Hi Laurent, On Tue, 6 Dec 2016, Laurent Pinchart wrote:
> Hi Guennadi, > > On Monday 05 Dec 2016 23:13:53 Guennadi Liakhovetski wrote: > > Just one question: > > > > On Tue, 6 Dec 2016, Laurent Pinchart wrote: > > >>>> + /* > > >>>> + * Register a metadata node. TODO: shall this only be enabled > > >>>> for some > > >>>> + * cameras? > > >>>> + */ > > >>>> + if (!(dev->quirks & UVC_QUIRK_BUILTIN_ISIGHT)) > > >>>> + uvc_meta_register(stream); > > >>>> + > > >>> > > >>> I think so, only for the cameras that can produce metadata. > > >> > > >> Every UVC camera produces metadata, but most cameras only have standard > > >> fields there. Whether we should stream standard header fields from the > > >> metadata node will be discussed later. If we do decide to stream > > >> standard header fields, then every USB camera gets metadata nodes. If we > > >> decide not to include standard fields, how do we know whether the camera > > >> has any private fields in headers without streaming from it? Do you want > > >> a quirk for such cameras? > > > > > > Unless they can be detected in a standard way that's probably the best > > > solution. Please remember that the UVC specification doesn't allow vendors > > > to extend headers in a vendor-specific way. > > > > Does it not? Where is that specified? I didn't find that anywhere. > > It's the other way around, it document a standard way to extend the payload > header. Any option you pick risks being incompatible with future revisions of > the specification. For instance the payload header's bmHeaderInfo field can > be > extended through the use of the EOF bit, but a future version of the > specification could also extend it, in a way that would make a > vendor-specific > extension be mistaken for the standard extension. > > Now, you could add data after the standard payload without touching the > bmHeaderInfo field, but I'm still worried it could be broken by future > versions of the specification. Exactly - using "free" space in payload headers is not a part of the spec, but is also not prohibited by it. As for future versions - cameras specify which version of the spec they implement, and existing versions cannot change. If a camera decides to upgrade and in future UVC versions there won't be enough space left for the private data, only then the camera manufacturer will have a problem, that they will have to solve. The user-space software will have to know, that UVC 5.1 metadata has a different format, than UVC 1.5, that's true. Thanks Guennadi -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
