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

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

Hi Laurent,

On Thu, 9 Nov 2017, Laurent Pinchart wrote:

> Hi Guennadi,
> 
> On Wednesday, 8 November 2017 12:43:46 EET Guennadi Liakhovetski wrote:

[snip]

> > To recall the metadata buffer layout should be
> > 
> > struct uvc_meta_buf {
> > 	uint64_t ns;
> > 	uint16_t sof;
> > 	uint8_t length;
> > 	uint8_t flags;
> > 	uint8_t buf[];
> > } __attribute__((packed));
> > 
> > where all the fields, beginning with "length" are a direct copy from the
> > UVC payload header. If multiple such payload headers have arrived for a
> > single frame, they will be appended and .bytesused will as usually have
> > the total byte count, used up in this frame. An application would then
> > calculate lengths of those individual metadata blocks as
> > 
> > sizeof(.ns) + sizeof(.sof) + .length
> > 
> > But this won't work with the "standard" UVC metadata format where any
> > private data, following standard fields, are dropped. In that case
> > applications would have to look at .flags and calculate the block length
> > based on them. Another possibility would be to rewrite the .length field
> > in the driver to only include standard fields, but I really don't think
> > that would be a good idea.
> 
> For the standard header the length can indeed be easily computed from the 
> flags. I wonder, however, why you think rewriting length would be a bad idea ?

Because I like the guarantee of the least intrusion. We (so far) 
guarantee, that the buffer contents beyond the system and the USB 
timestamps are a direct unmodified copy from the camera. This seems to be 
a good principle to stick to.

Thanks
Guennadi