Re: [RFC v2 09/10] media: uapi: h264: Add DPB entry field reference flags

[Jonas Karlman]

On 2019-11-15 08:45, Boris Brezillon wrote:
> On Fri, 15 Nov 2019 06:26:54 +0000
> Jonas Karlman <jonas@xxxxxxxxx> wrote:
>
>> On 2019-10-31 11:20, Boris Brezillon wrote:
>>> On Tue, 29 Oct 2019 01:26:01 +0000
>>> Jonas Karlman <jonas@xxxxxxxxx> wrote:
>>>  
>>>> Add DPB entry flags to help indicate when a reference frame is a field picture
>>>> and how the DPB entry is referenced, top or bottom field or full frame.
>>>>
>>>> Signed-off-by: Jonas Karlman <jonas@xxxxxxxxx>
>>>> ---
>>>>  Documentation/media/uapi/v4l/ext-ctrls-codec.rst | 12 ++++++++++++
>>>>  include/media/h264-ctrls.h                       |  4 ++++
>>>>  2 files changed, 16 insertions(+)
>>>>
>>>> diff --git a/Documentation/media/uapi/v4l/ext-ctrls-codec.rst b/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
>>>> index 28313c0f4e7c..d472a54d1c4d 100644
>>>> --- a/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
>>>> +++ b/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
>>>> @@ -2028,6 +2028,18 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
>>>>      * - ``V4L2_H264_DPB_ENTRY_FLAG_LONG_TERM``
>>>>        - 0x00000004
>>>>        - The DPB entry is a long term reference frame
>>>> +    * - ``V4L2_H264_DPB_ENTRY_FLAG_FIELD_PICTURE``
>>>> +      - 0x00000008
>>>> +      - The DPB entry is a field picture
>>>> +    * - ``V4L2_H264_DPB_ENTRY_FLAG_REF_TOP``
>>>> +      - 0x00000010
>>>> +      - The DPB entry is a top field reference
>>>> +    * - ``V4L2_H264_DPB_ENTRY_FLAG_REF_BOTTOM``
>>>> +      - 0x00000020
>>>> +      - The DPB entry is a bottom field reference
>>>> +    * - ``V4L2_H264_DPB_ENTRY_FLAG_REF_FRAME``
>>>> +      - 0x00000030
>>>> +      - The DPB entry is a reference frame
>>>>  
>>>>  ``V4L2_CID_MPEG_VIDEO_H264_DECODE_MODE (enum)``
>>>>      Specifies the decoding mode to use. Currently exposes slice-based and
>>>> diff --git a/include/media/h264-ctrls.h b/include/media/h264-ctrls.h
>>>> index e877bf1d537c..76020ebd1e6c 100644
>>>> --- a/include/media/h264-ctrls.h
>>>> +++ b/include/media/h264-ctrls.h
>>>> @@ -185,6 +185,10 @@ struct v4l2_ctrl_h264_slice_params {
>>>>  #define V4L2_H264_DPB_ENTRY_FLAG_VALID		0x01
>>>>  #define V4L2_H264_DPB_ENTRY_FLAG_ACTIVE		0x02
>>>>  #define V4L2_H264_DPB_ENTRY_FLAG_LONG_TERM	0x04
>>>> +#define V4L2_H264_DPB_ENTRY_FLAG_FIELD_PICTURE	0x08
>>>> +#define V4L2_H264_DPB_ENTRY_FLAG_REF_TOP	0x10
>>>> +#define V4L2_H264_DPB_ENTRY_FLAG_REF_BOTTOM	0x20
>>>> +#define V4L2_H264_DPB_ENTRY_FLAG_REF_FRAME	0x30  
>>> I don't remember all the details, but do we really need 3 flags?
>>> Maybe I'm wrong, but it looks like the following combination doesn't
>>> make sense:
>>>
>>> - FIELD_PICTURE + REF_FRAME: if it's a full frame ref it should
>>>   contain both top and bottom fields right, so it's no longer a
>>>   FIELD_PICTURE, is it?
>>>
>>> Can't we just have 2 flags?
>>>
>>> FIELD_PICTURE		0x08
>>> FIELD_REF_TOP		0x10 (meaning that FIELD_REF_BOTTOM is
>>> 			      0x00)
>>>
>>> and then have the following combinations:
>>>
>>> top field ref => FIELD_PICTURE | FIELD_REF_TOP
>>> bottom field ref => FIELD_PICTURE
>>> full frame ref => 0x0  
>> I am not sure and will need to look closer at spec and what ffmpeg is doing.
>> These flags was mostly inspired by the information ffmpeg stores in
>> H264Picture->reference and H264Picture->field_picture.
>>
>> I also believe that the new FLAG_REF_TOP/BOTTOM may make FLAG_ACTIVE obsolete.
>>
>> active => flags & FLAG_REF_FRAME
> Can't we keep the ACTIVE flag and drop the REF_FRAME one then? AFAIU,
> all we need to know in addition to what we already have is:
>
> * is the reference a full-frame or a field?
> * if it is a field, which one (top or bottom)?
>
> Hence my suggestion to keep only 2 flags.

I think we also need to know if the reference frame was a field or a full-frame picture
not just how it is referenced, I think there was an issue when I originally tried a similar
approach as you suggested, but it was too long time ago as I seem to have forgotten the details.

Will do more testing and see if I can refresh my memory.

Best regards,
Jonas