Re: [PATCH] v4l2-ctrl: add control for thumnails

[Tomasz Figa <tfiga@chromium.org>]

On Tue, Oct 13, 2020 at 2:52 PM Stanimir Varbanov
<stanimir.varbanov@linaro.org> wrote:
>
> Hi,
>
> On 6/4/20 3:57 PM, Tomasz Figa wrote:
> > On Thu, Jun 4, 2020 at 2:56 PM Hans Verkuil <hverkuil-cisco@xs4all.nl> wrote:
> >>
> >> On 04/06/2020 14:34, Stanimir Varbanov wrote:
> >>> Hi Hans,
> >>>
> >>> On 6/4/20 12:08 PM, Hans Verkuil wrote:
> >>>> On 04/06/2020 11:02, Stanimir Varbanov wrote:
> >>>>> Hi Hans,
> >>>>>
> >>>>> On 5/27/20 12:53 AM, Stanimir Varbanov wrote:
> >>>>>> Hi Hans,
> >>>>>>
> >>>>>> On 5/26/20 3:04 PM, Hans Verkuil wrote:
> >>>>>>> On 26/05/2020 10:54, Stanimir Varbanov wrote:
> >>>>>>>> Add v4l2 control for decoder thumbnail.
> >>>>>>>>
> >>>>>>>> Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
> >>>>>>>> ---
> >>>>>>>>  Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst | 7 +++++++
> >>>>>>>>  drivers/media/v4l2-core/v4l2-ctrls.c                      | 2 ++
> >>>>>>>>  include/uapi/linux/v4l2-controls.h                        | 2 ++
> >>>>>>>>  3 files changed, 11 insertions(+)
> >>>>>>>>
> >>>>>>>> diff --git a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
> >>>>>>>> index d0d506a444b1..e838e410651b 100644
> >>>>>>>> --- a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
> >>>>>>>> +++ b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
> >>>>>>>> @@ -3726,6 +3726,13 @@ enum v4l2_mpeg_video_hevc_size_of_length_field -
> >>>>>>>>      disables generating SPS and PPS at every IDR. Setting it to one enables
> >>>>>>>>      generating SPS and PPS at every IDR.
> >>>>>>>>
> >>>>>>>> +``V4L2_CID_MPEG_VIDEO_DECODER_THUMBNAIL (button)``
> >>>>>>>> +    Instructs the decoder to produce immediate output. The decoder should
> >>>>>>>> +    consume first input buffer for progressive stream (or first two buffers
> >>>>>>>> +    for interlace). Decoder should not allocate more output buffers that it
> >>>>>>>> +    is required to consume one input frame. Usually the decoder input
> >>>>>>>> +    buffers will contain only I/IDR frames but it is not mandatory.
> >>>>>>>
> >>>>>>> This is very vague. It doesn't explain why the control is called 'THUMBNAIL',
> >>>>>>> but more importantly it doesn't explain how this relates to normal decoding.
> >>>>>>
> >>>>>> If in the normal decode the capture queue buffers are 5, in the
> >>>>>> thumbnail mode the number of buffers will be only 1 (if the bitstream is
> >>>>>> progressive) and this will guarantee low memory usage. The other
> >>>>>> difference is that the decoder will produce decoded frames (without
> >>>>>> errors) only for I/IDR (sync frames).
> >>>>
> >>>> Isn't this really a "DECODE_SYNC_FRAMES_ONLY" control? That's what it does,
> >>>> right? Skip any B/P frames and only decode sync frames.
> >>>
> >>> Yes, it is.
> >>> To me V4L2_CID_MPEG_VIDEO_DECODE_SYNC_FRAMES sounds better. If you are
> >>> fine I can send a new patch.
> >>>
> >>> The definition of "sync frames" is a bit difficult for codec-agnostic
> >>> controls. Is it sound better "INTRA", DECODE_INTRA_FRAMES (ONLY)?
> >>
> >> INTRA is better. DECODE_INTRA_FRAMES_ONLY is a good name, I think.
> >>
> >> Thumbnail creation can be given as an example in the description of the
> >> control, but that's just a use-case.
> >
> > How about the other aspect of the behavior?
> >
> > "Decoder should not allocate more output buffers that it
> > is required to consume one input frame."
> >
>
> In fact I have to refine this; It looks like that picture type decode vs
> thumbnail are two different modes.
>
> Thumbnail mode - first frame decode without additional memory (one input
> buffer and one output buffer). The first frame can be even non-intra
> frame as well.

How comes it can be decoded without additional memory to store the
reference frames then?

> Also no matter frame parser is sending, the decoder will
> try to produce output for thumbnail generation.

Well, thinking of it now, actually DECODE_INTRA_FRAMES_ONLY makes
sense here - if it is set, only 1 CAPTURE buffer could be allowed
indeed, because no reference frames are needed for decoding.

If inter frames are needed, I believe full DPB needs to be allocated,
because it all depends on the stream how the references are set, so
this is equivalent to normal decoding.

best regards,
Tomasz

>
> > Best regards,
> > Tomasz
> >
> >>
> >> Regards,
> >>
> >>         Hans
> >>
> >>>
> >>>>
> >>>> That this is useful for creating thumbnails is just a specific use-case.
> >>>>
> >>>> Regards,
> >>>>
> >>>>      Hans
> >>>>
> >>>>>>
> >>>>>>>
> >>>>>>> I.e. if you are decoding and 'press' this control, what happens then?
> >>>>>>
> >>>>>> Might be the button type wasn't great idea. In fact the control should
> >>>>>> be set before streamon so that the driver returns min_capture_bufs 1.
> >>>>>>
> >>>>>>>
> >>>>>>> What exactly is the use-case?
> >>>>>>
> >>>>>> It could be used to generate thumbnails of all video clips in a folder
> >>>>>> or when you open a Gallery application on your phone.
> >>>>>>
> >>>>>
> >>>>> What is your opinion on that control? I could consider to make it Venus
> >>>>> custom control but from the use-case it looks other drivers also can
> >>>>> benefit of it.
> >>>>>
> >>>>> I tried to make more generic one [1] but it looks it will be too difficult.
> >>>>>
> >>>>
> >>>
> >>
>
> --
> regards,
> Stan