Re: [PATCH 2/6 v5] V4L: Add a UVC Metadata format

[Guennadi Liakhovetski <g.liakhovetski@gmx.de>]

Hi Hans,

Thanks for the comments.

On Mon, 30 Oct 2017, Hans Verkuil wrote:

> Hi Guennadi,
> 
> On 07/28/2017 02:46 PM, Hans Verkuil wrote:
> > On 07/28/2017 02:33 PM, Guennadi Liakhovetski wrote:
> >> Add a pixel format, used by the UVC driver to stream metadata.
> >>
> >> Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@intel.com>
> >> ---
> >>  Documentation/media/uapi/v4l/meta-formats.rst    |  1 +
> >>  Documentation/media/uapi/v4l/pixfmt-meta-uvc.rst | 39 ++++++++++++++++++++++++
> >>  include/uapi/linux/videodev2.h                   |  1 +
> >>  3 files changed, 41 insertions(+)
> >>  create mode 100644 Documentation/media/uapi/v4l/pixfmt-meta-uvc.rst

[snip]

> >> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> >> index 45cf735..0aad91c 100644
> >> --- a/include/uapi/linux/videodev2.h
> >> +++ b/include/uapi/linux/videodev2.h
> >> @@ -682,6 +682,7 @@ struct v4l2_pix_format {
> >>  /* Meta-data formats */
> >>  #define V4L2_META_FMT_VSP1_HGO    v4l2_fourcc('V', 'S', 'P', 'H') /* R-Car VSP1 1-D Histogram */
> >>  #define V4L2_META_FMT_VSP1_HGT    v4l2_fourcc('V', 'S', 'P', 'T') /* R-Car VSP1 2-D Histogram */
> >> +#define V4L2_META_FMT_UVC         v4l2_fourcc('U', 'V', 'C', 'H') /* UVC Payload Header metadata */
> 
> I discussed this with Laurent last week and since the metadata for UVC starts
> with a standard header followed by vendor-specific data it makes sense to
> use V4L2_META_FMT_UVC for just the standard header. Any vendor specific formats
> should have their own fourcc which starts with the standard header followed by
> the custom data. The UVC driver would enumerate both the standard and the vendor
> specific fourcc. This would allow generic UVC applications to use the standard
> header. Applications that know about the vendor specific data can select the
> vendor specific format.
> 
> This change would make this much more convenient to use.

Then the driver should be able to decide, which private fourcc code to use 
for each of those devices. A natural way to do that seems to be to put 
that in the .driver_info field of struct usb_device_id. For that I'd 
replace the current use of that field for quirks with a pointer to a 
struct in a separate patch. Laurent, would that be acceptable? Then add a 
field to that struct for a private metadata fourcc code.

Thanks
Guennadi

> Regards,
> 
> 	Hans
> 
> >>  /* priv field value to indicates that subsequent fields are valid. */
> >>  #define V4L2_PIX_FMT_PRIV_MAGIC		0xfeedcafe