linux-kernel - Re: [PATCH] media: v4l2-ctrls: add encoder skip frames control

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Date:   Tue, 19 May 2020 08:53:09 +0200
From:   Hans Verkuil <hverkuil-cisco@...all.nl>
To:     Maheshwar Ajja <majja@...eaurora.org>, mchehab@...nel.org,
        ezequiel@...labora.com, p.zabel@...gutronix.de,
        paul.kocialkowski@...tlin.com, jonas@...boo.se,
        posciak@...omium.org, boris.brezillon@...labora.com,
        ribalda@...nel.org, tglx@...utronix.de, sumitg@...dia.com,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media: v4l2-ctrls: add encoder skip frames control

On 19/05/2020 04:05, Maheshwar Ajja wrote:
> If V4L2_CID_MPEG_VIDEO_ENC_SKIP_FRAMES control is enabled
> encoder can drop frames, if required, to achieve target bitrate
> instead of modifying the quantization parameter which lowers
> the encoded frame quality.
> 
> Reference: 4.3.8.1 OMX_Video_ControlRateConstantSkipFrames
> https://www.khronos.org/registry/OpenMAX-IL/specs/OpenMAX_IL_1_1_2_Specification.pdf
> 
> Signed-off-by: Maheshwar Ajja <majja@...eaurora.org>
> ---
>  Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst | 7 +++++++
>  drivers/media/v4l2-core/v4l2-ctrls.c                      | 2 ++
>  include/uapi/linux/v4l2-controls.h                        | 1 +
>  3 files changed, 10 insertions(+)
> 
> diff --git a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
> index d0d506a..bc9265d 100644
> --- a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
> +++ b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
> @@ -1081,6 +1081,13 @@ enum v4l2_mpeg_video_h264_entropy_mode -
>      Macroblock level rate control enable. Applicable to the MPEG4 and
>      H264 encoders.
>  
> +``V4L2_CID_MPEG_VIDEO_ENC_SKIP_FRAMES (boolean)``
> +    Encoder skip frames enable. This control is applicable only if
> +    ``V4L2_CID_MPEG_VIDEO_BITRATE_MODE`` control is set. If this control
> +    is enabled encoder can drop frames, if required, to achieve target
> +    bitrate instead of modifying the quantization parameter which lowers
> +    the encoded frame quality.

It's a bit unclear for which bitrate modes this control is valid: only for
MODE_CBR, or also for the constant quality bitrate mode? Or both?

The phrase 'control is set' is meaningless for a menu control: it really is
always 'set'. So that needs to be reworked so it is more explicit.

Also note that there is an Exynos MFC control that appears to do something
similar: V4L2_CID_MPEG_MFC51_VIDEO_FRAME_SKIP_MODE.

To what extent does that overlap with the functionality proposed here?

It looks like this proposed control is basically the equivalent of setting
V4L2_CID_MPEG_MFC51_VIDEO_FRAME_SKIP_MODE to V4L2_MPEG_MFC51_FRAME_SKIP_MODE_LEVEL_LIMIT.

So perhaps this MFC control should be promoted to a standard MPEG control instead
of inventing a new control?

Regards,

	Hans

> +
>  ``V4L2_CID_MPEG_VIDEO_MPEG4_QPEL (boolean)``
>      Quarter pixel motion estimation for MPEG4. Applicable to the MPEG4
>      encoder.
> diff --git a/drivers/media/v4l2-core/v4l2-ctrls.c b/drivers/media/v4l2-core/v4l2-ctrls.c
> index 1c617b4..d2cb766 100644
> --- a/drivers/media/v4l2-core/v4l2-ctrls.c
> +++ b/drivers/media/v4l2-core/v4l2-ctrls.c
> @@ -914,6 +914,7 @@ const char *v4l2_ctrl_get_name(u32 id)
>  	case V4L2_CID_MPEG_VIDEO_FWHT_PARAMS:			return "FWHT Stateless Parameters";
>  	case V4L2_CID_FWHT_I_FRAME_QP:				return "FWHT I-Frame QP Value";
>  	case V4L2_CID_FWHT_P_FRAME_QP:				return "FWHT P-Frame QP Value";
> +	case V4L2_CID_MPEG_VIDEO_ENC_SKIP_FRAMES:		return "Encoder Skip Frames";
>  
>  	/* VPX controls */
>  	case V4L2_CID_MPEG_VIDEO_VPX_NUM_PARTITIONS:		return "VPX Number of Partitions";
> @@ -1180,6 +1181,7 @@ void v4l2_ctrl_fill(u32 id, const char **name, enum v4l2_ctrl_type *type,
>  	case V4L2_CID_MPEG_VIDEO_DECODER_SLICE_INTERFACE:
>  	case V4L2_CID_MPEG_VIDEO_FRAME_RC_ENABLE:
>  	case V4L2_CID_MPEG_VIDEO_MB_RC_ENABLE:
> +	case V4L2_CID_MPEG_VIDEO_ENC_SKIP_FRAMES:
>  	case V4L2_CID_MPEG_VIDEO_H264_8X8_TRANSFORM:
>  	case V4L2_CID_MPEG_VIDEO_H264_VUI_SAR_ENABLE:
>  	case V4L2_CID_MPEG_VIDEO_MPEG4_QPEL:
> diff --git a/include/uapi/linux/v4l2-controls.h b/include/uapi/linux/v4l2-controls.h
> index 0ba1005..d3bc015 100644
> --- a/include/uapi/linux/v4l2-controls.h
> +++ b/include/uapi/linux/v4l2-controls.h
> @@ -742,6 +742,7 @@ enum v4l2_cid_mpeg_video_hevc_size_of_length_field {
>  #define V4L2_CID_MPEG_VIDEO_HEVC_HIER_CODING_L6_BR	(V4L2_CID_MPEG_BASE + 642)
>  #define V4L2_CID_MPEG_VIDEO_REF_NUMBER_FOR_PFRAMES	(V4L2_CID_MPEG_BASE + 643)
>  #define V4L2_CID_MPEG_VIDEO_PREPEND_SPSPPS_TO_IDR	(V4L2_CID_MPEG_BASE + 644)
> +#define V4L2_CID_MPEG_VIDEO_ENC_SKIP_FRAMES		(V4L2_CID_MPEG_BASE + 645)
>  
>  /*  MPEG-class control IDs specific to the CX2341x driver as defined by V4L2 */
>  #define V4L2_CID_MPEG_CX2341X_BASE				(V4L2_CTRL_CLASS_MPEG | 0x1000)
>