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

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Message-ID: <c10bb0992a8bfb751edf66bdb7752da842877281.camel@collabora.com>
Date:   Sun, 12 Jul 2020 19:59:07 -0300
From:   Ezequiel Garcia <ezequiel@...labora.com>
To:     Jonas Karlman <jonas@...boo.se>,
        Nicolas Dufresne <nicolas.dufresne@...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 Sat, 2020-07-11 at 10:21 +0000, Jonas Karlman wrote:
> 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".
> 

I'm not sure why "used for short-term reference" instead
of "used for 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
> 

How about this:

#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          0x08

enum v4l2_h264_dpb_reference {
        V4L2_H264_DPB_TOP_REF = 0x1,
        V4L2_H264_DPB_BOTTOM_REF = 0x2,
        V4L2_H264_DPB_FRAME_REF = 0x3,
};

With the following semantics (which should be
specified in the documentation):

* VALID: non-empty DPB entry.
* ACTIVE: picture is marked as "used for reference" (short-term or long-term).
* LONG_TERM: picture is marked as "used for long-term".
* FIELD: picture is a single field, or a complementary field pair. 

The v4l2_h264_dpb_reference enum would flag which
of the fields as used for reference.

This enum seems less ambiguous and easier to use for both
drivers and applications.

I am not exactly sure why a driver would ever need to
configure an "unused for reference" decoded picture
(i.e. VALID=1, ACTIVE=0), but I guess it's just clearer
to include this in the interface.

Thanks,
Ezequiel

> 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
> > > 
> > > 
> > >