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

[Benjamin Gaignard <benjamin.gaignard@collabora.com>]

Le 05/05/2021 à 16:55, Hans Verkuil a écrit :
> 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?

Without that information the video can't be decoded.

>
> The problem is that a generic decoder would have to know that the HW is a hantro,
> 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.

Even if I put this parameter in decode_params structure that would means that a generic
userland decoder will have to know how the compute this value for hantro HW since it
isn't something that could be done on kernel side.


Regards,
Benjamin

>
> Regards,
>
> 	Hans
>
>> +
>> +.. note::
>> +
>> +        This control is not yet part of the public kernel API and
>> +        it is expected to change.
>> 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/media/hevc-ctrls.h b/include/media/hevc-ctrls.h
>> index 8e0109eea454..b713eeed1915 100644
>> --- a/include/media/hevc-ctrls.h
>> +++ b/include/media/hevc-ctrls.h
>> @@ -224,4 +224,17 @@ struct v4l2_ctrl_hevc_decode_params {
>>   	__u64	flags;
>>   };
>>   
>> +/*  MPEG-class control IDs specific to the Hantro driver as defined by V4L2 */
>> +#define V4L2_CID_CODEC_HANTRO_BASE				(V4L2_CTRL_CLASS_CODEC | 0x1200)
>> +/*
>> + * V4L2_CID_HANTRO_HEVC_SLICE_HEADER_SKIP -
>> + * 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).
>> + */
>> +#define V4L2_CID_HANTRO_HEVC_SLICE_HEADER_SKIP	(V4L2_CID_CODEC_HANTRO_BASE + 0)
>> +
>>   #endif
>>
>