Re: [PATCH 08/10] media: uapi: h264: Clean slice invariants syntax elements

[Tomasz Figa <tfiga@chromium.org>]

On Mon, Jul 27, 2020 at 4:39 PM Ezequiel Garcia <ezequiel@collabora.com> wrote:
>
> Hi Alexandre,
>
> Thanks a lot for the review.
>
> On Sat, 2020-07-25 at 23:34 +0900, Alexandre Courbot wrote:
> > On Thu, Jul 16, 2020 at 5:23 AM Ezequiel Garcia <ezequiel@collabora.com> wrote:
> > > The H.264 specification requires in its "Slice header semantics"
> > > section that the following values shall be the same in all slice headers:
> > >
> > >   pic_parameter_set_id
> > >   frame_num
> > >   field_pic_flag
> > >   bottom_field_flag
> > >   idr_pic_id
> > >   pic_order_cnt_lsb
> > >   delta_pic_order_cnt_bottom
> > >   delta_pic_order_cnt[ 0 ]
> > >   delta_pic_order_cnt[ 1 ]
> > >   sp_for_switch_flag
> > >   slice_group_change_cycle
> > >
> > > and can therefore be moved to the per-frame decode parameters control.
> >
> > I am really not a H.264 expert, so this question may not be relevant,
>
> All questions are welcome. I'm more than happy to discuss this patchset.
>
> > but are these values specified for every slice header in the
> > bitstream, or are they specified only once per frame?
> >
> > I am asking this because it would certainly make user-space code
> > simpler if we could remain as close to the bitstream as possible. If
> > these values are specified once per slice, then factorizing them would
> > leave user-space with the burden of deciding what to do if they change
> > across slices.
> >
> > Note that this is a double-edged sword, because it is not necessarily
> > better to leave the firmware in charge of deciding what to do in such
> > a case. :) So hopefully these are only specified once per frame in the
> > bitstream, in which case your proposal makes complete sense.
>
> Frame-based hardwares accelerators such as Hantro and Rockchip VDEC
> are doing the slice header parsing themselves. Therefore, the
> driver is not really parsing these fields on each slice header.
>
> Currently, we are already using only the first slice in a frame,
> as you can see from:
>
>         if (slices[0].flags & V4L2_H264_SLICE_FLAG_FIELD_PIC)
>                 reg |= G1_REG_DEC_CTRL0_PIC_FIELDMODE_E;
>
> Even if these fields are transported in the slice header,
> I think it makes sense for us to split them into the decode params
> (per-frame) control.
>
> They are really specified to be the same across all slices,
> so even I'd say if a bitstream violates this, it's likely
> either a corrupted bitstream or an encoder bug.
>
> OTOH, one thing this makes me realize is that the slice params control
> is wrongly specified as an array.

It is _not_.

> Namely, this text
> should be removed:
>
>        This structure is expected to be passed as an array, with one
>        entry for each slice included in the bitstream buffer.
>
> As the API is really not defined that way.
>
> I'll remove that on next iteration.

The v4l2_ctrl_h264_slice_params struct has more data than those that
are deemed to be the same across all the slices. A remarkable example
are the size and start_byte_offset fields.