power: reset: reboot-mode: Expose sysfs for registered reboot_modes

Currently, there is no standardized mechanism for userspace to discover
supported reboot modes on a platform. This limits userspace scripts, to
rely on hardcoded assumptions about the available reboot-modes.

Create a class 'reboot-mode' and a device under it. Use the name of the
registering driver as device name. Expose a sysfs interface under this
device to show available reboot mode arguments.

 This results in the creation of:
   /sys/class/reboot-mode/<driver>/reboot_modes

This read-only sysfs file will exposes the supported reboot mode
arguments provided by the registering driver, enabling userspace to
query the list of arguments.

Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Signed-off-by: Shivendra Pratap <shivendra.pratap@oss.qualcomm.com>
Link: https://patch.msgid.link/20260224-next-15nov_expose_sysfs-v24-2-4ee5b49d5a06@oss.qualcomm.com
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

drivers/power/reset/reboot-mode.c

@@ -4,12 +4,16 @@
  */
 
 #include <linux/device.h>
+#include <linux/err.h>
 #include <linux/init.h>
 #include <linux/kernel.h>
+#include <linux/list.h>
 #include <linux/module.h>
 #include <linux/of.h>
 #include <linux/reboot.h>
 #include <linux/reboot-mode.h>
+#include <linux/slab.h>
+#include <linux/string.h>
 
 #define PREFIX "mode-"
 
@@ -19,6 +23,54 @@ struct mode_info {
 	struct list_head list;
 };
 
+struct reboot_mode_sysfs_data {
+	struct device *reboot_mode_device;
+	struct list_head head;
+};
+
+static inline void reboot_mode_release_list(struct reboot_mode_sysfs_data *priv)
+{
+	struct mode_info *info;
+	struct mode_info *next;
+
+	list_for_each_entry_safe(info, next, &priv->head, list) {
+		list_del(&info->list);
+		kfree_const(info->mode);
+		kfree(info);
+	}
+}
+
+static ssize_t reboot_modes_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	struct reboot_mode_sysfs_data *priv;
+	struct mode_info *sysfs_info;
+	ssize_t size = 0;
+
+	priv = dev_get_drvdata(dev);
+	if (!priv)
+		return -ENODATA;
+
+	list_for_each_entry(sysfs_info, &priv->head, list)
+		size += sysfs_emit_at(buf, size, "%s ", sysfs_info->mode);
+
+	if (!size)
+		return -ENODATA;
+
+	return size + sysfs_emit_at(buf, size - 1, "\n");
+}
+static DEVICE_ATTR_RO(reboot_modes);
+
+static struct attribute *reboot_mode_attrs[] = {
+	&dev_attr_reboot_modes.attr,
+	NULL,
+};
+ATTRIBUTE_GROUPS(reboot_mode);
+
+static const struct class reboot_mode_class = {
+	.name = "reboot-mode",
+	.dev_groups = reboot_mode_groups,
+};
+
 static unsigned int get_reboot_mode_magic(struct reboot_mode_driver *reboot,
 					  const char *cmd)
 {
@@ -62,6 +114,51 @@ static int reboot_mode_notify(struct notifier_block *this,
 	return NOTIFY_DONE;
 }
 
+static int reboot_mode_create_device(struct reboot_mode_driver *reboot)
+{
+	struct reboot_mode_sysfs_data *priv;
+	struct mode_info *sysfs_info;
+	struct mode_info *info;
+	int ret;
+
+	priv = kzalloc_obj(*priv, GFP_KERNEL);
+	if (!priv)
+		return -ENOMEM;
+
+	INIT_LIST_HEAD(&priv->head);
+
+	list_for_each_entry(info, &reboot->head, list) {
+		sysfs_info = kzalloc_obj(*sysfs_info, GFP_KERNEL);
+		if (!sysfs_info) {
+			ret = -ENOMEM;
+			goto error;
+		}
+
+		sysfs_info->mode = kstrdup_const(info->mode, GFP_KERNEL);
+		if (!sysfs_info->mode) {
+			kfree(sysfs_info);
+			ret = -ENOMEM;
+			goto error;
+		}
+
+		list_add_tail(&sysfs_info->list, &priv->head);
+	}
+
+	priv->reboot_mode_device = device_create(&reboot_mode_class, NULL, 0,
+						 (void *)priv, reboot->dev->driver->name);
+	if (IS_ERR(priv->reboot_mode_device)) {
+		ret = PTR_ERR(priv->reboot_mode_device);
+		goto error;
+	}
+
+	return 0;
+
+error:
+	reboot_mode_release_list(priv);
+	kfree(priv);
+	return ret;
+}
+
 /**
  * reboot_mode_register - register a reboot mode driver
  * @reboot: reboot mode driver
@@ -113,16 +210,49 @@ int reboot_mode_register(struct reboot_mode_driver *reboot)
 	reboot->reboot_notifier.notifier_call = reboot_mode_notify;
 	register_reboot_notifier(&reboot->reboot_notifier);
 
+	ret = reboot_mode_create_device(reboot);
+	if (ret)
+		goto error;
+
 	return 0;
 
 error:
-	list_for_each_entry(info, &reboot->head, list)
+	reboot_mode_unregister(reboot);
-		kfree_const(info->mode);
-
 	return ret;
 }
 EXPORT_SYMBOL_GPL(reboot_mode_register);
 
+static int reboot_mode_match_by_name(struct device *dev, const void *data)
+{
+	const char *name = data;
+
+	if (!dev || !data)
+		return 0;
+
+	return dev_name(dev) && strcmp(dev_name(dev), name) == 0;
+}
+
+static inline void reboot_mode_unregister_device(struct reboot_mode_driver *reboot)
+{
+	struct reboot_mode_sysfs_data *priv;
+	struct device *reboot_mode_device;
+
+	reboot_mode_device = class_find_device(&reboot_mode_class, NULL, reboot->dev->driver->name,
+					       reboot_mode_match_by_name);
+
+	if (!reboot_mode_device)
+		return;
+
+	priv = dev_get_drvdata(reboot_mode_device);
+	device_unregister(reboot_mode_device);
+
+	if (!priv)
+		return;
+
+	reboot_mode_release_list(priv);
+	kfree(priv);
+}
+
 /**
  * reboot_mode_unregister - unregister a reboot mode driver
  * @reboot: reboot mode driver
@@ -132,6 +262,7 @@ int reboot_mode_unregister(struct reboot_mode_driver *reboot)
 	struct mode_info *info;
 
 	unregister_reboot_notifier(&reboot->reboot_notifier);
+	reboot_mode_unregister_device(reboot);
 
 	list_for_each_entry(info, &reboot->head, list)
 		kfree_const(info->mode);
@@ -199,6 +330,19 @@ void devm_reboot_mode_unregister(struct device *dev,
 }
 EXPORT_SYMBOL_GPL(devm_reboot_mode_unregister);
 
+static int __init reboot_mode_init(void)
+{
+	return class_register(&reboot_mode_class);
+}
+
+static void __exit reboot_mode_exit(void)
+{
+	class_unregister(&reboot_mode_class);
+}
+
+subsys_initcall(reboot_mode_init);
+module_exit(reboot_mode_exit);
+
 MODULE_AUTHOR("Andy Yan <andy.yan@rock-chips.com>");
 MODULE_DESCRIPTION("System reboot mode core library");
 MODULE_LICENSE("GPL v2");