refactor: use low priority workqueue for underglow and battery reporting

Blocking operations on the high priority system workqueue may result in
deadlocks, particularly when Bluetooth is in use.

app/src/battery.c

@@ -18,6 +18,7 @@ LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
 #include <zmk/event_manager.h>
 #include <zmk/battery.h>
 #include <zmk/events/battery_state_changed.h>
+#include <zmk/workqueue.h>
 
 static uint8_t last_state_of_charge = 0;
 
@@ -77,7 +78,9 @@ static void zmk_battery_work(struct k_work *work) {
 
 K_WORK_DEFINE(battery_work, zmk_battery_work);
 
-static void zmk_battery_timer(struct k_timer *timer) { k_work_submit(&battery_work); }
+static void zmk_battery_timer(struct k_timer *timer) {
+    k_work_submit_to_queue(zmk_workqueue_lowprio_work_q(), &battery_work);
+}
 
 K_TIMER_DEFINE(battery_timer, zmk_battery_timer, NULL);
 
@@ -97,14 +100,7 @@ static int zmk_battery_init(const struct device *_arg) {
         return -ENODEV;
     }
 
-    int rc = zmk_battery_update(battery);
-
-    if (rc != 0) {
-        LOG_DBG("Failed to update battery value: %d.", rc);
-        return rc;
-    }
-
-    k_timer_start(&battery_timer, K_MINUTES(1), K_SECONDS(CONFIG_ZMK_BATTERY_REPORT_INTERVAL));
+    k_timer_start(&battery_timer, K_NO_WAIT, K_SECONDS(CONFIG_ZMK_BATTERY_REPORT_INTERVAL));
 
     return 0;
 }

app/src/rgb_underglow.c

@@ -24,6 +24,7 @@
 #include <zmk/event_manager.h>
 #include <zmk/events/activity_state_changed.h>
 #include <zmk/events/usb_conn_state_changed.h>
+#include <zmk/workqueue.h>
 
 LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
 
@@ -203,7 +204,7 @@ static void zmk_rgb_underglow_tick_handler(struct k_timer *timer) {
         return;
     }
 
-    k_work_submit(&underglow_work);
+    k_work_submit_to_queue(zmk_workqueue_lowprio_work_q(), &underglow_work);
 }
 
 K_TIMER_DEFINE(underglow_tick, zmk_rgb_underglow_tick_handler, NULL);

app/src/workqueue.c

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2023 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <zephyr/kernel.h>
+#include <zephyr/device.h>
+
+#include <zmk/workqueue.h>
+
+K_THREAD_STACK_DEFINE(lowprio_q_stack, CONFIG_ZMK_LOW_PRIORITY_THREAD_STACK_SIZE);
+
+static struct k_work_q lowprio_work_q;
+
+struct k_work_q *zmk_workqueue_lowprio_work_q() {
+    return &lowprio_work_q;
+}
+
+static int workqueue_init() {
+    static const struct k_work_queue_config queue_config = {.name = "Low Priority Work Queue"};
+    k_work_queue_start(&lowprio_work_q, lowprio_q_stack, K_THREAD_STACK_SIZEOF(lowprio_q_stack),
+                       CONFIG_ZMK_LOW_PRIORITY_THREAD_PRIORITY, &queue_config);
+    return 0;
+}
+
+SYS_INIT(workqueue_init, POST_KERNEL, CONFIG_KERNEL_INIT_PRIORITY_DEFAULT);