Re: [PATCH 06/10] media: uapi: h264: Cleanup DPB entry interface

[Ezequiel Garcia <ezequiel@collabora.com>]

On Wed, 2020-07-22 at 18:09 +0200, Jernej Škrabec wrote:
> Hi!
> 
> Dne sreda, 15. julij 2020 ob 22:22:29 CEST je Ezequiel Garcia napisal(a):
> > As discussed recently, the current interface for the
> > Decoded Picture Buffer is not enough to properly
> > support field coding.
> > 
> > This commit introduces enough semantics to support
> > frame and field coding, and to signal how DPB entries
> > are "used for reference".
> > 
> > Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
> > ---
> >  .../media/v4l/ext-ctrls-codec.rst             | 46 ++++++++++++-------
> >  drivers/media/v4l2-core/v4l2-h264.c           |  4 +-
> >  drivers/staging/media/rkvdec/rkvdec-h264.c    |  8 ++--
> >  include/media/h264-ctrls.h                    |  8 +++-
> >  4 files changed, 42 insertions(+), 24 deletions(-)
> > 
> > diff --git a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
> > b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst index
> > dd8e5a2e8986..46d4c8c6ad47 100644
> > --- a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
> > +++ b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
> > @@ -2058,10 +2058,35 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
> > * - __s32
> >        - ``bottom_field_order_cnt``
> >        -
> > +    * - enum :c:type:`v4l2_h264_dpb_reference`
> > +      - ``reference``
> > +      - Specifies how the DPB entry is referenced.
> >      * - __u32
> >        - ``flags``
> >        - See :ref:`DPB Entry Flags <h264_dpb_flags>`
> > 
> > +.. c:type:: v4l2_h264_dpb_reference
> > +
> > +.. cssclass:: longtable
> > +
> > +.. flat-table::
> > +    :header-rows:  0
> > +    :stub-columns: 0
> > +    :widths:       1 1 2
> > +
> > +    * - ``V4L2_H264_DPB_TOP_REF``
> > +      - 0x1
> > +      - The top field in field pair is used for
> > +        short-term reference.
> > +    * - ``V4L2_H264_DPB_BOTTOM_REF``
> > +      - 0x2
> > +      - The bottom field in field pair is used for
> > +        short-term reference.
> > +    * - ``V4L2_H264_DPB_FRAME_REF``
> > +      - 0x3
> > +      - The frame (or the top/bottom fields, if it's a field pair)
> > +        is used for short-term reference.
> > +
> >  .. _h264_dpb_flags:
> > 
> >  ``DPB Entries Flags``
> > @@ -2075,29 +2100,16 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
> > 
> >      * - ``V4L2_H264_DPB_ENTRY_FLAG_VALID``
> >        - 0x00000001
> > -      - The DPB entry is valid and should be considered
> > +      - The DPB entry is valid (non-empty) and should be considered.
> >      * - ``V4L2_H264_DPB_ENTRY_FLAG_ACTIVE``
> 
> I'm still not sure that we actually need both flags. Technically, if entry is 
> not used for reference then doesn't need to be present. Am I missing 
> something?
> 

Indeed, that's exactly what I raised during the review of these flags.

However, note that the Cedrus driver seem to do something with
VALID but not ACTIVE DPB entries, although maybe that's not really needed.

If you could check the code and drop the VALID usage, and confirm
ACTIVE is the only flag needed, that would be nice.

Now, OTOH, having an extra flag is not hurting us.
It might be more consistent for applications, as it
would mean simpler setup of the DPB, and more consistent mapping
between application kept DPB and kernel DPB.

Thanks,
Ezequiel

> Best regards,
> Jernej
> 
> >        - 0x00000002
> > -      - The DPB entry is currently being used as a reference frame
> > +      - The DPB entry is used for reference.
> >      * - ``V4L2_H264_DPB_ENTRY_FLAG_LONG_TERM``
> >        - 0x00000004
> > -      - The DPB entry is a long term reference frame
> > +      - The DPB entry is used for long-term reference.
> >      * - ``V4L2_H264_DPB_ENTRY_FLAG_FIELD``
> >        - 0x00000008
> > -      - The DPB entry is a field reference, which means only one of the
> > field -        will be used when decoding the new frame/field. When not set
> > the DPB -        entry is a frame reference (both fields will be used).
> > Note that this -        flag does not say anything about the number of
> > fields contained in the -        reference frame, it just describes the one
> > used to decode the new -        field/frame
> > -    * - ``V4L2_H264_DPB_ENTRY_FLAG_BOTTOM_FIELD``
> > -      - 0x00000010
> > -      - The DPB entry is a bottom field reference (only the bottom field of
> > the -        reference frame is needed to decode the new frame/field). Only
> > valid if -        V4L2_H264_DPB_ENTRY_FLAG_FIELD is set. When
> > -        V4L2_H264_DPB_ENTRY_FLAG_FIELD is set but
> > -        V4L2_H264_DPB_ENTRY_FLAG_BOTTOM_FIELD is not, that means the
> > -        DPB entry is a top field reference
> > +      - The DPB entry is a single field or a complementary field pair.
> > 
> >  ``V4L2_CID_MPEG_VIDEO_H264_DECODE_MODE (enum)``
> >      Specifies the decoding mode to use. Currently exposes slice-based and
> > diff --git a/drivers/media/v4l2-core/v4l2-h264.c
> > b/drivers/media/v4l2-core/v4l2-h264.c index edf6225f0522..306a51683606
> > 100644
> > --- a/drivers/media/v4l2-core/v4l2-h264.c
> > +++ b/drivers/media/v4l2-core/v4l2-h264.c
> > @@ -66,10 +66,10 @@ v4l2_h264_init_reflist_builder(struct
> > v4l2_h264_reflist_builder *b, else
> >  			b->refs[i].frame_num = dpb[i].frame_num;
> > 
> > -		if (!(dpb[i].flags & V4L2_H264_DPB_ENTRY_FLAG_FIELD))
> > +		if (dpb[i].reference & V4L2_H264_DPB_FRAME_REF)
> >  			pic_order_count = 
> min(dpb[i].top_field_order_cnt,
> >  					      
> dpb[i].bottom_field_order_cnt);
> > -		else if (dpb[i].flags & 
> V4L2_H264_DPB_ENTRY_FLAG_BOTTOM_FIELD)
> > +		else if (dpb[i].reference & V4L2_H264_DPB_BOTTOM_REF)
> >  			pic_order_count = 
> dpb[i].bottom_field_order_cnt;
> >  		else
> >  			pic_order_count = dpb[i].top_field_order_cnt;
> > diff --git a/drivers/staging/media/rkvdec/rkvdec-h264.c
> > b/drivers/staging/media/rkvdec/rkvdec-h264.c index
> > 7b66e2743a4f..57539c630422 100644
> > --- a/drivers/staging/media/rkvdec/rkvdec-h264.c
> > +++ b/drivers/staging/media/rkvdec/rkvdec-h264.c
> > @@ -953,11 +953,11 @@ static void config_registers(struct rkvdec_ctx *ctx,
> >  			     RKVDEC_COLMV_USED_FLAG_REF;
> > 
> >  		if (!(dpb[i].flags & V4L2_H264_DPB_ENTRY_FLAG_FIELD))
> > -			refer_addr |= RKVDEC_TOPFIELD_USED_REF |
> > -				      RKVDEC_BOTFIELD_USED_REF;
> > -		else if (dpb[i].flags & 
> V4L2_H264_DPB_ENTRY_FLAG_BOTTOM_FIELD)
> > +			refer_addr |= RKVDEC_FIELD_REF;
> > +
> > +		if (dpb[i].reference & V4L2_H264_DPB_TOP_REF)
> >  			refer_addr |= RKVDEC_BOTFIELD_USED_REF;
> > -		else
> > +		else if (dpb[i].reference & V4L2_H264_DPB_BOTTOM_REF)
> >  			refer_addr |= RKVDEC_TOPFIELD_USED_REF;
> > 
> >  		writel_relaxed(dpb[i].top_field_order_cnt,
> > diff --git a/include/media/h264-ctrls.h b/include/media/h264-ctrls.h
> > index 620ee8863d74..52f3976b986c 100644
> > --- a/include/media/h264-ctrls.h
> > +++ b/include/media/h264-ctrls.h
> > @@ -202,7 +202,12 @@ struct v4l2_ctrl_h264_slice_params {
> >  #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
> > -#define V4L2_H264_DPB_ENTRY_FLAG_BOTTOM_FIELD	0x10
> > +
> > +enum v4l2_h264_dpb_reference {
> > +	V4L2_H264_DPB_TOP_REF = 0x1,
> > +	V4L2_H264_DPB_BOTTOM_REF = 0x2,
> > +	V4L2_H264_DPB_FRAME_REF = 0x3,
> > +};
> > 
> >  struct v4l2_h264_dpb_entry {
> >  	__u64 reference_ts;
> > @@ -211,6 +216,7 @@ struct v4l2_h264_dpb_entry {
> >  	/* Note that field is indicated by v4l2_buffer.field */
> >  	__s32 top_field_order_cnt;
> >  	__s32 bottom_field_order_cnt;
> > +	enum v4l2_h264_dpb_reference reference;
> >  	__u32 flags; /* V4L2_H264_DPB_ENTRY_FLAG_* */
> >  };
> 
> 
>