Re: [PATCH v10 6/9] media: uapi: Add a control for HANTRO driver

[Ezequiel Garcia <ezequiel@collabora.com>]

On Wed, 2021-05-05 at 16:55 +0200, Hans Verkuil wrote:
> On 20/04/2021 14:10, Benjamin Gaignard wrote:
> > The HEVC HANTRO driver needs to know the number of bits to skip at
> > the beginning of the slice header.
> > That is a hardware specific requirement so create a dedicated control
> > for this purpose.
> > 
> > Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
> > ---
> >  .../userspace-api/media/drivers/hantro.rst    | 19 +++++++++++++++++++
> >  .../userspace-api/media/drivers/index.rst     |  1 +
> >  include/media/hevc-ctrls.h                    | 13 +++++++++++++
> >  3 files changed, 33 insertions(+)
> >  create mode 100644 Documentation/userspace-api/media/drivers/hantro.rst
> > 
> > diff --git a/Documentation/userspace-api/media/drivers/hantro.rst b/Documentation/userspace-api/media/drivers/hantro.rst
> > new file mode 100644
> > index 000000000000..cd9754b4e005
> > --- /dev/null
> > +++ b/Documentation/userspace-api/media/drivers/hantro.rst
> > @@ -0,0 +1,19 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +Hantro video decoder driver
> > +===========================
> > +
> > +The Hantro video decoder driver implements the following driver-specific controls:
> > +
> > +``V4L2_CID_HANTRO_HEVC_SLICE_HEADER_SKIP (integer)``
> > +    Specifies to Hantro HEVC video decoder driver the number of data (in bits) to
> > +    skip in the slice segment header.
> > +    If non-IDR, the bits to be skipped go from syntax element "pic_output_flag"
> > +    to before syntax element "slice_temporal_mvp_enabled_flag".
> > +    If IDR, the skipped bits are just "pic_output_flag"
> > +    (separate_colour_plane_flag is not supported).
> 
> I'm not very keen on this. Without this information the video data cannot be
> decoded, or will it just be suboptimal?
> 
> The problem is that a generic decoder would have to know that the HW is a hantro,

Applications can just query which controls are exposed by a video device,
and if this control is found, then it means it needs to be set.

> and then call this control. If they don't (and are testing on non-hantro HW), then
> it won't work, thus defeating the purpose of the HW independent decoder API.
> 
> Since hantro is widely used, and if there is no other way to do this beside explitely
> setting this control, then perhaps this should be part of the standard HEVC API.
> Non-hantro drivers that do not need this can just skip it.
> 

The decision to move it out of the HEVC API is not really to avoid setting it.
In the end, most/all applications will end up required to set this