Re: [PATCH v4 03/11] media: uapi: h264: Add the concept of decoding mode

From: Hans Verkuil
Date: Mon Aug 12 2019 - 06:19:45 EST

On 8/8/19 12:34 PM, Ezequiel Garcia wrote:
> From: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
>
> Some stateless decoders don't support per-slice decoding granularity
> (or at least not in a way that would make them efficient or easy to use).
>
> Expose a menu to control the supported decoding modes. Drivers are
> allowed to support only one decoding but they can support both too.
>
> Signed-off-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
> Reviewed-by: Paul Kocialkowski <paul.kocialkowski@xxxxxxxxxxx>
> ---
> Changes in v4:
> * Typos/rewording fixes
>
> Changes in v3:
> * s/per-{slice,frame} decoding/{slice,frame}-based decoding/
> * Add Paul's R-b
>
> Changes in v2:
> * Allow decoding multiple slices in per-slice decoding mode
> * Minor doc improvement/fixes
> ---
> .../media/uapi/v4l/ext-ctrls-codec.rst | 43 ++++++++++++++++++-
> drivers/media/v4l2-core/v4l2-ctrls.c | 9 ++++
> include/media/h264-ctrls.h | 11 +++++
> 3 files changed, 62 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/media/uapi/v4l/ext-ctrls-codec.rst b/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
> index c5f39dd50043..ea0455957149 100644
> --- a/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
> +++ b/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
> @@ -1747,6 +1747,11 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
> * - __u32
> - ``size``
> -
> + * - __u32
> + - ``start_byte_offset``
> + - Where the slice payload starts in the output buffer. Useful when the
> + OUTPUT buffer contains more than one slice (some codecs need to know
> + where each slice starts in this buffer).
> * - __u32
> - ``header_bit_size``
> -
> @@ -1930,7 +1935,10 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
> -
> * - __u16
> - ``num_slices``
> - - Number of slices needed to decode the current frame
> + - Number of slices needed to decode the current frame/field. When
> + operating in slice-based decoding mode (see
> + :c:type:`v4l2_mpeg_video_h264_decoding_mode`), this field
> + should always be set to one.
> * - __u16
> - ``nal_ref_idc``
> - NAL reference ID value coming from the NAL Unit header
> @@ -2021,6 +2029,39 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
> - 0x00000004
> - The DPB entry is a long term reference frame
>
> +``V4L2_CID_MPEG_VIDEO_H264_DECODING_MODE (enum)``
> + Specifies the decoding mode to use. Currently exposes slice-based and
> + frame-based decoding but new modes might be added later on.
> + Drivers may expose a single or multiple decoding modes,
> + depending on what they can support.

This should mention that this is valid for V4L2_PIX_FMT_H264_SLICE pixelformat
only. And it is a required control (i.e. it must always be present for that
pixelformat). Ditto for the start code control.

Also, the V4L2_PIX_FMT_H264_SLICE should refer to this control and the start
code control as well since these controls effectively are modifiers for the
pixelformat.

Regards,

Hans

> +
> + .. note::
> +
> + This menu control is not yet part of the public kernel API and
> + it is expected to change.
> +
> +.. c:type:: v4l2_mpeg_video_h264_decoding_mode
> +
> +.. cssclass:: longtable
> +
> +.. flat-table::
> + :header-rows: 0
> + :stub-columns: 0
> + :widths: 1 1 2
> +
> + * - ``V4L2_MPEG_VIDEO_H264_SLICE_BASED_DECODING``
> + - 0
> + - The decoding is done at the slice granularity.
> + v4l2_ctrl_h264_decode_params->num_slices should be set to 1.
> + The OUTPUT buffer must contain a single slice.
> + * - ``V4L2_MPEG_VIDEO_H264_FRAME_BASED_DECODING``
> + - 1
> + - The decoding is done at the frame granularity.
> + v4l2_ctrl_h264_decode_params->num_slices should be set to the number of
> + slices forming a frame.

Actually, I wonder if there shouldn't be a third option: FIELD_BASED_DECODING
to indicate that the hardware expects the slices of a single field in a buffer
instead of both fields.

Or just stick to FRAME_BASED_DECODING for now but explicitly mention that
in the case of interlaced decoding both fields must be present in the buffer.

A FIELD_BASED_DECODING option can always be added later if necessary.

> + The OUTPUT buffer should contain all slices needed to decode the
> + frame/field.
> +
> .. _v4l2-mpeg-mpeg2:
>
> ``V4L2_CID_MPEG_VIDEO_MPEG2_SLICE_PARAMS (struct)``
> diff --git a/drivers/media/v4l2-core/v4l2-ctrls.c b/drivers/media/v4l2-core/v4l2-ctrls.c
> index cd1ae016706f..c3194299bfac 100644
> --- a/drivers/media/v4l2-core/v4l2-ctrls.c
> +++ b/drivers/media/v4l2-core/v4l2-ctrls.c
> @@ -402,6 +402,11 @@ const char * const *v4l2_ctrl_get_menu(u32 id)
> "Explicit",
> NULL,
> };
> + static const char * const h264_decoding_mode[] = {
> + "Slice-Based",
> + "Frame-Based",
> + NULL,
> + };
> static const char * const mpeg_mpeg2_level[] = {
> "Low",
> "Main",
> @@ -633,6 +638,8 @@ const char * const *v4l2_ctrl_get_menu(u32 id)
> return h264_fp_arrangement_type;
> case V4L2_CID_MPEG_VIDEO_H264_FMO_MAP_TYPE:
> return h264_fmo_map_type;
> + case V4L2_CID_MPEG_VIDEO_H264_DECODING_MODE:
> + return h264_decoding_mode;
> case V4L2_CID_MPEG_VIDEO_MPEG2_LEVEL:
> return mpeg_mpeg2_level;
> case V4L2_CID_MPEG_VIDEO_MPEG2_PROFILE:
> @@ -852,6 +859,7 @@ const char *v4l2_ctrl_get_name(u32 id)
> case V4L2_CID_MPEG_VIDEO_H264_SCALING_MATRIX: return "H264 Scaling Matrix";
> case V4L2_CID_MPEG_VIDEO_H264_SLICE_PARAMS: return "H264 Slice Parameters";
> case V4L2_CID_MPEG_VIDEO_H264_DECODE_PARAMS: return "H264 Decode Parameters";
> + case V4L2_CID_MPEG_VIDEO_H264_DECODING_MODE: return "H264 Decoding Mode";
> case V4L2_CID_MPEG_VIDEO_MPEG2_LEVEL: return "MPEG2 Level";
> case V4L2_CID_MPEG_VIDEO_MPEG2_PROFILE: return "MPEG2 Profile";
> case V4L2_CID_MPEG_VIDEO_MPEG4_I_FRAME_QP: return "MPEG4 I-Frame QP Value";
> @@ -1220,6 +1228,7 @@ void v4l2_ctrl_fill(u32 id, const char **name, enum v4l2_ctrl_type *type,
> case V4L2_CID_MPEG_VIDEO_H264_VUI_SAR_IDC:
> case V4L2_CID_MPEG_VIDEO_H264_SEI_FP_ARRANGEMENT_TYPE:
> case V4L2_CID_MPEG_VIDEO_H264_FMO_MAP_TYPE:
> + case V4L2_CID_MPEG_VIDEO_H264_DECODING_MODE:
> case V4L2_CID_MPEG_VIDEO_MPEG2_LEVEL:
> case V4L2_CID_MPEG_VIDEO_MPEG2_PROFILE:
> case V4L2_CID_MPEG_VIDEO_MPEG4_LEVEL:
> diff --git a/include/media/h264-ctrls.h b/include/media/h264-ctrls.h
> index 6160a69c0143..e6c510877f67 100644
> --- a/include/media/h264-ctrls.h
> +++ b/include/media/h264-ctrls.h
> @@ -26,6 +26,7 @@
> #define V4L2_CID_MPEG_VIDEO_H264_SCALING_MATRIX (V4L2_CID_MPEG_BASE+1002)
> #define V4L2_CID_MPEG_VIDEO_H264_SLICE_PARAMS (V4L2_CID_MPEG_BASE+1003)
> #define V4L2_CID_MPEG_VIDEO_H264_DECODE_PARAMS (V4L2_CID_MPEG_BASE+1004)
> +#define V4L2_CID_MPEG_VIDEO_H264_DECODING_MODE (V4L2_CID_MPEG_BASE+1005)
>
> /* enum v4l2_ctrl_type type values */
> #define V4L2_CTRL_TYPE_H264_SPS 0x0110
> @@ -33,6 +34,12 @@
> #define V4L2_CTRL_TYPE_H264_SCALING_MATRIX 0x0112
> #define V4L2_CTRL_TYPE_H264_SLICE_PARAMS 0x0113
> #define V4L2_CTRL_TYPE_H264_DECODE_PARAMS 0x0114
> +#define V4L2_CTRL_TYPE_H264_DECODING_MODE 0x0115
> +
> +enum v4l2_mpeg_video_h264_decoding_mode {
> + V4L2_MPEG_VIDEO_H264_SLICE_BASED_DECODING,
> + V4L2_MPEG_VIDEO_H264_FRAME_BASED_DECODING,
> +};
>
> #define V4L2_H264_SPS_CONSTRAINT_SET0_FLAG 0x01
> #define V4L2_H264_SPS_CONSTRAINT_SET1_FLAG 0x02
> @@ -125,6 +132,10 @@ struct v4l2_h264_pred_weight_table {
> struct v4l2_ctrl_h264_slice_params {
> /* Size in bytes, including header */
> __u32 size;
> +
> + /* Offset in bytes to the start of slice in the OUTPUT buffer. */
> + __u32 start_byte_offset;
> +
> /* Offset in bits to slice_data() from the beginning of this slice. */
> __u32 header_bit_size;
>
>

Regards,

Hans