linux-kernel - Re: [RFC 07/12] media: uapi: h264: Add DPB entry field reference flags

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Message-ID: <cf4a5d3f-934f-de59-b5df-97b23e30cb1b@kwiboo.se>
Date:   Sat, 11 Jul 2020 10:21:57 +0000 (UTC)
From:   Jonas Karlman <jonas@...boo.se>
To:     Nicolas Dufresne <nicolas.dufresne@...labora.com>,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Boris Brezillon <boris.brezillon@...labora.com>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil@...all.nl>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "linux-rockchip@...ts.infradead.org" 
        <linux-rockchip@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 07/12] media: uapi: h264: Add DPB entry field reference
 flags

On 2020-07-10 23:49, Nicolas Dufresne wrote:
> Le vendredi 10 juillet 2020 à 09:25 -0300, Ezequiel Garcia a écrit :
>> +Nicolas
>>
>> On Fri, 2020-07-10 at 14:05 +0200, Boris Brezillon wrote:
>>> On Fri, 10 Jul 2020 08:50:28 -0300
>>> Ezequiel Garcia <ezequiel@...labora.com> wrote:
>>>
>>>> On Fri, 2020-07-10 at 10:13 +0200, Boris Brezillon wrote:
>>>>> On Fri, 10 Jul 2020 01:21:07 -0300
>>>>> Ezequiel Garcia <ezequiel@...labora.com> wrote:
>>>>>   
>>>>>> Hello Jonas,
>>>>>>
>>>>>> In the context of the uAPI cleanup,
>>>>>> I'm revisiting this patch.
>>>>>>
>>>>>> On Sun, 2019-09-01 at 12:45 +0000, Jonas Karlman 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@...boo.se>
>>>>>>> ---
>>>>>>>  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 bc5dd8e76567..eb6c32668ad7 100644
>>>>>>> --- a/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
>>>>>>> +++ b/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
>>>>>>> @@ -2022,6 +2022,18 @@ 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've been going thru the H264 spec and I'm unsure,
>>>>>> are all these flags semantically needed?
>>>>>>
>>>>>> For instance, if one of REF_BOTTOM or REF_TOP (or both)
>>>>>> are set, doesn't that indicate it's a field picture?
>>>>>>
>>>>>> Or conversely, if neither REF_BOTTOM or REF_TOP are set,
>>>>>> then it's a frame picture?  
>>>>>
>>>>> I think that's what I was trying to do here [1]
>>>>>
>>>>> [1]https://patchwork.kernel.org/patch/11392095/  
>>>>
>>>> Right. Aren't we missing a DPB_ENTRY_FLAG_TOP_FIELD?
>>>>
>>>> If I understand correctly, the DPB can contain:
>>>>
>>>> * frames (FLAG_FIELD not set)
>>>> * a field pair, with a single field (FLAG_FIELD and either TOP or BOTTOM).
>>>> * a field pair, with boths fields (FLAG_FIELD and both TOP or BOTTOM).
>>>
>>> Well, my understand is that, if the buffer contains both a TOP and
>>> BOTTOM field, it actually becomes a full frame, so you actually have
>>> those cases:
>>>
>>> * FLAG_FIELD not set: this a frame (note that a TOP/BOTTOM field
>>>   decoded buffer can become of frame if it's complemented with the
>>>   missing field later during the decoding)
>>> * FLAG_FIELD set + BOTTOM_FIELD not set: this is a TOP field
>>> * FLAG_FIELD set + BOTTOM_FIELD set: this is a BOTTOM field
>>> * FLAG_FIELD not set + BOTTOM_FIELD set: invalid combination
> 
> Let's admit, while this work, it's odd. Can we just move to that instewad ?
> 
>   FLAG_TOP_FIELD
>   FLAG_BOTTOM_FIELD
>   FLAG_FRAME = (FLAG_TOP_FIELD | FLAG_BOTTOM_FIELD)
> 
> So it can be used as a flag, but also is a proper enum and there is no longer an
> invalid combination.
>   
>>>
>>> but I might be wrong.

There seems to be some misunderstanding here, the top/bottom flagging should
not be used to describe if the picture is a field, field pair or frame, it
should be used to flag if a frame or the top and/or bottom field (in case of
a field pair) is "used for short-term reference".

FLAG_TOP_REF
FLAG_BOTTOM_REF
FLAG_FRAME_REF = (FLAG_TOP_REF | FLAG_BOTTOM_REF)

Would be a more appropriate naming.

The FIELD_PIC flag would then be used to describe if the picture is a
reference frame or a complementary reference field pair.

As described in hantro h264 driver [1] the MV buffer is split in two
for field encoded frames, and I guess the rkvdec block does something
similar and therefore the HW blocks probably needs to know if the reference
picture is a reference frame or a complementary reference field pair.
It should be possible to keep such state in driver but since such information
was easily available in ffmpeg and the driver being "stateless" using a flag
seamed like a good choice at the time.

Please note that I have not done any test without the "field pic" flagging
but both mpp and the imx/hantro reference code are configuring this bit.

[1] https://git.linuxtv.org/media_tree.git/tree/drivers/staging/media/hantro/hantro_g1_h264_dec.c#n265

Regards,
Jonas

>>
>> Yes, perhaps that's correct. I was trying to think strictly
>> in terms of the H264 semantics, to define a clean interface.
>>
>> From the mpp code, looks like the above is enough for rkvdec
>> (although I haven't done any tests).
>>
>> Ezequiel
>>
>>
>>
>