On 11/10/2025 8:44 PM, Bartosz Golaszewski wrote:

On Sun, 9 Nov 2025 at 15:38, Shivendra Pratap
[off-list ref] wrote:

quoted

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 <redacted>
---
 drivers/power/reset/reboot-mode.c | 62 ++++++++++++++++++++++++++++++++++++++-
 include/linux/reboot-mode.h       |  2 ++
 2 files changed, 63 insertions(+), 1 deletion(-)

diff --git a/drivers/power/reset/reboot-mode.c b/drivers/power/reset/reboot-mode.c
index 873ac45cd7659b214b7c21958f580ca381e0a63d..582aa7f8ed7fa485c5a67877558c9b15d3600ef4 100644
--- a/drivers/power/reset/reboot-mode.c
+++ b/drivers/power/reset/reboot-mode.c

@@ -6,6 +6,7 @@
 #define pr_fmt(fmt)    "reboot-mode: " fmt

 #include <linux/device.h>
+#include <linux/err.h>
 #include <linux/init.h>
 #include <linux/kernel.h>
 #include <linux/list.h>

@@ -23,6 +24,8 @@ struct mode_info {
        struct list_head list;
 };

+static struct class *rb_class;
+

I know C is a spartan language but the rb_ prefix makes me think of
the red-black tree. Please call it reboot_mode_class.

sure will make it reboot_mode_class.

quoted

 static u64 get_reboot_mode_magic(struct reboot_mode_driver *reboot, const char *cmd)
 {
        const char *normal = "normal";

@@ -65,6 +68,51 @@ static int reboot_mode_notify(struct notifier_block *this,
        return NOTIFY_DONE;
 }

+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 = (struct reboot_mode_driver *)dev_get_drvdata(dev);

No need for the cast.

Ack.

quoted

+       if (!reboot)
+               return -ENODATA;
+
+       list_for_each_entry(info, &reboot->head, list)
+               size += sysfs_emit_at(buf, size, "%s ", info->mode);
+
+       if (size) {
+               size += sysfs_emit_at(buf, size - 1, "\n");
+               return size;
+       }

This is a weird logic inversion. Just do:

if (!size)
    return -ENODATA;

return size + sysfs_emit_at(buf, size - 1, "\n");

Ack.

quoted

+
+       return -ENODATA;
+}
+static DEVICE_ATTR_RO(reboot_modes);
+
+static int create_reboot_mode_device(struct reboot_mode_driver *reboot)
+{
+       int ret = 0;
+
+       if (!rb_class) {
+               rb_class = class_create("reboot-mode");
+               if (IS_ERR(rb_class))
+                       return PTR_ERR(rb_class);
+       }

Why the lazy initialization here? Is there any reason you can't
statically define the class? Don't you need synchronization here if
multiple drivers try to do this?

reboot-mode framework does not has a init. Should i add a inti_call
here and create a class? Can you suggest a bit more.

For if we at-all continue this lazy init, will add a lock.

thanks,
Shivendra