Re: [PATCH v3 4/4] uvcvideo: add a metadata device node

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

Hi Guennadi,

On Friday 30 Dec 2016 14:04:34 Guennadi Liakhovetski wrote:
> On Fri, 30 Dec 2016, Laurent Pinchart wrote:
> > On Friday 30 Dec 2016 11:43:02 Guennadi Liakhovetski wrote:
> >> Hi Laurent,
> >> 
> >> I'd like to discuss extending this patch a bit, preferably as an
> >> incremental patch.
> >> 
> >> First let me confirm my current understanding of the way the UVC driver
> >> creates its media device topology. Do I understand it correctly, that
> >> the driver allocates UVC entities (not media controller entities) for all
> >> UVC units and terminals, but then uses subdevices for all such UVC
> >> entities, except terminals, i.e. only for UVC units? struct uvc_entity
> >> has an embedded struct v4l2_subdev object, but it's unused for UVC
> >> terminals. Instead terminals are associated to video devices, which are
> >> then linked into the MC topology? Is this my understanding correct?
> > 
> > That's correct, but looking at the code now, I think the driver should use
> > a struct media_entity directly instead of a struct v4l2_subdev as it
> > doesn't need any of the infrastructure provided by subdevs.
> > 
> >> I have a problem with the current version of this patch, that there is
> >> no way to associate video device nodes with respepctive metadata nodes.
> >> Would it be acceptable to use an MC link for this association?
> > 
> > No, links describe data connections.
> 
> Well, it is data - it's metadata, extracted from USB buffers.
> 
> >> Is it allowed for video device MC entities to have source pads
> >> additionally to their (usually single) sink pad(s) (in case of input
> >> video devices)? If that would be acceptable, I could create an additional
> >> patch to add a source pad to output terminal video nodes to link it to
> >> metadata nodes.
> > 
> > That's a hack, I don't think it's a good idea.
> 
> Ok, would a completely specialised one-off sysfs solution be better? Maybe
> a link under the metadata node to the main node?

Come on, I know you're better than that. Stop thinking short term about the 
quickest hack that can provide the feature you need.

-- 
Regards,

Laurent Pinchart