linux-kernel - Re: [PATCH v17 05/12] power: reset: reboot-mode: Expose sysfs for registered reboot

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Message-ID: <CACMJSesMK37D7Qy+rVq7w6bUt6bYGXykUid6bUKXvh7M9mntZA@mail.gmail.com>
Date: Mon, 10 Nov 2025 16:14:53 +0100
From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
To: Shivendra Pratap <shivendra.pratap@....qualcomm.com>
Cc: Bjorn Andersson <andersson@...nel.org>, Sebastian Reichel <sre@...nel.org>, Rob Herring <robh@...nel.org>, 
	Sudeep Holla <sudeep.holla@....com>, Souvik Chakravarty <Souvik.Chakravarty@....com>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Andy Yan <andy.yan@...k-chips.com>, Mark Rutland <mark.rutland@....com>, 
	Lorenzo Pieralisi <lpieralisi@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	Konrad Dybcio <konradybcio@...nel.org>, cros-qcom-dts-watchers@...omium.org, 
	Vinod Koul <vkoul@...nel.org>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <will@...nel.org>, Florian Fainelli <florian.fainelli@...adcom.com>, 
	Moritz Fischer <moritz.fischer@...us.com>, John Stultz <john.stultz@...aro.org>, 
	Matthias Brugger <matthias.bgg@...il.com>, Krzysztof Kozlowski <krzk@...nel.org>, 
	Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>, 
	Mukesh Ojha <mukesh.ojha@....qualcomm.com>, Stephen Boyd <swboyd@...omium.org>, 
	Andre Draszik <andre.draszik@...aro.org>, 
	Kathiravan Thirumoorthy <kathiravan.thirumoorthy@....qualcomm.com>, linux-pm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-arm-msm@...r.kernel.org, 
	Elliot Berman <quic_eberman@...cinc.com>, Xin Liu <xin.liu@....qualcomm.com>, 
	Srinivas Kandagatla <srini@...nel.org>
Subject: Re: [PATCH v17 05/12] power: reset: reboot-mode: Expose sysfs for
 registered reboot_modes

On Sun, 9 Nov 2025 at 15:38, 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 | 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.

>  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.

> +       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");

> +
> +       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?

Bart