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Message-ID: <CACMJSeu6BGS+AyEXyR9S7d6qGkbP3GiEzq6qy1860QaOQ-peQA@mail.gmail.com>
Date: Mon, 17 Nov 2025 14:25:42 +0100
From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
To: Shivendra Pratap <shivendra.pratap@....qualcomm.com>
Cc: Sebastian Reichel <sre@...nel.org>, Bjorn Andersson <andersson@...nel.org>, linux-kernel@...r.kernel.org,
linux-arm-msm@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH 2/2] power: reset: reboot-mode: Expose sysfs for
registered reboot_modes
On Sun, 16 Nov 2025 at 16:20, Shivendra Pratap
<shivendra.pratap@....qualcomm.com> wrote:
>
> Currently, there is no standardized mechanism for userspace to discover
> which reboot-modes are supported on a given platform. This limitation
> forces tools and scripts to rely on hardcoded assumptions about the
> supported reboot-modes.
>
> Create a class 'reboot-mode' and a device under it to expose a sysfs
> interface to show the available reboot mode arguments to userspace. Use
> the driver_name field of the struct reboot_mode_driver to create the
> device. For device-based drivers, configure the device driver name as
> driver_name.
>
> This results in the creation of:
> /sys/class/reboot-mode/<driver>/reboot_modes
>
> This read-only sysfs file will exposes the list of supported reboot
> modes arguments provided by the driver, enabling userspace to query the
> list of arguments.
>
> Signed-off-by: Shivendra Pratap <shivendra.pratap@....qualcomm.com>
> ---
> drivers/power/reset/reboot-mode.c | 72 +++++++++++++++++++++++++++++++++++++++
> include/linux/reboot-mode.h | 3 ++
> 2 files changed, 75 insertions(+)
>
> diff --git a/drivers/power/reset/reboot-mode.c b/drivers/power/reset/reboot-mode.c
> index fba53f638da04655e756b5f8b7d2d666d1379535..062df67735c4818cfeb894941e537f19ea9d4ccb 100644
> --- a/drivers/power/reset/reboot-mode.c
> +++ b/drivers/power/reset/reboot-mode.c
> @@ -7,18 +7,77 @@
> #include <linux/init.h>
> #include <linux/kernel.h>
> #include <linux/module.h>
> +#include <linux/mutex.h>
> #include <linux/of.h>
> #include <linux/reboot.h>
> #include <linux/reboot-mode.h>
>
> #define PREFIX "mode-"
>
> +static DEFINE_MUTEX(reboot_mode_mutex);
> +
> struct mode_info {
> const char *mode;
> u32 magic;
> struct list_head list;
> };
>
> +static ssize_t reboot_modes_show(struct device *dev, struct device_attribute *attr, char *buf)
> +{
> + struct reboot_mode_driver *reboot;
> + struct mode_info *info;
> + ssize_t size = 0;
> +
> + reboot = container_of(dev, struct reboot_mode_driver, reboot_mode_device);
> + if (!reboot)
> + return -ENODATA;
> +
> + list_for_each_entry(info, &reboot->head, list)
> + size += sysfs_emit_at(buf, size, "%s ", 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 void reboot_mode_device_release(struct device *dev)
> +{
> + /* place holder to avoid warning on device_unregister. nothing to free */
> +}
> +
> +static void reboot_mode_create_device(struct reboot_mode_driver *reboot)
> +{
> + static bool is_class_registered;
> +
> + reboot->reboot_mode_device_registered = false;
> +
> + scoped_guard(mutex, &reboot_mode_mutex) {
> + if (!is_class_registered) {
> + if (!class_register(&reboot_mode_class))
> + is_class_registered = true;
> + }
> + }
This could be achieved with DO_ONCE() but you still haven't explained
why this needs to be done here. Why not in the module's
subsys_initcall()? As of now, the class will not appear in sysfs until
the first device is registered which isn't a very common behavior.
Bart
> +
> + reboot->reboot_mode_device.class = &reboot_mode_class;
> + reboot->reboot_mode_device.release = reboot_mode_device_release;
> + dev_set_name(&reboot->reboot_mode_device, reboot->driver_name);
> + if (!device_register(&reboot->reboot_mode_device))
> + reboot->reboot_mode_device_registered = true;
> +}
> +
> static unsigned int get_reboot_mode_magic(struct reboot_mode_driver *reboot,
> const char *cmd)
> {
> @@ -78,6 +137,8 @@ int reboot_mode_register(struct reboot_mode_driver *reboot)
>
> INIT_LIST_HEAD(&reboot->head);
>
> + reboot_mode_create_device(reboot);
> +
> for_each_property_of_node(np, prop) {
> if (strncmp(prop->name, PREFIX, len))
> continue;
> @@ -119,6 +180,11 @@ int reboot_mode_register(struct reboot_mode_driver *reboot)
> list_for_each_entry(info, &reboot->head, list)
> kfree_const(info->mode);
>
> + if (reboot->reboot_mode_device_registered) {
> + device_unregister(&reboot->reboot_mode_device);
> + reboot->reboot_mode_device_registered = false;
> + }
> +
> return ret;
> }
> EXPORT_SYMBOL_GPL(reboot_mode_register);
> @@ -136,6 +202,11 @@ int reboot_mode_unregister(struct reboot_mode_driver *reboot)
> list_for_each_entry(info, &reboot->head, list)
> kfree_const(info->mode);
>
> + if (reboot->reboot_mode_device_registered) {
> + device_unregister(&reboot->reboot_mode_device);
> + reboot->reboot_mode_device_registered = false;
> + }
> +
> return 0;
> }
> EXPORT_SYMBOL_GPL(reboot_mode_unregister);
> @@ -162,6 +233,7 @@ int devm_reboot_mode_register(struct device *dev,
> if (!dr)
> return -ENOMEM;
>
> + reboot->driver_name = reboot->dev->driver->name;
> rc = reboot_mode_register(reboot);
> if (rc) {
> devres_free(dr);
> diff --git a/include/linux/reboot-mode.h b/include/linux/reboot-mode.h
> index 4a2abb38d1d612ec0fdf05eb18c98b210f631b7f..400cfde0e029aef14ff90a11b9d12d0c3ce8dee6 100644
> --- a/include/linux/reboot-mode.h
> +++ b/include/linux/reboot-mode.h
> @@ -5,6 +5,9 @@
> struct reboot_mode_driver {
> struct device *dev;
> struct list_head head;
> + const char *driver_name;
> + struct device reboot_mode_device;
> + bool reboot_mode_device_registered;
> int (*write)(struct reboot_mode_driver *reboot, unsigned int magic);
> struct notifier_block reboot_notifier;
> };
>
> --
> 2.34.1
>
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