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

[Stanimir Varbanov]

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@xxxxxxxxxx>
>>> ---
>>>  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).
> 
>>
>> 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

[1] https://www.spinics.net/lists/linux-media/msg167794.html