Re: [PATCH 2/7] driver core: create class_is_registered()

["Rafael J. Wysocki" <rafael@xxxxxxxxxx>]

On Fri, Mar 31, 2023 at 11:33 AM Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> Some classes (i.e. gpio), want to know if they have been registered or
> not, and poke around in the class's internal structures to try to figure
> this out.  Because this is not really a good idea, provide a function
> for classes to call to try to figure this out.
>
> Note, this is racy as the state of the class could change at any moment
> in time after the call is made, but as usually a class only wants to
> know if it has been registered yet or not, it should be fairly safe to
> use, and is just as safe as the previous "poke at the class internals"
> check was.
>
> Move the gpiolib code to use this function as proof that it works
> properly.
>
> Cc: Linus Walleij <linus.walleij@xxxxxxxxxx>
> Cc: Bartosz Golaszewski <brgl@xxxxxxxx>
> Cc: Sebastian Reichel <sre@xxxxxxxxxx>
> Cc: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>
> Cc: linux-gpio@xxxxxxxxxxxxxxx
> Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

Reviewed-by: Rafael J. Wysocki <rafael@xxxxxxxxxx>

> ---
>  drivers/base/class.c         | 25 +++++++++++++++++++++++++
>  drivers/gpio/gpiolib-sysfs.c |  4 ++--
>  include/linux/device/class.h |  1 +
>  3 files changed, 28 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/base/class.c b/drivers/base/class.c
> index 68a6f9b56d19..a8a1bf976290 100644
> --- a/drivers/base/class.c
> +++ b/drivers/base/class.c
> @@ -634,6 +634,31 @@ void class_compat_remove_link(struct class_compat *cls, struct device *dev,
>  }
>  EXPORT_SYMBOL_GPL(class_compat_remove_link);
>
> +/**
> + * class_is_registered - determine if at this moment in time, a class is
> + *                      registered in the driver core or not.
> + * @class: the class to check
> + *
> + * Returns a boolean to state if the class is registered in the driver core
> + * or not.  Note that the value could switch right after this call is made,
> + * so only use this in places where you "know" it is safe to do so (usually
> + * to determine if the specific class has been registered yet or not).
> + *
> + * Be careful in using this.
> + */
> +bool class_is_registered(const struct class *class)
> +{
> +       struct subsys_private *sp = class_to_subsys(class);
> +       bool is_initialized = false;
> +
> +       if (sp) {
> +               is_initialized = true;
> +               subsys_put(sp);
> +       }
> +       return is_initialized;
> +}
> +EXPORT_SYMBOL_GPL(class_is_registered);
> +
>  int __init classes_init(void)
>  {
>         class_kset = kset_create_and_add("class", NULL, NULL);
> diff --git a/drivers/gpio/gpiolib-sysfs.c b/drivers/gpio/gpiolib-sysfs.c
> index a895915affa5..1a9b21731cc9 100644
> --- a/drivers/gpio/gpiolib-sysfs.c
> +++ b/drivers/gpio/gpiolib-sysfs.c
> @@ -554,7 +554,7 @@ int gpiod_export(struct gpio_desc *desc, bool direction_may_change)
>         int                     offset;
>
>         /* can't export until sysfs is available ... */
> -       if (!gpio_class.p) {
> +       if (!class_is_registered(&gpio_class)) {
>                 pr_debug("%s: called too early!\n", __func__);
>                 return -ENOENT;
>         }
> @@ -728,7 +728,7 @@ int gpiochip_sysfs_register(struct gpio_device *gdev)
>          * register later, in gpiolib_sysfs_init() ... here we just
>          * verify that _some_ field of gpio_class got initialized.
>          */
> -       if (!gpio_class.p)
> +       if (!class_is_registered(&gpio_class))
>                 return 0;
>
>         /*
> diff --git a/include/linux/device/class.h b/include/linux/device/class.h
> index b53728ca56fb..9cb5db0588c8 100644
> --- a/include/linux/device/class.h
> +++ b/include/linux/device/class.h
> @@ -84,6 +84,7 @@ extern struct kobject *sysfs_dev_block_kobj;
>
>  int __must_check class_register(struct class *class);
>  void class_unregister(const struct class *class);
> +bool class_is_registered(const struct class *class);
>
>  struct class_compat;
>  struct class_compat *class_compat_register(const char *name);
> --
> 2.40.0
>