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Subject: Re: [PATCH] v4l2-ctrl: add control for thumnails
To:     Stanimir Varbanov <stanimir.varbanov@linaro.org>,
        linux-media@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     Philipp Zabel <p.zabel@pengutronix.de>,
        Ezequiel Garcia <ezequiel@collabora.com>,
        Paul Kocialkowski <paul.kocialkowski@bootlin.com>,
        Nicolas Dufresne <nicolas@ndufresne.ca>,
        Tomasz Figa <tfiga@chromium.org>
References: <20200526085446.30956-1-stanimir.varbanov@linaro.org>
 <65ac9697-a43f-7025-e6fe-69c4a44c6d9a@xs4all.nl>
 <ce28a840-a763-6700-16dd-d708e570d55c@linaro.org>
 <d02c6cd0-a502-dc52-519e-54b6328d5373@linaro.org>
 <544b19dd-4883-bae0-b488-46c856eb207d@xs4all.nl>
 <6f71931a-9060-a399-835c-a1cf1f05dc79@linaro.org>
From:   Hans Verkuil <hverkuil-cisco@xs4all.nl>
Message-ID: <1bc10e88-8cbe-3da9-daeb-d015f42d7acc@xs4all.nl>
Date:   Thu, 4 Jun 2020 14:56:31 +0200
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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.

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.
>>>
>>
>