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

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Message-ID: <20191115084548.45ea5b8d@collabora.com>
Date:   Fri, 15 Nov 2019 08:45:48 +0100
From:   Boris Brezillon <boris.brezillon@...labora.com>
To:     Jonas Karlman <jonas@...boo.se>
Cc:     Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Hans Verkuil <hverkuil@...all.nl>,
        Tomasz Figa <tfiga@...omium.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v2 09/10] media: uapi: h264: Add DPB entry field reference
 flags

On Fri, 15 Nov 2019 06:26:54 +0000
Jonas Karlman <jonas@...boo.se> wrote:

> On 2019-10-31 11:20, Boris Brezillon wrote:
> > On Tue, 29 Oct 2019 01:26:01 +0000
> > Jonas Karlman <jonas@...boo.se> 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 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.