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

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Message-ID: <ec409a20cf266938fa804bd685a8ed3f1e30b0be.camel@collabora.com>
Date:   Fri, 10 Jul 2020 09:18:20 -0300
From:   Ezequiel Garcia <ezequiel@...labora.com>
To:     Jonas Karlman <jonas@...boo.se>,
        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 Fri, 2020-07-10 at 08:48 +0000, Jonas Karlman wrote:
> On 2020-07-10 10:13, 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?
> 
> These flags would only indicate how the frame / field pair / field is
> referenced and not if the DPB entry was decoded as a frame or field pair.
> 

I believe _how_ the picture is referenced shouldn't (or can't?) be signaled
in the DPB representation. It seems Jernej's [1] which properly adds a
flag for each entry in ref_pic_list0 is the right way.

https://patchwork.linuxtv.org/patch/64289/

> Both hantro and rkvdec needs to know how the referenced frame / field pair
> was decoded (not how it is referenced), my best guess is that MV is stored
> differently for a frame (linear) and field pair (buffer split in two).
> 
> I think we should be able to track how the buffer was decoded similar to
> how VP9 keep track of buffer width/height.
> 
> When I played with interlaced decoding of rkvdec a few weeks ago I
> reverted flags to something similar as my initial rfc patch, see [1].
> I guess it should be possible to keep current flags and track field_pic
> in driver, some macro to simplify check for top/bottom ref could be
> useful if flags is kept as-is.
> 
> I am hoping to find some time next week to revisit hantro interlaced
> and refine rkvdec interlaced support.
> 
> [1] https://github.com/Kwiboo/linux-rockchip/compare/da52ca6f8d2284aebea2d0b99d254b64922faa2d...c9f04cd9bc65eda0da713f4ce1c77eeb1960bd70
> 

Yup, I noticed this and it's why I started looking at the uAPI side
of the DPB.

It seems to me all we are missing is further clarification
of the meaning of each DPB_ENTRY_FLAG (possibly adding/removing
flags).

>From this snippet:

		if (dpb[i].flags & V4L2_H264_DPB_ENTRY_FLAG_FIELD_PIC)
			refer_addr |= RKVDEC_FIELD_REF;
		if (dpb[i].flags & V4L2_H264_DPB_ENTRY_FLAG_TOP_REF)
			refer_addr |= RKVDEC_TOPFIELD_USED_REF;
		if (dpb[i].flags & V4L2_H264_DPB_ENTRY_FLAG_BOTTOM_REF)
			refer_addr |= RKVDEC_BOTFIELD_USED_REF;
		if (dpb[i].flags & V4L2_H264_DPB_ENTRY_FLAG_ACTIVE)
			refer_addr |= RKVDEC_COLMV_USED_FLAG_REF;

Which of these flags are needed, i.e. which are required to fully
describe a picture stored in the DPB?

Also, since we are here, I wonder what is exactly RKVDEC COLMV
and what's the condition for RKVDEC_COLMV_USED_FLAG_REF.

Thanks a lot!
Ezequiel

> Regards,
> Jonas
> 
> > > 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/
> >