Re: [PATCH 3/3 v7] uvcvideo: add a metadata device node

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

Hi Laurent,

On Tue, 5 Dec 2017, Laurent Pinchart wrote:

> Hi Guennadi,
> 
> On Tuesday, 5 December 2017 02:24:30 EET Laurent Pinchart wrote:
> > On Wednesday, 8 November 2017 18:00:14 EET Guennadi Liakhovetski wrote:
> 
> [snip]
> 
> > > +static void uvc_video_decode_meta(struct uvc_streaming *stream,
> > > +			struct uvc_buffer *buf, struct uvc_buffer *meta_buf,
> > > +			u8 *mem, unsigned int length)
> > 
> > The buf parameter is unused, you can remove it. mem isn't modified, I would
> > make it const.
> > 
> > > +{
> > > +	struct uvc_meta_buf *meta;
> > > +	size_t len_std = 2;
> > > +	bool has_pts, has_scr;
> > > +	unsigned long flags;
> > > +	struct timespec ts;
> > > +	u8 *scr;
> > 
> > And scr should be const too.
> > 
> > > +
> > > +	if (!meta_buf || length == 2 ||
> > > +	    meta_buf->length - meta_buf->bytesused <
> > > +	    length + sizeof(meta->ns) + sizeof(meta->sof))
> > > +		return;
> > 
> > If the metadata buffer overflows should we also set the error bit like we do
> > for video buffers ? I have mixed feelings about this, I'd appreciate your
> > input.
> > 
> > > +	has_pts = mem[1] & UVC_STREAM_PTS;
> > > +	has_scr = mem[1] & UVC_STREAM_SCR;
> > > +
> > > +	if (has_pts) {
> > > +		len_std += 4;
> > > +		scr = mem + 6;
> > > +	} else {
> > > +		scr = mem + 2;
> > > +	}
> > > +
> > > +	if (has_scr)
> > > +		len_std += 6;
> > > +
> > > +	if (stream->cur_meta_format == V4L2_META_FMT_UVC)
> > > +		length = len_std;
> > > +
> > > +	if (length == len_std && (!has_scr ||
> > > +				  !memcmp(scr, stream->clock.last_scr, 6)))
> > > +		return;
> > > +
> > > +	meta = (struct uvc_meta_buf *)((u8 *)meta_buf->mem +
> > > meta_buf->bytesused); +	local_irq_save(flags);
> > > +	uvc_video_get_ts(&ts);
> > 
> > FYI, Arnd has posted https://patchwork.kernel.org/patch/10076887/. If the
> > patch gets merged first I can help with the rebasing.
> 
> I've reviewed and merged Arnd patches in my tree, and...
> 
> > > +	meta->sof = usb_get_current_frame_number(stream->dev->udev);
> > > +	local_irq_restore(flags);
> > > +	meta->ns = timespec_to_ns(&ts);
> > 
> > The meta pointer can be unaligned as the structure is packed and its size
> > isn't a multiple of the size of the largest field (and it can contain an
> > unspecified amount of vendor data anyway). You thus can't access it directly
> > on all architectures, you will need to use the put_unaligned macro. As I
> > haven't checked whether all architectures can handle unaligned accesses
> > without generating a trap, I would store the USB frame number in a local
> > variable and use the put_unaligned macro output of the IRQ disabled section
> > (feel free to show me that I'm unnecessarily cautious :-)).
> > 
> > > +	if (has_scr)
> > > +		memcpy(stream->clock.last_scr, scr, 6);
> > > +
> > > +	memcpy(&meta->length, mem, length);
> > > +	meta_buf->bytesused += length + sizeof(meta->ns) + sizeof(meta->sof);
> > > +
> > > +	uvc_trace(UVC_TRACE_FRAME,
> > > +		  "%s(): t-sys %lu.%09lus, SOF %u, len %u, flags 0x%x, PTS %u, STC 
> %u
> > > frame SOF %u\n",
> > > +		  __func__, ts.tv_sec, ts.tv_nsec, meta->sof,
> > > +		  meta->length, meta->flags, has_pts ? *(u32 *)meta->buf : 0,
> > > +		  has_scr ? *(u32 *)scr : 0,
> > > +		  has_scr ? *(u32 *)(scr + 4) & 0x7ff : 0);
> > > +}
> 
> [snip]
> 
> > For your convenience I've rebased your patch series on top of the two
> > patches I mentioned and added another patch on top that contains fixes for
> > all the small issues mentioned above. The result is available at

In your rebased version you've also dropped the hunk for 
drivers/media/v4l2-core/v4l2-ioctl.c adding a description for the new 
V4L2_META_FMT_UVC format - is that on purpose?

Thanks
Guennadi

> > 
> > 	git://linuxtv.org/pinchartl/media.git uvc/metadata
> > 
> > There are just a handful of issues or questions I haven't addressed, if we
> > handle them I think we'll be good to go.
> 
> ... updated the above branch with a rebased version of the series.
> 
> -- 
> Regards,
> 
> Laurent Pinchart
>