Re: [PATCH v8 09/13] media: uapi: Add a control for HANTRO driver

[Ezequiel Garcia <ezequiel@collabora.com>]

Hi Benjamin,

Thanks for the patch.

On Thu, 2021-04-01 at 17:59 +0200, 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
> that this purpose.
> 
> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
> ---
> version 5:
>  - Be even more verbose in control documentation.
>  - Do not create class for the control.
> version 4:
> - The control is now an integer which is enough to provide the numbers
>   of bits to skip.
> version 3:
> - Fix typo in field name
> 
>  .../userspace-api/media/drivers/hantro.rst         | 14 ++++++++++++++
>  .../userspace-api/media/drivers/index.rst          |  1 +
>  include/uapi/linux/v4l2-controls.h                 | 13 +++++++++++++
>  3 files changed, 28 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..78dcd2a44a03
> --- /dev/null
> +++ b/Documentation/userspace-api/media/drivers/hantro.rst
> @@ -0,0 +1,14 @@
> +.. 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).
> diff --git a/Documentation/userspace-api/media/drivers/index.rst b/Documentation/userspace-api/media/drivers/index.rst
> index 1a9038f5f9fa..12e3c512d718 100644
> --- a/Documentation/userspace-api/media/drivers/index.rst
> +++ b/Documentation/userspace-api/media/drivers/index.rst
> @@ -33,6 +33,7 @@ For more details see the file COPYING in the source distribution of Linux.
>  
>         ccs
>         cx2341x-uapi
> +        hantro
>         imx-uapi
>         max2175
>         meye-uapi
> diff --git a/include/uapi/linux/v4l2-controls.h b/include/uapi/linux/v4l2-controls.h
> index f3376aafea65..1dfb874b6272 100644
> --- a/include/uapi/linux/v4l2-controls.h
> +++ b/include/uapi/linux/v4l2-controls.h
> @@ -869,6 +869,19 @@ enum v4l2_mpeg_mfc51_video_force_frame_type {
>  #define V4L2_CID_MPEG_MFC51_VIDEO_H264_ADAPTIVE_RC_STATIC              (V4L2_CID_CODEC_MFC51_BASE+53)
>  #define V4L2_CID_MPEG_MFC51_VIDEO_H264_NUM_REF_PIC_FOR_P               (V4L2_CID_CODEC_MFC51_BASE+54)
>  
> +/*  MPEG-class control IDs specific to the Hantro driver as defined by V4L2 */

We are moving away from "MPEG" terminology for codecs.

> +#define V4L2_CID_CODEC_HANTRO_BASE                             (V4L2_CTRL_CLASS_CODEC | 0x1200)

Using V4L2_CTRL_CLASS_CODEC_STATELESS is IMO better,
since this belongs to a stateless decoder.

And also, since we are still a bit unsure about the
syntax of this parameter (given it's not documented):

how about keeping the V4L2_CID_HANTRO_HEVC_SLICE_HEADER_SKIP
definition in drivers/staging/media/hantro/hantro.h ?

This would be a hint for applications that this control
is a quirk.

Hans, Philipp, any thoughts on this?

Regards,
Ezequiel

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