endif
 
 if !ZMK_SPLIT_BLE_ROLE_CENTRAL
 
+config ZMK_SPLIT_BLE_PERIPHERAL_STACK_SIZE
+	int "BLE split peripheral notify thread stack size"
+	default 512
+
+config ZMK_SPLIT_BLE_PERIPHERAL_PRIORITY
+	int "BLE split peripheral notify thread priority"
+	default 5
+
+config ZMK_SPLIT_BLE_PERIPHERAL_POSITION_QUEUE_SIZE
+	int "Max number of key position state events to queue to send to the central"
+	default 10
+
 config ZMK_USB
 	default n
 

app/src/split/bluetooth/service.c

@@ -6,6 +6,7 @@
 
 #include <zephyr/types.h>
 #include <sys/util.h>
+#include <init.h>
 
 #include <logging/log.h>
 
@@ -18,8 +19,10 @@ LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
 #include <zmk/split/bluetooth/uuid.h>
 #include <zmk/split/bluetooth/service.h>
 
+#define POS_STATE_LEN 16
+
 static uint8_t num_of_positions = ZMK_KEYMAP_LEN;
-static uint8_t position_state[16];
+static uint8_t position_state[POS_STATE_LEN];
 
 static ssize_t split_svc_pos_state(struct bt_conn *conn, const struct bt_gatt_attr *attrs,
                                    void *buf, uint16_t len, uint16_t offset) {
@@ -45,12 +48,62 @@ BT_GATT_SERVICE_DEFINE(
     BT_GATT_DESCRIPTOR(BT_UUID_NUM_OF_DIGITALS, BT_GATT_PERM_READ, split_svc_num_of_positions, NULL,
                        &num_of_positions), );
 
+K_THREAD_STACK_DEFINE(service_q_stack, CONFIG_ZMK_SPLIT_BLE_PERIPHERAL_STACK_SIZE);
+
+struct k_work_q service_work_q;
+
+K_MSGQ_DEFINE(position_state_msgq, sizeof(char[POS_STATE_LEN]),
+              CONFIG_ZMK_SPLIT_BLE_PERIPHERAL_POSITION_QUEUE_SIZE, 4);
+
+void send_position_state_callback(struct k_work *work) {
+    uint8_t state[POS_STATE_LEN];
+
+    while (k_msgq_get(&position_state_msgq, &state, K_NO_WAIT) == 0) {
+        int err = bt_gatt_notify(NULL, &split_svc.attrs[1], &state, sizeof(state));
+        if (err) {
+            LOG_DBG("Error notifying %d", err);
+        }
+    }
+};
+
+K_WORK_DEFINE(service_position_notify_work, send_position_state_callback);
+
+int send_position_state() {
+    int err = k_msgq_put(&position_state_msgq, position_state, K_MSEC(100));
+    if (err) {
+        switch (err) {
+        case -EAGAIN: {
+            LOG_WRN("Position state message queue full, popping first message and queueing again");
+            uint8_t discarded_state[POS_STATE_LEN];
+            k_msgq_get(&position_state_msgq, &discarded_state, K_NO_WAIT);
+            return send_position_state();
+        }
+        default:
+            LOG_WRN("Failed to queue position state to send (%d)", err);
+            return err;
+        }
+    }
+
+    k_work_submit_to_queue(&service_work_q, &service_position_notify_work);
+
+    return 0;
+}
+
 int zmk_split_bt_position_pressed(uint8_t position) {
     WRITE_BIT(position_state[position / 8], position % 8, true);
-    return bt_gatt_notify(NULL, &split_svc.attrs[1], &position_state, sizeof(position_state));
+    return send_position_state();
 }
 
 int zmk_split_bt_position_released(uint8_t position) {
     WRITE_BIT(position_state[position / 8], position % 8, false);
-    return bt_gatt_notify(NULL, &split_svc.attrs[1], &position_state, sizeof(position_state));
+    return send_position_state();
 }
+
+int service_init(const struct device *_arg) {
+    k_work_q_start(&service_work_q, service_q_stack, K_THREAD_STACK_SIZEOF(service_q_stack),
+                   CONFIG_ZMK_SPLIT_BLE_PERIPHERAL_PRIORITY);
+
+    return 0;
+}
+
+SYS_INIT(service_init, APPLICATION, CONFIG_ZMK_BLE_INIT_PRIORITY);