Re: [PATCH 1/4] media: v4l2-ctrl: Add frame-skip std encoder control

[Stanimir Varbanov <stanimir.varbanov@linaro.org>]

Hi Nicolas,

On 7/7/20 11:53 PM, Nicolas Dufresne wrote:
> Le dimanche 05 juillet 2020 à 15:11 +0300, Stanimir Varbanov a écrit :
>> Adds encoders standard v4l2 control for frame-skip. The control
>> is a copy of a custom encoder control so that other v4l2 encoder
>> drivers can use it.
>>
>> Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
>> ---
>>  .../media/v4l/ext-ctrls-codec.rst             | 32 +++++++++++++++++++
>>  drivers/media/v4l2-core/v4l2-ctrls.c          | 10 ++++++
>>  include/uapi/linux/v4l2-controls.h            |  6 ++++
>>  3 files changed, 48 insertions(+)
>>
>> diff --git a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
>> index d0d506a444b1..a8b4c0b40747 100644
>> --- a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
>> +++ b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
>> @@ -592,6 +592,38 @@ enum v4l2_mpeg_video_bitrate_mode -
>>      the average video bitrate. It is ignored if the video bitrate mode
>>      is set to constant bitrate.
>>  
>> +``V4L2_CID_MPEG_VIDEO_FRAME_SKIP_MODE (enum)``
>> +
>> +enum v4l2_mpeg_video_frame_skip_mode -
>> +    Indicates in what conditions the encoder should skip frames. If
>> +    encoding a frame would cause the encoded stream to be larger then a
>> +    chosen data limit then the frame will be skipped. Possible values
>> +    are:
> 
> I have nothing against this API, in fact it's really nice to generalize
> as this is very common. Though, I think we are missing two things. This
> documentation refer to the "chosen data limit". Is there controls to
> configure these *chosen* limit ? The other issue is the vagueness of
> the documented mode, see lower...
> 
>> +
>> +
>> +.. tabularcolumns:: |p{9.2cm}|p{8.3cm}|
>> +
>> +.. raw:: latex
>> +
>> +    \small
>> +
>> +.. flat-table::
>> +    :header-rows:  0
>> +    :stub-columns: 0
>> +
>> +    * - ``V4L2_MPEG_FRAME_SKIP_MODE_DISABLED``
>> +      - Frame skip mode is disabled.
>> +    * - ``V4L2_MPEG_FRAME_SKIP_MODE_LEVEL_LIMIT``
>> +      - Frame skip mode enabled and buffer limit is set by the chosen
>> +	level and is defined by the standard.
> 
> At least for H.264, a level is compose of 3 limits. One is the maximum
> number of macroblocks, this is is evidently not use for frame skipping
> and already constrained in V4L2 (assuming the driver does not ignore
> the level control of course). The two other limits are decoded
> macroblocks/s and encoded kbits/s. Both are measure over time, which
> means the M2M encoder needs to be timing aware. I think the time source
> should be documented. Perhaps it is mandatory to set a frame interval
> for this to work ? Or we need some timestamp to allow variable frame
> interval ? (I don't think the second is really an option without
> extending the API again, and confusingly, since I think we have used
> the timestamp for other purpose already)

Do you want to say that the encoder input timestamp, bitrate control
(V4L2_CID_MPEG_VIDEO_BITRATE) and S_PARM is not enough to describe
FRAME_SKIP_MODE_LEVEL_LIMIT mode?

> 
>> +    * - ``V4L2_MPEG_FRAME_SKIP_MODE_BUF_LIMIT``
>> +      - Frame skip mode enabled and buffer limit is set by the VBV
>> +	(MPEG1/2/4) or CPB (H264) buffer size control.
> 
> The notion of VBV an CPB is unlikely well known. If my memory is right,
> these are constrained in buffering: in bytes (VBV) or bits per frame
> over a window of n-frames (or the gop size for some less flexible
> encoder) (CPB). I think these should be somehow chosen by application
> (with controls), directly or indirectly, and documented here to ensure
> we get consistent implementation across drivers.

I guess you want me to add here references to the following controls:

V4L2_CID_MPEG_VIDEO_VBV_SIZE
V4L2_CID_MPEG_VIDEO_VBV_DELAY
V4L2_CID_MPEG_VIDEO_H264_CPB_SIZE

?

> 
>> +
>> +.. raw:: latex
>> +
>> +    \normalsize
>> +
>>  ``V4L2_CID_MPEG_VIDEO_TEMPORAL_DECIMATION (integer)``
>>      For every captured frame, skip this many subsequent frames (default
>>      0).
>> diff --git a/drivers/media/v4l2-core/v4l2-ctrls.c b/drivers/media/v4l2-core/v4l2-ctrls.c
>> index 3f3fbcd60cc6..d088acfa6dd8 100644
>> --- a/drivers/media/v4l2-core/v4l2-ctrls.c
>> +++ b/drivers/media/v4l2-core/v4l2-ctrls.c
>> @@ -590,6 +590,12 @@ const char * const *v4l2_ctrl_get_menu(u32 id)
>>  		"External",
>>  		NULL,
>>  	};
>> +	static const char * const mpeg_video_frame_skip[] = {
>> +		"Disabled",
>> +		"Level Limit",
>> +		"VBV/CPB Limit",
>> +		NULL,
>> +	};
>>  
>>  	switch (id) {
>>  	case V4L2_CID_MPEG_AUDIO_SAMPLING_FREQ:
>> @@ -651,6 +657,8 @@ const char * const *v4l2_ctrl_get_menu(u32 id)
>>  		return flash_strobe_source;
>>  	case V4L2_CID_MPEG_VIDEO_HEADER_MODE:
>>  		return header_mode;
>> +	case V4L2_CID_MPEG_VIDEO_FRAME_SKIP_MODE:
>> +		return mpeg_video_frame_skip;
>>  	case V4L2_CID_MPEG_VIDEO_MULTI_SLICE_MODE:
>>  		return multi_slice;
>>  	case V4L2_CID_MPEG_VIDEO_H264_ENTROPY_MODE:
>> @@ -844,6 +852,7 @@ const char *v4l2_ctrl_get_name(u32 id)
>>  	case V4L2_CID_MPEG_VIDEO_MB_RC_ENABLE:			return "H264 MB Level Rate Control";
>>  	case V4L2_CID_MPEG_VIDEO_HEADER_MODE:			return "Sequence Header Mode";
>>  	case V4L2_CID_MPEG_VIDEO_MAX_REF_PIC:			return "Max Number of Reference Pics";
>> +	case V4L2_CID_MPEG_VIDEO_FRAME_SKIP_MODE:		return "Frame Skip Mode";
>>  	case V4L2_CID_MPEG_VIDEO_H263_I_FRAME_QP:		return "H263 I-Frame QP Value";
>>  	case V4L2_CID_MPEG_VIDEO_H263_P_FRAME_QP:		return "H263 P-Frame QP Value";
>>  	case V4L2_CID_MPEG_VIDEO_H263_B_FRAME_QP:		return "H263 B-Frame QP Value";
>> @@ -1265,6 +1274,7 @@ void v4l2_ctrl_fill(u32 id, const char **name, enum v4l2_ctrl_type *type,
>>  	case V4L2_CID_FLASH_LED_MODE:
>>  	case V4L2_CID_FLASH_STROBE_SOURCE:
>>  	case V4L2_CID_MPEG_VIDEO_HEADER_MODE:
>> +	case V4L2_CID_MPEG_VIDEO_FRAME_SKIP_MODE:
>>  	case V4L2_CID_MPEG_VIDEO_MULTI_SLICE_MODE:
>>  	case V4L2_CID_MPEG_VIDEO_H264_ENTROPY_MODE:
>>  	case V4L2_CID_MPEG_VIDEO_H264_LEVEL:
>> diff --git a/include/uapi/linux/v4l2-controls.h b/include/uapi/linux/v4l2-controls.h
>> index 62271418c1be..4e1526175a4c 100644
>> --- a/include/uapi/linux/v4l2-controls.h
>> +++ b/include/uapi/linux/v4l2-controls.h
>> @@ -742,6 +742,12 @@ 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_FRAME_SKIP_MODE		(V4L2_CID_MPEG_BASE + 645)
>> +enum v4l2_mpeg_video_frame_skip_mode {
>> +	V4L2_MPEG_VIDEO_FRAME_SKIP_MODE_DISABLED	= 0,
>> +	V4L2_MPEG_VIDEO_FRAME_SKIP_MODE_LEVEL_LIMIT	= 1,
>> +	V4L2_MPEG_VIDEO_FRAME_SKIP_MODE_BUF_LIMIT	= 2,
>> +};
>>  
>>  /*  MPEG-class control IDs specific to the CX2341x driver as defined by V4L2 */
>>  #define V4L2_CID_MPEG_CX2341X_BASE				(V4L2_CTRL_CLASS_MPEG | 0x1000)
> 

-- 
regards,
Stan