battery: Add the battery hooking API

This is a patch that implements a generic hooking API for the
generic ACPI battery driver.

With this new generic API, drivers can expose platform specific
behaviour via sysfs attributes in /sys/class/power_supply/BATn/
in a generic way.

A perfect example of the need for this API are Lenovo ThinkPads.

Lenovo ThinkPads have a ACPI extension that allows the setting of
start and stop charge thresholds in the EC and battery firmware
via ACPI. The thinkpad_acpi module can use this API to expose
sysfs attributes that it controls inside the ACPI battery driver
sysfs tree, under /sys/class/power_supply/BATN/.

The file drivers/acpi/battery.h has been moved to
include/acpi/battery.h and the includes inside ac.c, sbs.c, and
battery.c have been adjusted to reflect that.

When drivers hooks into the API, the API calls add_battery() for
each battery in the system that passes it a acpi_battery
struct. Then, the drivers can use device_create_file() to create
new sysfs attributes with that struct and identify the batteries
for per-battery attributes.

Signed-off-by: Ognjen Galic <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

drivers/acpi/ac.c

···
 #include <linux/platform_device.h>
 #include <linux/power_supply.h>
 #include <linux/acpi.h>
-#include "battery.h"
+#include <acpi/battery.h>
 
 #define PREFIX "ACPI: "
 

drivers/acpi/battery.c

···
  * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  */
 
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
 #include <linux/kernel.h>
+#include <linux/list.h>
 #include <linux/module.h>
+#include <linux/mutex.h>
 #include <linux/init.h>
 #include <linux/types.h>
 #include <linux/jiffies.h>
···
 #include <linux/acpi.h>
 #include <linux/power_supply.h>
 
-#include "battery.h"
+#include <acpi/battery.h>
 
 #define PREFIX "ACPI: "
 
···
 	struct power_supply_desc bat_desc;
 	struct acpi_device *device;
 	struct notifier_block pm_nb;
+	struct list_head list;
 	unsigned long update_time;
 	int revision;
 	int rate_now;
···
 	.store = acpi_battery_alarm_store,
 };
 
+/*
+ * The Battery Hooking API
+ *
+ * This API is used inside other drivers that need to expose
+ * platform-specific behaviour within the generic driver in a
+ * generic way.
+ *
+ */
+
+static LIST_HEAD(acpi_battery_list);
+static LIST_HEAD(battery_hook_list);
+static DEFINE_MUTEX(hook_mutex);
+
+void __battery_hook_unregister(struct acpi_battery_hook *hook, int lock)
+{
+	struct acpi_battery *battery;
+	/*
+	 * In order to remove a hook, we first need to
+	 * de-register all the batteries that are registered.
+	 */
+	if (lock)
+		mutex_lock(&hook_mutex);
+	list_for_each_entry(battery, &acpi_battery_list, list) {
+		hook->remove_battery(battery->bat);
+	}
+	list_del(&hook->list);
+	if (lock)
+		mutex_unlock(&hook_mutex);
+	pr_info("extension unregistered: %s\n", hook->name);
+}
+
+void battery_hook_unregister(struct acpi_battery_hook *hook)
+{
+	__battery_hook_unregister(hook, 1);
+}
+EXPORT_SYMBOL_GPL(battery_hook_unregister);
+
+void battery_hook_register(struct acpi_battery_hook *hook)
+{
+	struct acpi_battery *battery;
+
+	mutex_lock(&hook_mutex);
+	INIT_LIST_HEAD(&hook->list);
+	list_add(&hook->list, &battery_hook_list);
+	/*
+	 * Now that the driver is registered, we need
+	 * to notify the hook that a battery is available
+	 * for each battery, so that the driver may add
+	 * its attributes.
+	 */
+	list_for_each_entry(battery, &acpi_battery_list, list) {
+		if (hook->add_battery(battery->bat)) {
+			/*
+			 * If a add-battery returns non-zero,
+			 * the registration of the extension has failed,
+			 * and we will not add it to the list of loaded
+			 * hooks.
+			 */
+			pr_err("extension failed to load: %s", hook->name);
+			__battery_hook_unregister(hook, 0);
+			return;
+		}
+	}
+	pr_info("new extension: %s\n", hook->name);
+	mutex_unlock(&hook_mutex);
+}
+EXPORT_SYMBOL_GPL(battery_hook_register);
+
+/*
+ * This function gets called right after the battery sysfs
+ * attributes have been added, so that the drivers that
+ * define custom sysfs attributes can add their own.
+*/
+static void battery_hook_add_battery(struct acpi_battery *battery)
+{
+	struct acpi_battery_hook *hook_node;
+
+	mutex_lock(&hook_mutex);
+	INIT_LIST_HEAD(&battery->list);
+	list_add(&battery->list, &acpi_battery_list);
+	/*
+	 * Since we added a new battery to the list, we need to
+	 * iterate over the hooks and call add_battery for each
+	 * hook that was registered. This usually happens
+	 * when a battery gets hotplugged or initialized
+	 * during the battery module initialization.
+	 */
+	list_for_each_entry(hook_node, &battery_hook_list, list) {
+		if (hook_node->add_battery(battery->bat)) {
+			/*
+			 * The notification of the extensions has failed, to
+			 * prevent further errors we will unload the extension.
+			 */
+			__battery_hook_unregister(hook_node, 0);
+			pr_err("error in extension, unloading: %s",
+					hook_node->name);
+		}
+	}
+	mutex_unlock(&hook_mutex);
+}
+
+static void battery_hook_remove_battery(struct acpi_battery *battery)
+{
+	struct acpi_battery_hook *hook;
+
+	mutex_lock(&hook_mutex);
+	/*
+	 * Before removing the hook, we need to remove all
+	 * custom attributes from the battery.
+	 */
+	list_for_each_entry(hook, &battery_hook_list, list) {
+		hook->remove_battery(battery->bat);
+	}
+	/* Then, just remove the battery from the list */
+	list_del(&battery->list);
+	mutex_unlock(&hook_mutex);
+}
+
+static void __exit battery_hook_exit(void)
+{
+	struct acpi_battery_hook *hook;
+	struct acpi_battery_hook *ptr;
+	/*
+	 * At this point, the acpi_bus_unregister_driver()
+	 * has called remove for all batteries. We just
+	 * need to remove the hooks.
+	 */
+	list_for_each_entry_safe(hook, ptr, &battery_hook_list, list) {
+		__battery_hook_unregister(hook, 1);
+	}
+	mutex_destroy(&hook_mutex);
+}
+
 static int sysfs_add_battery(struct acpi_battery *battery)
 {
 	struct power_supply_config psy_cfg = { .drv_data = battery, };
···
 		battery->bat = NULL;
 		return result;
 	}
+	battery_hook_add_battery(battery);
 	return device_create_file(&battery->bat->dev, &alarm_attr);
 }
 
···
 		mutex_unlock(&battery->sysfs_lock);
 		return;
 	}
-
+	battery_hook_remove_battery(battery);
 	device_remove_file(&battery->bat->dev, &alarm_attr);
 	power_supply_unregister(battery->bat);
 	battery->bat = NULL;
···
 static void __exit acpi_battery_exit(void)
 {
 	async_synchronize_cookie(async_cookie + 1);
-	if (battery_driver_registered)
+	if (battery_driver_registered) {
 		acpi_bus_unregister_driver(&acpi_battery_driver);
+		battery_hook_exit();
+	}
 #ifdef CONFIG_ACPI_PROCFS_POWER
 	if (acpi_battery_dir)
 		acpi_unlock_battery_dir(acpi_battery_dir);

drivers/acpi/battery.h

···
-/* SPDX-License-Identifier: GPL-2.0 */
-#ifndef __ACPI_BATTERY_H
-#define __ACPI_BATTERY_H
-
-#define ACPI_BATTERY_CLASS "battery"
-
-#define ACPI_BATTERY_NOTIFY_STATUS	0x80
-#define ACPI_BATTERY_NOTIFY_INFO	0x81
-#define ACPI_BATTERY_NOTIFY_THRESHOLD   0x82
-
-#endif

drivers/acpi/sbs.c

···
 #include <linux/delay.h>
 #include <linux/power_supply.h>
 #include <linux/platform_data/x86/apple.h>
+#include <acpi/battery.h>
 
 #include "sbshc.h"
-#include "battery.h"
 
 #define PREFIX "ACPI: "
 

include/acpi/battery.h

···
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ACPI_BATTERY_H
+#define __ACPI_BATTERY_H
+
+#define ACPI_BATTERY_CLASS "battery"
+
+#define ACPI_BATTERY_NOTIFY_STATUS	0x80
+#define ACPI_BATTERY_NOTIFY_INFO	0x81
+#define ACPI_BATTERY_NOTIFY_THRESHOLD   0x82
+
+struct acpi_battery_hook {
+	const char *name;
+	int (*add_battery)(struct power_supply *battery);
+	int (*remove_battery)(struct power_supply *battery);
+	struct list_head list;
+};
+
+void battery_hook_register(struct acpi_battery_hook *hook);
+void battery_hook_unregister(struct acpi_battery_hook *hook);
+
+#endif