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Message-ID: <CAMRc=Me=F5gTmkfO+2_3_pqebsz9GvabCK+xv6bfQeGgkSs6Dw@mail.gmail.com>
Date: Tue, 25 Nov 2025 06:44:12 -0800
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Shivendra Pratap <shivendra.pratap@....qualcomm.com>
Cc: linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
linux-pm@...r.kernel.org, Sebastian Reichel <sre@...nel.org>,
Bartosz Golaszewski <bartosz.golaszewski@...aro.org>, Bjorn Andersson <andersson@...nel.org>
Subject: Re: [PATCH v19 2/2] power: reset: reboot-mode: Expose sysfs for
registered reboot_modes
On Sat, 22 Nov 2025 19:05:45 +0100, Shivendra Pratap
<shivendra.pratap@....qualcomm.com> said:
> 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 | 86 +++++++++++++++++++++++++++++++++++++++
> include/linux/reboot-mode.h | 3 ++
> 2 files changed, 89 insertions(+)
>
> diff --git a/drivers/power/reset/reboot-mode.c b/drivers/power/reset/reboot-mode.c
> index fba53f638da04655e756b5f8b7d2d666d1379535..71cd97ffb3ee8da530514ab918bc6d58a122b4ce 100644
> --- a/drivers/power/reset/reboot-mode.c
> +++ b/drivers/power/reset/reboot-mode.c
> @@ -13,12 +13,61 @@
>
> #define PREFIX "mode-"
>
> +static bool reboot_mode_class_registered;
You don't need this, please see below.
> +
> 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);
> +
Don't you need locking here? What if we'll end up adding a new reboot mode at
the same time?
> + 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_register_device(struct reboot_mode_driver *reboot)
> +{
> + 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;
> + else
> + reboot->reboot_mode_device_registered = false;
Just use device_create(). I would also suggest creating a private structure
that embeds the pointer to the struct device created by device_create() and
the pointer to the reboot_mode_driver. If you pass it as driver data to
device_create(), you'll be able to retrieve it with dev_get_drvdata() in
sysfs callbacks.
> +}
> +
> static unsigned int get_reboot_mode_magic(struct reboot_mode_driver *reboot,
> const char *cmd)
> {
> @@ -78,6 +127,9 @@ int reboot_mode_register(struct reboot_mode_driver *reboot)
>
> INIT_LIST_HEAD(&reboot->head);
>
> + if (reboot_mode_class_registered)
> + reboot_mode_register_device(reboot);
> +
> for_each_property_of_node(np, prop) {
> if (strncmp(prop->name, PREFIX, len))
> continue;
> @@ -119,6 +171,11 @@ 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 +193,11 @@ 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 +224,7 @@ 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);
> @@ -199,6 +262,29 @@ void devm_reboot_mode_unregister(struct device *dev,
> }
> EXPORT_SYMBOL_GPL(devm_reboot_mode_unregister);
>
> +static int __init reboot_mode_init(void)
> +{
> + if (class_register(&reboot_mode_class))
> + reboot_mode_class_registered = false;
> + else
> + reboot_mode_class_registered = true;
> +
> + return 0;
> +}
> +
> +static void __exit reboot_mode_exit(void)
> +{
> + if (reboot_mode_class_registered)
> + class_unregister(&reboot_mode_class);
Please take a moment to read the code of the functions you're calling. The
class_unregister() function will check if the class you're passing as argument
was previously registered. If not, it will do nothing. You don't need all this
checking.
> +}
> +
> +#ifdef MODULE
> +module_init(reboot_mode_init);
> +module_exit(reboot_mode_exit);
> +#else
> +subsys_initcall(reboot_mode_init);
> +#endif
You don't need to do this. It's perfectly fine to just do:
subsys_initcall(reboot_mode_init);
module_exit(reboot_mode_exit);
This is handled in the module.h header[1].
> +
> MODULE_AUTHOR("Andy Yan <andy.yan@...k-chips.com>");
> MODULE_DESCRIPTION("System reboot mode core library");
> MODULE_LICENSE("GPL v2");
> 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;
No need for this, just use device_is_registered() wherever you need to check
it.
> int (*write)(struct reboot_mode_driver *reboot, unsigned int magic);
> struct notifier_block reboot_notifier;
> };
>
> --
> 2.34.1
>
>
>
Thanks,
Bart
[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/module.h#n103
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