feat(ble): Support perhipheral battery levels.

* Add ability to fetch and report peripheral battery levels
  on split centrals.
* Add additional support for adding a new Battery Level
  service to split centrals that exposes fetched peripheral
  battery levels to connected hosts.

Co-authored-by: Peter Johanson <peter@peterjohanson.com>

app/src/split/bluetooth/central_bas_proxy.c

@@ -0,0 +1,98 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <zephyr/device.h>
+#include <zephyr/init.h>
+#include <sys/types.h>
+#include <zephyr/kernel.h>
+#include <zephyr/drivers/sensor.h>
+#include <zephyr/bluetooth/gatt.h>
+
+#include <zephyr/logging/log.h>
+
+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/split/bluetooth/central.h>
+
+static void blvl_ccc_cfg_changed(const struct bt_gatt_attr *attr, uint16_t value) {
+    ARG_UNUSED(attr);
+
+    bool notif_enabled = (value == BT_GATT_CCC_NOTIFY);
+
+    LOG_INF("BAS Notifications %s", notif_enabled ? "enabled" : "disabled");
+}
+
+static ssize_t read_blvl(struct bt_conn *conn, const struct bt_gatt_attr *attr, void *buf,
+                         uint16_t len, uint16_t offset) {
+    const uint8_t source = (uint8_t)(uint32_t)attr->user_data;
+    uint8_t level = 0;
+    int rc = zmk_split_get_peripheral_battery_level(source, &level);
+
+    if (rc == -EINVAL) {
+        LOG_ERR("Invalid peripheral index requested for battery level read: %d", source);
+        return 0;
+    }
+
+    return bt_gatt_attr_read(conn, attr, buf, len, offset, &level, sizeof(uint8_t));
+}
+
+static const struct bt_gatt_cpf aux_level_cpf = {
+    .format = 0x04, // uint8
+    .exponent = 0x0,
+    .unit = 0x27AD,        // Percentage
+    .name_space = 0x01,    // Bluetooth SIG
+    .description = 0x0108, // "auxiliary"
+};
+
+#define PERIPH_CUD_(x) "Peripheral " #x
+#define PERIPH_CUD(x) PERIPH_CUD_(x)
+
+// How many GATT attributes each battery level adds to our service
+#define PERIPH_BATT_LEVEL_ATTR_COUNT 5
+// The second generated attribute is the one used to send GATT notifications
+#define PERIPH_BATT_LEVEL_ATTR_NOTIFY_IDX 1
+
+#define PERIPH_BATT_LEVEL_ATTRS(i, _)                                                              \
+    BT_GATT_CHARACTERISTIC(BT_UUID_BAS_BATTERY_LEVEL, BT_GATT_CHRC_READ | BT_GATT_CHRC_NOTIFY,     \
+                           BT_GATT_PERM_READ, read_blvl, NULL, i),                                 \
+        BT_GATT_CCC(blvl_ccc_cfg_changed, BT_GATT_PERM_READ | BT_GATT_PERM_WRITE),                 \
+        BT_GATT_CPF(&aux_level_cpf), BT_GATT_CUD(PERIPH_CUD(i), BT_GATT_PERM_READ),
+
+BT_GATT_SERVICE_DEFINE(bas_aux, BT_GATT_PRIMARY_SERVICE(BT_UUID_BAS),
+                       LISTIFY(CONFIG_ZMK_SPLIT_BLE_CENTRAL_PERIPHERALS, PERIPH_BATT_LEVEL_ATTRS,
+                               ()));
+
+int peripheral_batt_lvl_listener(const zmk_event_t *eh) {
+    const struct zmk_peripheral_battery_state_changed *ev =
+        as_zmk_peripheral_battery_state_changed(eh);
+    if (ev == NULL) {
+        return ZMK_EV_EVENT_BUBBLE;
+    };
+
+    if (ev->source >= CONFIG_ZMK_SPLIT_BLE_CENTRAL_PERIPHERALS) {
+        LOG_WRN("Got battery level event for an out of range peripheral index");
+        return ZMK_EV_EVENT_BUBBLE;
+    }
+
+    LOG_DBG("Peripheral battery level event: %u", ev->state_of_charge);
+
+    // Offset by the index of the source plus the specific offset to find the attribute to notify
+    // on.
+    int index = (PERIPH_BATT_LEVEL_ATTR_COUNT * ev->source) + PERIPH_BATT_LEVEL_ATTR_NOTIFY_IDX;
+
+    int rc = bt_gatt_notify(NULL, &bas_aux.attrs[index], &ev->state_of_charge, sizeof(uint8_t));
+    if (rc < 0 && rc != -ENOTCONN) {
+        LOG_WRN("Failed to notify hosts of peripheral battery level: %d", rc);
+    }
+
+    return ZMK_EV_EVENT_BUBBLE;
+};
+
+ZMK_LISTENER(peripheral_batt_lvl_listener, peripheral_batt_lvl_listener);
+ZMK_SUBSCRIPTION(peripheral_batt_lvl_listener, zmk_peripheral_battery_state_changed);