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

From: Nicolas Dufresne <nicolas.dufresne@collabora.com>
To: Ezequiel Garcia <ezequiel@collabora.com>,
	Boris Brezillon <boris.brezillon@collabora.com>
Cc: Jonas Karlman <jonas@kwiboo.se>,
	Mauro Carvalho Chehab <mchehab@kernel.org>,
	Hans Verkuil <hverkuil@xs4all.nl>,
	Philipp Zabel <p.zabel@pengutronix.de>,
	Paul Kocialkowski <paul.kocialkowski@bootlin.com>,
	"linux-media@vger.kernel.org" <linux-media@vger.kernel.org>,
	"linux-rockchip@lists.infradead.org" 
	<linux-rockchip@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [RFC 07/12] media: uapi: h264: Add DPB entry field reference flags
Date: Fri, 10 Jul 2020 17:49:07 -0400	[thread overview]
Message-ID: <9a897ef99048077233685b121b03bd750e4d4a83.camel@collabora.com> (raw)
In-Reply-To: <05b6cff6ba230c0ab6a562e17926d8503e2dfadd.camel@collabora.com>

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@collabora.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@collabora.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@kwiboo.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.
> 
> 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
> 
> 
> 