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

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@linux.intel.com>
---
 drivers/base/core.c    | 24 ++++++++++++++++++++++++
 include/linux/device.h |  1 +
 2 files changed, 25 insertions(+)

On Tue, Jun 7, 2022 at 10:22 PM Andy Shevchenko
<andriy.shevchenko@linux.intel.com> 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@linux.intel.com>
> ---
>  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
>

On Tue, Jun 7, 2022 at 10:22 PM Andy Shevchenko
<andriy.shevchenko@linux.intel.com> wrote:
>
> We have already a helper to get the first child device, use it and
> drop custom approach.
>
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

Nice cleanup.

Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

> ---
>  drivers/spi/spi.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
> index ea09d1b42bf6..87dc8773108b 100644
> --- a/drivers/spi/spi.c
> +++ b/drivers/spi/spi.c
> @@ -2613,11 +2613,6 @@ int spi_slave_abort(struct spi_device *spi)
>  }
>  EXPORT_SYMBOL_GPL(spi_slave_abort);
>
> -static int match_true(struct device *dev, void *data)
> -{
> -       return 1;
> -}
> -
>  static ssize_t slave_show(struct device *dev, struct device_attribute *attr,
>                           char *buf)
>  {
> @@ -2625,7 +2620,7 @@ static ssize_t slave_show(struct device *dev, struct device_attribute *attr,
>                                                    dev);
>         struct device *child;
>
> -       child = device_find_child(&ctlr->dev, NULL, match_true);
> +       child = device_find_first_child(&ctlr->dev);
>         return sprintf(buf, "%s\n",
>                        child ? to_spi_device(child)->modalias : NULL);
>  }
> @@ -2644,7 +2639,7 @@ static ssize_t slave_store(struct device *dev, struct device_attribute *attr,
>         if (rc != 1 || !name[0])
>                 return -EINVAL;
>
> -       child = device_find_child(&ctlr->dev, NULL, match_true);
> +       child = device_find_first_child(&ctlr->dev);
>         if (child) {
>                 /* Remove registered slave */
>                 device_unregister(child);
> --
> 2.35.1
>

On Tue, Jun 07, 2022 at 11:20:58PM +0300, Andy Shevchenko wrote:
> We have already a helper to get the first child device, use it and
> drop custom approach.
> 
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> ---
>  drivers/spi/spi.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
> index ea09d1b42bf6..87dc8773108b 100644
> --- a/drivers/spi/spi.c
> +++ b/drivers/spi/spi.c
> @@ -2613,11 +2613,6 @@ int spi_slave_abort(struct spi_device *spi)
>  }
>  EXPORT_SYMBOL_GPL(spi_slave_abort);
>  
> -static int match_true(struct device *dev, void *data)
> -{
> -	return 1;
> -}
> -
>  static ssize_t slave_show(struct device *dev, struct device_attribute *attr,
>  			  char *buf)
>  {
> @@ -2625,7 +2620,7 @@ static ssize_t slave_show(struct device *dev, struct device_attribute *attr,
>  						   dev);
>  	struct device *child;
>  
> -	child = device_find_child(&ctlr->dev, NULL, match_true);
> +	child = device_find_first_child(&ctlr->dev);
>  	return sprintf(buf, "%s\n",
>  		       child ? to_spi_device(child)->modalias : NULL);
>  }

Horrible naming convention asside, what is this really showing?  I do
not see this documented in Documentation/ABI/ anywhere, so can it just
be dropped entirely?

Ah, it's in Documentation/spi/spi-summary.rst not where it belongs...

Looks like "any" of the child devices could match here, so it's just
finding the first one by default.  So you aren't explicitly asking for
the real first device, you could return the last one as well, and it
would still work as there is just "one" device in this list from what I
can tell.

So is does this really deserve a new driver core api call?

thanks,

greg k-h

On Wed, Jun 08, 2022 at 01:29:08PM +0200, Rafael J. Wysocki wrote:
> On Tue, Jun 7, 2022 at 10:22 PM Andy Shevchenko
> <andriy.shevchenko@linux.intel.com> wrote:

...

> 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.

With this we actually may simply provide a match function and it will make the
clean ups (like patch 2 in the series) almost the same without introducing a
device core call.

Something like

int device_match_any_for_find(struct device *dev, void *unused)
{
	return 1;
}

As I replied to Greg it's pity we can't use device_match_any()...

On Wed, Jun 08, 2022 at 02:53:28PM +0300, Andy Shevchenko wrote:
> On Wed, Jun 08, 2022 at 01:29:08PM +0200, Rafael J. Wysocki wrote:
> > On Tue, Jun 7, 2022 at 10:22 PM Andy Shevchenko
> > <andriy.shevchenko@linux.intel.com> wrote:
> 
> ...
> 
> > 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.
> 
> With this we actually may simply provide a match function and it will make the
> clean ups (like patch 2 in the series) almost the same without introducing a
> device core call.
> 
> Something like
> 
> int device_match_any_for_find(struct device *dev, void *unused)
> {
> 	return 1;
> }
> 
> As I replied to Greg it's pity we can't use device_match_any()...

	int device_match_any(struct device *dev, const void *unused)

How is that not ok to use here?

thanks,

greg k-h

On Wed, Jun 8, 2022 at 1:54 PM Andy Shevchenko
<andriy.shevchenko@linux.intel.com> wrote:
>
> On Wed, Jun 08, 2022 at 01:29:08PM +0200, Rafael J. Wysocki wrote:
> > On Tue, Jun 7, 2022 at 10:22 PM Andy Shevchenko
> > <andriy.shevchenko@linux.intel.com> wrote:
>
> ...
>
> > 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.
>
> With this we actually may simply provide a match function and it will make the
> clean ups (like patch 2 in the series) almost the same without introducing a
> device core call.

That works too, but IMO it would be a bit cleaner to have the wrapper
defined as a proper function.

>
> Something like
>
> int device_match_any_for_find(struct device *dev, void *unused)
> {
>         return 1;
> }
>
> As I replied to Greg it's pity we can't use device_match_any()...

Well, that only is a matter of adding one more variant of _match_any_ ...

On Wed, Jun 08, 2022 at 02:03:32PM +0200, Greg Kroah-Hartman wrote:
> On Wed, Jun 08, 2022 at 02:49:14PM +0300, Andy Shevchenko wrote:

...

> Why not exactly?  match_true() above and device_match_any() have the
> same signature from what I can tell:
> 	static int match_true(struct device *dev, void *data)
> 	int device_match_any(struct device *dev, const void *unused)
> 
> What am I missing, the const?

Yep! Compiler is very unhappy about it.

> > I agree that all thing should be using _any instead of _first.
> 
> Yes, so let's fix it please, don't propagate bad patterns.

Will do, thanks!