Re: [RFC PATCH 1/2] media: docs-rst: Add decoder UAPI specification to Codec Interfaces

[Tomasz Figa]

On Thu, Jun 7, 2018 at 4:22 PM Hans Verkuil <hverkuil@xxxxxxxxx> wrote:
>
> On 06/05/2018 03:10 PM, Dave Stevenson wrote:
> > Hi Tomasz.
> >
> > Thanks for formalising this.
> > I'm working on a stateful V4L2 codec driver on the Raspberry Pi and
> > was having to deduce various implementation details from other
> > drivers. I know how much we all tend to hate having to write
> > documentation, but it is useful to have.
> >
> > On 5 June 2018 at 11:33, Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:
> >> Due to complexity of the video decoding process, the V4L2 drivers of
> >> stateful decoder hardware require specific sequencies of V4L2 API calls
> >> to be followed. These include capability enumeration, initialization,
> >> decoding, seek, pause, dynamic resolution change, flush and end of
> >> stream.
> >>
> >> Specifics of the above have been discussed during Media Workshops at
> >> LinuxCon Europe 2012 in Barcelona and then later Embedded Linux
> >> Conference Europe 2014 in DÃsseldorf. The de facto Codec API that
> >> originated at those events was later implemented by the drivers we already
> >> have merged in mainline, such as s5p-mfc or mtk-vcodec.
> >>
> >> The only thing missing was the real specification included as a part of
> >> Linux Media documentation. Fix it now and document the decoder part of
> >> the Codec API.
> >>
> >> Signed-off-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> >> ---
> >>  Documentation/media/uapi/v4l/dev-codec.rst | 771 +++++++++++++++++++++
> >>  Documentation/media/uapi/v4l/v4l2.rst      |  14 +-
> >>  2 files changed, 784 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/Documentation/media/uapi/v4l/dev-codec.rst b/Documentation/media/uapi/v4l/dev-codec.rst
> >> index c61e938bd8dc..0483b10c205e 100644
> >> --- a/Documentation/media/uapi/v4l/dev-codec.rst
> >> +++ b/Documentation/media/uapi/v4l/dev-codec.rst
> >> @@ -34,3 +34,774 @@ the codec and reprogram it whenever another file handler gets access.
> >>  This is different from the usual video node behavior where the video
> >>  properties are global to the device (i.e. changing something through one
> >>  file handle is visible through another file handle).
> >
> > I know this isn't part of the changes, but raises a question in
> > v4l2-compliance (so probably one for Hans).
> > testUnlimitedOpens tries opening the device 100 times. On a normal
> > device this isn't a significant overhead, but when you're allocating
> > resources on a per instance basis it quickly adds up.
> > Internally I have state that has a limit of 64 codec instances (either
> > encode or decode), so either I allocate at start_streaming and fail on
> > the 65th one, or I fail on open. I generally take the view that
> > failing early is a good thing.
> > Opinions? Is 100 instances of an M2M device really sensible?
>
> Resources should not be allocated by the driver until needed (i.e. the
> queue_setup op is a good place for that).
>
> It is perfectly legal to open a video node just to call QUERYCAP to
> see what it is, and I don't expect that to allocate any hardware resources.
> And if I want to open it 100 times, then that should just work.
>
> It is *always* wrong to limit the number of open arbitrarily.

That's a valid point indeed. Besides the querying use case, userspace
might just want to pre-open a bigger number of instances, but it
doesn't mean that they would be streaming all at the same time indeed.

Best regards,
Tomasz