Re: [PATCH v2 1/3] media: v4l: async: add helper API media_async_register_subdev()

[Frank Li]

On Mon, Mar 02, 2026 at 03:21:39PM +0200, Sakari Ailus wrote:
> Hi Frank,
>
> Thanks for the set.
>
> On Thu, Feb 26, 2026 at 05:55:26PM -0500, Frank Li wrote:
> > Add the helper API media_async_register_subdev(), which
> > combines media_entity_pads_init(), v4l2_subdev_init_finalize(), and
> > v4l2_async_register_subdev() into a single call.
> >
> > Reduce code duplication and simplify error handling in drivers.
>
> I appreciate your efforts to try to simplify registering a sensor
> sub-device, but I'm not sure this gets far enough to make a notable
> difference: the new function calls two functions typically needed and
> requires the driver to implement a new one for registering a sub-device.

I not sure what's you means. Actually online one
media_async_register_subdev() funciton need be called at driver's probe,
which simple error handler much.  You can refer patch 2/3. (reduce 12 line
codes)

In file, drivers/media/platform/synopsys/dw-mipi-csi2rx.c, about 600 line
codes, only 20-30 line related hardware register access, others is overhead
of subdev driver.

Frank
>
> >
> > Signed-off-by: Frank Li <Frank.Li@xxxxxxx>
> > ---
> >  drivers/media/v4l2-core/v4l2-async.c | 33 +++++++++++++++++++++++++++++++++
> >  include/media/v4l2-async.h           | 22 ++++++++++++++++++++++
> >  2 files changed, 55 insertions(+)
> >
> > diff --git a/drivers/media/v4l2-core/v4l2-async.c b/drivers/media/v4l2-core/v4l2-async.c
> > index 1c08bba9ecb91f46b7479da613d6c1688d4b0b5c..e07173f566fbd8fa332b5e58be288e806b4c0482 100644
> > --- a/drivers/media/v4l2-core/v4l2-async.c
> > +++ b/drivers/media/v4l2-core/v4l2-async.c
> > @@ -19,6 +19,7 @@
> >  #include <linux/slab.h>
> >  #include <linux/types.h>
> >
> > +#include <media/media-entity.h>
> >  #include <media/v4l2-async.h>
> >  #include <media/v4l2-device.h>
> >  #include <media/v4l2-fwnode.h>
> > @@ -881,6 +882,38 @@ int __v4l2_async_register_subdev(struct v4l2_subdev *sd, struct module *module)
> >  }
> >  EXPORT_SYMBOL(__v4l2_async_register_subdev);
> >
> > +int __media_pad_init_and_register_subdev(struct v4l2_subdev *sd, u16 num_pads,
> > +	struct media_pad *pads,
> > +	int (*register_subdev)(struct v4l2_subdev *sd, struct module *module),
> > +	struct module *module)
> > +{
> > +	int ret;
> > +
> > +	if (!register_subdev)
> > +		return -EINVAL;
> > +
> > +	ret = media_entity_pads_init(&sd->entity, num_pads, pads);
> > +	if (ret)
> > +		return ret;
> > +
> > +	ret = v4l2_subdev_init_finalize(sd);
> > +	if (ret)
> > +		goto err_entity_cleanup;
> > +
> > +	ret = register_subdev(sd, module);
> > +	if (ret)
> > +		goto err_subdev_cleanup;
> > +
> > +	return 0;
> > +
> > +err_subdev_cleanup:
> > +	v4l2_subdev_cleanup(sd);
> > +err_entity_cleanup:
> > +	media_entity_cleanup(&sd->entity);
> > +	return ret;
> > +}
> > +EXPORT_SYMBOL(__media_pad_init_and_register_subdev);
> > +
> >  void v4l2_async_unregister_subdev(struct v4l2_subdev *sd)
> >  {
> >  	struct v4l2_async_connection *asc, *asc_tmp;
> > diff --git a/include/media/v4l2-async.h b/include/media/v4l2-async.h
> > index f26c323e9c963065fd7c19d6d9835df1194bc069..220a302a626732e15452f3efb19b03bdc51e64d5 100644
> > --- a/include/media/v4l2-async.h
> > +++ b/include/media/v4l2-async.h
> > @@ -336,6 +336,28 @@ int __v4l2_async_register_subdev(struct v4l2_subdev *sd, struct module *module);
> >  int __must_check
> >  v4l2_async_register_subdev_sensor(struct v4l2_subdev *sd);
> >
> > +struct media_pad;
> > +
> > +int __must_check
> > +__media_pad_init_and_register_subdev(struct v4l2_subdev *sd, u16 num_pads,
> > +	struct media_pad *pads,
> > +	int (*register_subdev)(struct v4l2_subdev *sd, struct module *module),
> > +	struct module *module);
> > +
> > +/**
> > + * media_async_register_subdev - Initialize the entity pads and
> > + *				 registers a sub-device to the
> > + *				 asynchronous subdevice framework
> > + * @sd: pointer to &struct v4l2_subdev
> > + * @num_pads: total number of sink and source pads
> > + * @pads: Array of @num_pads pads.
> > + *
> > + * Returns an error on failure, 0 on success.
> > + */
> > +#define media_async_register_subdev(sd, num_pads, pads)			\
> > +	__media_pad_init_and_register_subdev(sd, num_pads, pads,	\
> > +					     __v4l2_async_register_subdev, \
> > +					     THIS_MODULE)
> >  /**
> >   * v4l2_async_unregister_subdev - unregisters a sub-device to the asynchronous
> >   *	subdevice framework
> >
>
> --
> Kind regards,
>
> Sakari Ailus