Re: [PATCH v6 1/9] media: v4l2-subdev: Introduce [get|set]_mbus_config pad ops

[Janusz Krzysztofik <jmkrzyszt@gmail.com>]

Hi Hans,

On Wednesday, July 15, 2020 5:08:05 P.M. CEST Hans Verkuil wrote:
> On 14/07/2020 15:58, Jacopo Mondi wrote:
> > Introduce two new pad operations to allow retrieving and configuring the
> > media bus parameters on a subdevice pad.
> > 
> > The newly introduced operations aims to replace the s/g_mbus_config video
> > operations, which have been on their way for deprecation since a long
> > time.
> > 
> > Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
> > ---
> >  include/media/v4l2-subdev.h | 27 +++++++++++++++++++++++++++
> >  1 file changed, 27 insertions(+)
> > 
> > diff --git a/include/media/v4l2-subdev.h b/include/media/v4l2-subdev.h
> > index f7fe78a6f65a..d8b9d5735307 100644
> > --- a/include/media/v4l2-subdev.h
> > +++ b/include/media/v4l2-subdev.h
> > @@ -670,6 +670,29 @@ struct v4l2_subdev_pad_config {
> >   *
> >   * @set_frame_desc: set the low level media bus frame parameters, @fd array
> >   *                  may be adjusted by the subdev driver to device capabilities.
> > + *
> > + * @get_mbus_config: get the media bus configuration of a remote sub-device.
> > + *		     The media bus configuration is usually retrieved from the
> > + *		     firmware interface at sub-device probe time, immediately
> > + *		     applied to the hardware and eventually adjusted by the
> > + *		     driver. Remote sub-devices (usually video receivers) shall
> > + *		     use this operation to query the transmitting end bus
> > + *		     configuration in order to adjust their own one accordingly.
> > + *		     Callers should make sure they get the most up-to-date as
> > + *		     possible configuration from the remote end, likely calling
> > + *		     this operation as close as possible to stream on time. The
> > + *		     operation shall fail if the pad index it has been called on
> > + *		     is not valid.
> > + *
> > + * @set_mbus_config: set the media bus configuration of a remote sub-device.
> > + *		     This operations is intended to allow, in combination with
> > + *		     the get_mbus_config operation, the negotiation of media bus
> > + *		     configuration parameters between media sub-devices. The
> > + *		     operation shall not fail if the requested configuration is
> > + *		     not supported, but the driver shall update the content of
> > + *		     the %config argument to reflect what has been actually
> > + *		     applied to the hardware. The operation shall fail if the
> > + *		     pad index it has been called on is not valid.
> >   */
> 
> Hm, I'd hoped I could merge this, but I think include/media/v4l2-mediabus.h
> also needs to be updated.
> 
> So the old g_mbus_config returned all supported configurations, while the
> new get_mbus_config returns the *current* configuration.
> 
> That's fine, but that means that the meaning of the struct v4l2_mbus_config
> flags field changes as well and several comments in v4l2-mediabus.h need to
> be updated to reflect this.
> 
> E.g. instead of '/* How many lanes the client can use */' it becomes
> '/* How many lanes the client uses */'.
> 
> Frankly, these flags can be redesigned now since you only need a single
> e.g. V4L2_MBUS_HSYNC_ACTIVE_HIGH flag since if it is not set, then that
> means ACTIVE_LOW. And since it is now a single bit, it is also easy to
> make each flag a bit field.

Even if this makes sense for .get_mbus_config() which returns current 
configuration, how about keeping the old semantics of the flags and let 
.set_mbus_config() accept a potentially incomplete or redundant set of flags 
specified by the caller to select a supported configuration from?  That approach 
was actually proposed before by Jacopo when he argued against my suggestion to 
add a wrapper with a check for mutually exclusive flags[1], and I found it a 
very good alternative to my other rejected suggestion of adding TRY support.

[1] https://www.spinics.net/lists/linux-media/msg171878.html

Thanks,
Janusz

> 
> The CSI2 lanes/channels can just be a bit field for the number of lanes/channels,
> which is much easier to read. I strongly recommend making this change at the minimum.
> 
> Now all this can be done in a follow-up series, but the v4l2-mediabus.h
> definitely needs to be updated to reflect the new code.
> 
> This can be done as a v6 5.5/9 patch (since it should come right after the
> g/s_mbus_config removal).
> 
> Regards,
> 
> 	Hans
> 
> >  struct v4l2_subdev_pad_ops {
> >  	int (*init_cfg)(struct v4l2_subdev *sd,
> > @@ -710,6 +733,10 @@ struct v4l2_subdev_pad_ops {
> >  			      struct v4l2_mbus_frame_desc *fd);
> >  	int (*set_frame_desc)(struct v4l2_subdev *sd, unsigned int pad,
> >  			      struct v4l2_mbus_frame_desc *fd);
> > +	int (*get_mbus_config)(struct v4l2_subdev *sd, unsigned int pad,
> > +			       struct v4l2_mbus_config *config);
> > +	int (*set_mbus_config)(struct v4l2_subdev *sd, unsigned int pad,
> > +			       struct v4l2_mbus_config *config);
> >  };
> >  
> >  /**
> > 
> 
>