On Mon, Sep 9, 2019 at 4:30 PM Philipp Zabel <p.zabel@xxxxxxxxxxxxxx> wrote:
>
> On Mon, 2019-09-09 at 16:09 +0900, Tomasz Figa wrote:
> > On Thu, Sep 5, 2019 at 11:17 PM Nicolas Dufresne
> > <nicolas.dufresne@xxxxxxxxxxxxx> wrote:
> > >
> > > Le jeudi 05 septembre 2019 à 12:39 +0200, Philipp Zabel a écrit :
> > > > On Thu, 2019-09-05 at 19:31 +0900, Tomasz Figa wrote:
> > > > > On Thu, Sep 5, 2019 at 7:15 PM Philipp Zabel <p.zabel@xxxxxxxxxxxxxx> wrote:
> > > > > > This flag tells the kernel whether the slice header contained the
> > > > > > num_ref_idx_l[01]_active_minus1 syntax elements, or whether the
> > > > > > num_ref_idx_l[01]_default_active_minus1 from PPS should be used
> > > > > > instead.
> > > > > >
> > > > > > Signed-off-by: Philipp Zabel <p.zabel@xxxxxxxxxxxxxx>
> > > > > > ---
> > > > > > Documentation/media/uapi/v4l/ext-ctrls-codec.rst | 3 +++
> > > > > > include/media/h264-ctrls.h | 1 +
> > > > > > 2 files changed, 4 insertions(+)
> > > > > >
> > > > > > diff --git a/Documentation/media/uapi/v4l/ext-ctrls-codec.rst b/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
> > > > > > index bc5dd8e76567..451a5b0f2a35 100644
> > > > > > --- a/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
> > > > > > +++ b/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
> > > > > > @@ -1860,6 +1860,9 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
> > > > > > * - ``V4L2_H264_SLICE_FLAG_SP_FOR_SWITCH``
> > > > > > - 0x00000008
> > > > > > -
> > > > > > + * - ``V4L2_H264_SLICE_FLAG_NUM_REF_IDX_ACTIVE_OVERRIDE
> > > > > > + - 0x00000010
> > > > > > + - Corresponds to the num_ref_idx_active_override_flag syntax element.
> > > > > >
> > > > >
> > > > > As far as I remember, the idea was for the userspace to always put the
> > > > > right num_ref_idx in the slice_params and the drivers always use that.
> > > > > Was there any problem with that?
> > > >
> > > > I don't think so, at least for currently known hardware.
> > > >
> > > > In that case we should drop the unused
> > > > num_ref_idx_l[01]_default_active_minus1 fields from struct
> > > > v4l2_ctrl_h264_pps and document that userspace should fill
> > > > the defaults into v4l2_ctrl_h264_slice_params themselves if
> > > > num_ref_idx_active_override_flag wasn't set.
> > >
> > > It might have been added in a previous effort to allow reconstructing
> > > the bitstream from the structures.
> >
> > Wouldn't one still be able to reconstruct a valid (but not exact)
> > stream without that flag, given the assumption above?
>
> If the Hantro G1, as appears to be the case, parses the slice header and
> decides itself whether to use the override from the slice or the default
> that was written to a register, it needs the
> num_ref_idx_l[01]_default_active_minus1 field to fill the register, but
> doesn't need either the num_ref_idx_l[01]_active_minus1 override nor the
> flag.
>
> A decoder that doesn't parse the slice header can always be told to use
> the override (thus no need to have the flag in the uapi), if userspace
> fills the default into the override fields as a fallback. Such a decoder
> does need the num_ref_idx_l[01]_active_minus1 override, but doesn't need
> the num_ref_idx_l[01]_default_active_minus1 field nor the flag.
>
> That is my current understanding of the intention behind this interface,
> I hope this is accurate.
> I've tried to make the docs reflect this in ("media: uapi: h264: clarify
> num_ref_idx_l[01]_(default_)active fields") [1].
>
> [1] https://lore.kernel.org/linux-media/20190905114210.9232-1-p.zabel@xxxxxxxxxxxxxx/T/#u
Yes, makes sense for me.
Best regards,
Tomasz
