RE: [PATCH 3/3] v4l: Set proper timestamp type in selected drivers which use videobuf2

[Kamil Debski <k.debski@samsung.com>]

Hi,
> From: Sylwester Nawrocki [mailto:s.nawrocki@samsung.com]
> Sent: Tuesday, January 22, 2013 11:38 AM
> 
> Hi,
> 
> On 01/21/2013 03:07 PM, Kamil Debski wrote:
> >> How about making MONOTONIC timestamps default instead, or at least
> >> assigning all drivers something else than UNKNOWN?
> >
> > So why did you add the UNKNOWN flag?
> >
> > The way I see it - UNKNOWN is the default and the one who coded the
> > driver will set it to either MONOTONIC or COPY if it is one of these
> > two. It won't be changed otherwise. There are drivers, which do not
> > fill the timestamp field at all:
> > - drivers/media/platform/coda.c
> > - drivers/media/platform/exynos-gsc/gsc-m2m.c
> 
> Hmm, there is already a patch queued for this driver. It was intended
> for v3.8 but has been delayed to 3.9.
> 
> http://git.linuxtv.org/media_tree.git/commitdiff/f60e160e126bdd8f0d928c
> d8b3fce54659597394
> 
> > - drivers/media/platform/m2m-deinterlace.c
> > - drivers/media/platform/mx2_emmaprp.c
> > - drivers/media/platform/s5p-fimc/fimc-m2m.c
> > - drivers/media/platform/s5p-g2d.c
> > - drivers/media/platform/s5p-jpeg/jpeg-core.c
> >
> > The way you did it in your patches left no room for any kind of
> > choice. I did comment at least twice about mem-2-mem devices in your
> > RFCs, if I remember correctly. I think Sylwester was also writing
> > about this.
> > Still everything got marked as MONOTONIC.
> >
> > If we were to assume that there were no other timestamp types then
> > monotonic (which is not true, but this was your assumption), then
> what
> > was the reason to add this timestamp framework?
> 
> Hmm, we could likely leave MONOTONIC as the default timestamp type. It
> doesn't really matter what is the default, as long as drivers are
> provided with an API to override it.
> 
> The reason why the above drivers don't do anything with
> v4l2_buffer::timestamp field is there is no clear definitions at the
> specification for mem-to-mem devices. We are working here on a Video
> Memory-to-memory Interface DocBook documentation.
> 
> I think we will need a way to tell user space that timestamps are
> copied from OUTPUT to CAPTURE buffer queue. At least that's what seems
> more useful for applications. i.e. copying timestamps, rather than
> filling them with the monotonic clock value.
> 
> OTOH I'm not certain what's the main purpose of such copied timestamps,
> is it to identify which CAPTURE buffer comes from which OUTPUT buffer ?
> 

Yes, indeed. This is especially useful when the CAPTURE buffers can be
returned in an order different than the order of corresponding OUTPUT
buffers.

Best wishes,
-- 
Kamil Debski
Linux Platform Group
Samsung Poland R&D Center