HID: wacom: Allow dynamic battery creation/destruction

Tablets like the Intuos, Intuos Pro, and Bamboo have a connector for an
optional wireless module that can be connected on the fly. The presence
(or absence) of this module is indicated in a status report recieved
from the tablet. This patch adds a workqueue function that will create
or destroy a power_supply object at runtime to match the current state
of the WACOM_QUIRK_BATTERY flag.

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Acked-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/wacom.h

@@ -142,4 +142,5 @@ void wacom_wac_usage_mapping(struct hid_device *hdev,
 int wacom_wac_event(struct hid_device *hdev, struct hid_field *field,
 		struct hid_usage *usage, __s32 value);
 void wacom_wac_report(struct hid_device *hdev, struct hid_report *report);
+void wacom_battery_work(struct work_struct *work);
 #endif

drivers/hid/wacom_sys.c

@@ -1057,8 +1057,7 @@ static int wacom_initialize_battery(struct wacom *wacom)
 
 static void wacom_destroy_battery(struct wacom *wacom)
 {
-	if ((wacom->wacom_wac.features.quirks & WACOM_QUIRK_BATTERY) &&
-	     wacom->battery.dev) {
+	if (wacom->battery.dev) {
 		power_supply_unregister(&wacom->battery);
 		wacom->battery.dev = NULL;
 		power_supply_unregister(&wacom->ac);
@@ -1329,6 +1328,20 @@ fail:
 	return;
 }
 
+void wacom_battery_work(struct work_struct *work)
+{
+	struct wacom *wacom = container_of(work, struct wacom, work);
+
+	if ((wacom->wacom_wac.features.quirks & WACOM_QUIRK_BATTERY) &&
+	     !wacom->battery.dev) {
+		wacom_initialize_battery(wacom);
+	}
+	else if (!(wacom->wacom_wac.features.quirks & WACOM_QUIRK_BATTERY) &&
+		 wacom->battery.dev) {
+		wacom_destroy_battery(wacom);
+	}
+}
+
 /*
  * Not all devices report physical dimensions from HID.
  * Compute the default from hardcoded logical dimension