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

[Sakari Ailus <sakari.ailus@linux.intel.com>]

Hi Laurent,

On Sun, Dec 15, 2024 at 06:59:35PM +0200, Laurent Pinchart wrote:
> Hi Sakari,
> 
> Thank you for the patch.
> 
> 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.

> 
> >  	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 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().

> 
> >  }
> >  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,

Sakari Ailus