Re: [PATCH v1 1/2] driver core: Introduce device_find_first_child() helper

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

On Tue, Jun 7, 2022 at 10:22 PM Andy Shevchenko
<andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
>
> There are several places in the kernel where this kind of functionality is
> being used. Provide a generic helper for such cases.
>
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> ---
>  drivers/base/core.c    | 24 ++++++++++++++++++++++++
>  include/linux/device.h |  1 +
>  2 files changed, 25 insertions(+)
>
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 7cd789c4985d..972bfe975cd0 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -3832,6 +3832,30 @@ struct device *device_find_child_by_name(struct device *parent,
>  }
>  EXPORT_SYMBOL_GPL(device_find_child_by_name);
>
> +/**
> + * device_find_first_child - device iterator for locating the fist child device.
> + * @parent: parent struct device
> + *
> + * This is similar to the device_find_child() function above, but it
> + * returns a reference to the first child device.
> + *
> + * NOTE: you will need to drop the reference with put_device() after use.
> + */
> +struct device *device_find_first_child(struct device *parent)
> +{
> +       struct klist_iter i;
> +       struct device *child;
> +
> +       if (!parent)
> +               return NULL;
> +
> +       klist_iter_init(&parent->p->klist_children, &i);
> +       child = get_device(next_device(&i));
> +       klist_iter_exit(&i);
> +       return child;
> +}
> +EXPORT_SYMBOL_GPL(device_find_first_child);

I would define it as

static int match_first(struct device *dev, void *)
{
       return 1;
}

struct device *device_find_first_child(struct device *parent)
{
        return device_find_first_child(parent, NULL, match_first);
}
EXPORT_SYMBOL_GPL(device_find_first_child);

which is not that much more overhead.

> +
>  int __init devices_init(void)
>  {
>         devices_kset = kset_create_and_add("devices", &device_uevent_ops, NULL);
> diff --git a/include/linux/device.h b/include/linux/device.h
> index dc941997795c..20171a4358df 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -905,6 +905,7 @@ struct device *device_find_child(struct device *dev, void *data,
>                                  int (*match)(struct device *dev, void *data));
>  struct device *device_find_child_by_name(struct device *parent,
>                                          const char *name);
> +struct device *device_find_first_child(struct device *parent);
>  int device_rename(struct device *dev, const char *new_name);
>  int device_move(struct device *dev, struct device *new_parent,
>                 enum dpm_order dpm_order);
> --
> 2.35.1
>