Re: [PATCH v7 2/5] media: v4l: Support obtaining link frequency via get_mbus_config

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

On Mon, Dec 16, 2024 at 08:46:43AM +0000, Sakari Ailus wrote:
> On Sun, Dec 15, 2024 at 06:59:35PM +0200, Laurent Pinchart wrote:
> > On Tue, Dec 10, 2024 at 09:59:03AM +0200, Sakari Ailus wrote:
> > > Add link_freq field to struct v4l2_mbus_config in order to pass the link
> > > frequency to the receiving sub-device.
> > 
> > The documentation of v4l2_get_link_freq() should be expanded to explain
> > the new mode of operation. It should document which of the supported
> > methods are recommended for new drivers.
> > 
> > > 
> > > Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> > > ---
> > >  drivers/media/v4l2-core/v4l2-common.c | 11 ++++++++---
> > >  include/media/v4l2-mediabus.h         |  2 ++
> > >  2 files changed, 10 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/drivers/media/v4l2-core/v4l2-common.c b/drivers/media/v4l2-core/v4l2-common.c
> > > index 9fe74c7e064f..88fbd5608f00 100644
> > > --- a/drivers/media/v4l2-core/v4l2-common.c
> > > +++ b/drivers/media/v4l2-core/v4l2-common.c
> > > @@ -508,13 +508,18 @@ EXPORT_SYMBOL_GPL(__v4l2_get_link_freq_ctrl);
> > >  s64 __v4l2_get_link_freq_pad(struct media_pad *pad, unsigned int mul,
> > >  			     unsigned int div)
> > >  {
> > > +	struct v4l2_mbus_config mbus_config = {};
> > 
> > Isn't mbus_config fully populated by the .get_mbus_config() operation ?
> > That should be documented in the .get_mbus_config() operation
> > documentation.
> 
> It's a good practice to zero the memory before drivers get to work on it. I
> presume drivers will set the fields that are relevant for them and ignore
> the rest.
> 
> I can add a note on get_mbus_config() the drivers should set all struct
> fields to known values.

Thanks.

> > >  	struct v4l2_subdev *sd;
> > > +	int ret;
> > >  
> > >  	sd = media_entity_to_v4l2_subdev(pad->entity);
> > > -	if (!sd)
> > > -		return -ENODEV;
> > > +	ret = v4l2_subdev_call(sd, pad, get_mbus_config, pad->index,
> > > +			       &mbus_config);
> > > +	if (ret < 0 && ret != -ENOIOCTLCMD)
> > > +		return ret;
> > >  
> > > -	return __v4l2_get_link_freq_ctrl(sd->ctrl_handler, mul, div);
> > > +	return mbus_config.link_freq ?:
> > > +		__v4l2_get_link_freq_ctrl(sd->ctrl_handler, mul, div);
> > 
> > 	if (mbus_config.link_freq)
> > 		return mbus_config.link_freq;
> > 
> > 	return __v4l2_get_link_freq_ctrl(sd->ctrl_handler, mul, div);
> 
> Whether this would be cleaner is debatable at least. I can switch if you
> insist though.

I think it's nicer. You could even write

 	if (mbus_config.link_freq)
 		return mbus_config.link_freq;

	/*
	 * Fall back to using the link frequency control if the media bus config
	 * doesn't provide a link frequency.
	 */
 	return __v4l2_get_link_freq_ctrl(sd->ctrl_handler, mul, div);

> > I wonder if we should also add a message in case link_freq is 0, to get
> > drivers to convert to reporting the link frequency through
> > .get_mbus_config() if they already implement the operation.
> 
> I'm not sure this will be useful: in most cases the LINK_FREQ control is
> used and for a reason: it's user-configurable. Drivers should only populate
> the field if the link frequency is determined by the driver. For the
> receiver drivers it does not matter: they use v4l2_get_link_freq().

I think this depends on whether or not driver that have a configurable
link frequency should report the current value through
.get_mbus_config(). I think that drivers that implement
.get_mbus_config() should, for consistency.

> > >  }
> > >  EXPORT_SYMBOL_GPL(__v4l2_get_link_freq_pad);
> > >  #endif /* CONFIG_MEDIA_CONTROLLER */
> > > diff --git a/include/media/v4l2-mediabus.h b/include/media/v4l2-mediabus.h
> > > index 5bce6e423e94..cc5f776dc662 100644
> > > --- a/include/media/v4l2-mediabus.h
> > > +++ b/include/media/v4l2-mediabus.h
> > > @@ -148,6 +148,7 @@ enum v4l2_mbus_type {
> > >  /**
> > >   * struct v4l2_mbus_config - media bus configuration
> > >   * @type: interface type
> > > + * @link_freq: The link frequency. See also V4L2_CID_LINK_FREQ control.
> > 
> > Not a candidate for this series, but I'd like to simplify drivers by
> > implementing the LINK_FREQ control automatically.
> > 
> > >   * @bus: bus configuration data structure
> > >   * @bus.parallel: embedded &struct v4l2_mbus_config_parallel.
> > >   *		  Used if the bus is parallel or BT.656.
> > > @@ -162,6 +163,7 @@ enum v4l2_mbus_type {
> > >   */
> > >  struct v4l2_mbus_config {
> > >  	enum v4l2_mbus_type type;
> > > +	u64 link_freq;
> > >  	union {
> > >  		struct v4l2_mbus_config_parallel parallel;
> > >  		struct v4l2_mbus_config_mipi_csi1 mipi_csi1;

-- 
Regards,

Laurent Pinchart