Date:   Tue, 13 Oct 2020 14:33:00 -0400
From:   Nicolas Dufresne <nicolas.dufresne@...labora.com>
To:     Ezequiel Garcia <ezequiel@...labora.com>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     kernel@...labora.com, Hans Verkuil <hverkuil@...all.nl>
Subject: Re: [PATCH v2] media: uapi: h264: Add documentation to the
 interface header

Le lundi 28 septembre 2020 à 17:14 -0300, Ezequiel Garcia a écrit :
> In preparation for making the interface public,
> document all the structures. Special care is taken to
> annotate those fields that depart from the H264 syntax.
> 
> This commit only adds documentation and doesn't affect
> functionality in any way.
> 
> Signed-off-by: Ezequiel Garcia <ezequiel@...labora.com>
> ---
> v2:
> * Address the feedback, commenting every field, including
>   reserved and flags.
> * Clarify scaling list ordering.
> ---
>  include/media/h264-ctrls.h | 195 +++++++++++++++++++++++++++++++++++--
>  1 file changed, 185 insertions(+), 10 deletions(-)
> 
> diff --git a/include/media/h264-ctrls.h b/include/media/h264-ctrls.h
> index ec4799154438..893e21354a9b 100644
> --- a/include/media/h264-ctrls.h
> +++ b/include/media/h264-ctrls.h
> @@ -46,11 +46,38 @@
>  #define V4L2_CTRL_TYPE_H264_DECODE_PARAMS	0x0114
>  #define V4L2_CTRL_TYPE_H264_PRED_WEIGHTS	0x0115
>  
> +/**
> + * enum v4l2_mpeg_video_h264_decode_mode - Decoding mode
> + *
> + * @V4L2_MPEG_VIDEO_H264_DECODE_MODE_SLICE_BASED: indicates that decoding
> + * is performed one slice at a time. In this mode,
> + * V4L2_CID_MPEG_VIDEO_H264_SLICE_PARAMS must contain the parsed slice
> + * parameters and the OUTPUT buffer must contain a single slice.
> + * V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF feature is used
> + * in order to support multislice frames.
                          multi-slice or multiple slice per ?

> + * @V4L2_MPEG_VIDEO_H264_DECODE_MODE_FRAME_BASED: indicates that
> + * decoding is performed per frame. The OUTPUT buffer must contain
> + * all slices and also both fields. This mode is typically supported

I'm surprised that this must contain both fields without distinction of
MBAFF or alternate. We have a pretty limited interlacing coverage atm,
are we certain this is correct ? I'm fine with that, as long as it
matches the HW.

> + * by device drivers that are able to parse the slice(s) header(s)
> + * in hardware. When this mode is selected,
> + * V4L2_CID_MPEG_VIDEO_H264_SLICE_PARAMS is not used.
> + */
>  enum v4l2_mpeg_video_h264_decode_mode {
>  	V4L2_MPEG_VIDEO_H264_DECODE_MODE_SLICE_BASED,
>  	V4L2_MPEG_VIDEO_H264_DECODE_MODE_FRAME_BASED,
>  };
>  
> +/**
> + * enum v4l2_mpeg_video_h264_start_code - Start code
> + *
> + * @V4L2_MPEG_VIDEO_H264_START_CODE_NONE: slices are passed
> + * to the driver without any start code.
> + * @V4L2_MPEG_VIDEO_H264_START_CODE_ANNEX_B: slices are passed
> + * to the driver with an Annex B start code prefix
> + * (legal start codes can be 3-bytes 0x000001 or 4-bytes 0x00000001).
> + * This mode is typically supported by device drivers that parse
> + * the start code in hardware.
> + */

Do we need to mention that not all combinations are possible ? An
example, having START_CODE_NONE with FRAME_BASED would simply not work,
since it's impossible to find the start/end of NAL.

>  enum v4l2_mpeg_video_h264_start_code {
>  	V4L2_MPEG_VIDEO_H264_START_CODE_NONE,
>  	V4L2_MPEG_VIDEO_H264_START_CODE_ANNEX_B,
> @@ -71,6 +98,31 @@ enum v4l2_mpeg_video_h264_start_code {
>  #define V4L2_H264_SPS_FLAG_MB_ADAPTIVE_FRAME_FIELD		0x20
>  #define V4L2_H264_SPS_FLAG_DIRECT_8X8_INFERENCE			0x40
>  
> +/**
> + * struct v4l2_ctrl_h264_sps - H264 sequence parameter set
> + *
> + * All the members on this sequence parameter set structure match the
> + * sequence parameter set syntax as specified by the H264 specification.
> + *
> + * @profile_idc: see H264 specification.
> + * @constraint_set_flags: see H264 specification.
> + * @level_idc: see H264 specification.
> + * @seq_parameter_set_id: see H264 specification.
> + * @chroma_format_idc: see H264 specification.
> + * @bit_depth_luma_minus8: see H264 specification.
> + * @bit_depth_chroma_minus8: see H264 specification.
> + * @log2_max_frame_num_minus4: see H264 specification.
> + * @pic_order_cnt_type: see H264 specification.
> + * @log2_max_pic_order_cnt_lsb_minus4: see H264 specification.
> + * @max_num_ref_frames: see H264 specification.
> + * @num_ref_frames_in_pic_order_cnt_cycle: see H264 specification.
> + * @offset_for_ref_frame: see H264 specification.
> + * @offset_for_non_ref_pic: see H264 specification.
> + * @offset_for_top_to_bottom_field: see H264 specification.
> + * @pic_width_in_mbs_minus1: see H264 specification.
> + * @pic_height_in_map_units_minus1: see H264 specification.
> + * @flags: see V4L2_H264_SPS_FLAG_{}.
> + */
>  struct v4l2_ctrl_h264_sps {
>  	__u8 profile_idc;
>  	__u8 constraint_set_flags;
> @@ -101,6 +153,31 @@ struct v4l2_ctrl_h264_sps {
>  #define V4L2_H264_PPS_FLAG_TRANSFORM_8X8_MODE				0x0040
>  #define V4L2_H264_PPS_FLAG_SCALING_MATRIX_PRESENT			0x0080
>  
> +/**
> + * struct v4l2_ctrl_h264_pps - H264 picture parameter set
> + *
> + * Except where noted, all the members on this picture parameter set
> + * structure match the sequence parameter set syntax as specified
> + * by the H264 specification.
> + *
> + * In particular, V4L2_H264_PPS_FLAG_SCALING_MATRIX_PRESENT flag
> + * has a specific meaning. This flag should be set if a non-flat
> + * scaling matrix applies to the picture. In this case, applications
> + * are expected to use V4L2_CID_MPEG_VIDEO_H264_SCALING_MATRIX,
> + * to pass the values of the non-flat matrices.
> + *
> + * @pic_parameter_set_id: see H264 specification.
> + * @seq_parameter_set_id: see H264 specification.
> + * @num_slice_groups_minus1: see H264 specification.
> + * @num_ref_idx_l0_default_active_minus1: see H264 specification.
> + * @num_ref_idx_l1_default_active_minus1: see H264 specification.
> + * @weighted_bipred_idc: see H264 specification.
> + * @pic_init_qp_minus26: see H264 specification.
> + * @pic_init_qs_minus26: see H264 specification.
> + * @chroma_qp_index_offset: see H264 specification.
> + * @second_chroma_qp_index_offset: see H264 specification.
> + * @flags: see V4L2_H264_PPS_FLAG_{}.
> + */
>  struct v4l2_ctrl_h264_pps {
>  	__u8 pic_parameter_set_id;
>  	__u8 seq_parameter_set_id;
> @@ -115,6 +192,23 @@ struct v4l2_ctrl_h264_pps {
>  	__u16 flags;
>  };
>  
> +/**
> + * struct v4l2_ctrl_h264_scaling_matrix - H264 scaling matrices
> + *
> + * @scaling_list_4x4: scaling matrix after applying the inverse
> + * scanning process. Expected list order is Intra Y, Intra Cb,
> + * Intra Cr, Inter Y, Inter Cb, Inter Cr. The values on each
> + * scaling list are expected in raster scan order.
> + * @scaling_list_8x8: scaling matrix after applying the inverse
> + * scanning process. Expected list order is Intra Y, Inter Y,
> + * Intra Cb, Inter Cb, Intra Cr, Inter Cr. The values on each
> + * scaling list are expected in raster scan order.
> + *
> + * Note that the list order is different for the 4x4 and 8x8
> + * matrices as per the H264 specification, see table 7-2 "Assignment
> + * of mnemonic names to scaling list indices and specification of
> + * fall-back rule".
> + */
>  struct v4l2_ctrl_h264_scaling_matrix {
>  	__u8 scaling_list_4x4[6][16];
>  	__u8 scaling_list_8x8[6][64];
> @@ -134,6 +228,16 @@ struct v4l2_h264_weight_factors {
>  	 ((pps)->weighted_bipred_idc == 1 && \
>  	  (slice)->slice_type == V4L2_H264_SLICE_TYPE_B))
>  
> +/**
> + * struct v4l2_ctrl_h264_pred_weights - Prediction weight table
> + *
> + * Prediction weight table, which matches the syntax specified
> + * by the H264 specification.
> + *
> + * @luma_log2_weight_denom: see H264 specification.
> + * @chroma_log2_weight_denom: see H264 specification.
> + * @weight_factors: luma and chroma weight factors.
> + */
>  struct v4l2_ctrl_h264_pred_weights {
>  	__u16 luma_log2_weight_denom;
>  	__u16 chroma_log2_weight_denom;
> @@ -153,19 +257,55 @@ struct v4l2_ctrl_h264_pred_weights {
>  #define V4L2_H264_BOTTOM_FIELD_REF			0x2
>  #define V4L2_H264_FRAME_REF				0x3
>  
> +/**
> + * struct v4l2_h264_reference - H264 picture reference
> + *
> + * @fields: indicates how the picture is referenced.
> + * Valid values are V4L2_H264_{}_REF.
> + * @index: index into v4l2_ctrl_h264_decode_params.dpb[].
> + */
>  struct v4l2_h264_reference {
>  	__u8 fields;
> -
> -	/* Index into v4l2_ctrl_h264_decode_params.dpb[] */
>  	__u8 index;
>  };
>  
> +/**
> + * struct v4l2_ctrl_h264_slice_params - H264 slice parameters
> + *
> + * This structure holds the H264 syntax elements that are specified
> + * as non-invariant for the slices in a given frame.
> + *
> + * Slice invariant syntax elements are contained in struct
> + * v4l2_ctrl_h264_decode_params. This is done to reduce the API surface
> + * on frame-based decoders, where slice header parsing is done by the
> + * hardware.
> + *
> + * Slice invariant syntax elements are specified in specification section
> + * "7.4.3 Slice header semantics".
> + *
> + * Except where noted, the members on this struct match the slice header syntax.
> + *
> + * @header_bit_size: offset in bits to slice_data() from the beginning of this slice.
> + * @first_mb_in_slice: see H264 specification.
> + * @slice_type: see H264 specification.
> + * @colour_plane_id: see H264 specification.
> + * @redundant_pic_cnt: see H264 specification.
> + * @cabac_init_idc: see H264 specification.
> + * @slice_qp_delta: see H264 specification.
> + * @slice_qs_delta: see H264 specification.
> + * @disable_deblocking_filter_idc: see H264 specification.
> + * @slice_alpha_c0_offset_div2: see H264 specification.
> + * @slice_beta_offset_div2: see H264 specification.
> + * @num_ref_idx_l0_active_minus1: see H264 specification.
> + * @num_ref_idx_l1_active_minus1: see H264 specification.
> + * @reserved: padding field. Should be zeroed by applications.
> + * @ref_pic_list0: reference picture list 0 after applying the per-slice modifications.
> + * @ref_pic_list1: reference picture list 1 after applying the per-slice modifications.
> + * @flags: see V4L2_H264_SLICE_FLAG_{}.
> + */
>  struct v4l2_ctrl_h264_slice_params {
> -	/* Offset in bits to slice_data() from the beginning of this slice. */
>  	__u32 header_bit_size;
> -
>  	__u32 first_mb_in_slice;
> -
>  	__u8 slice_type;
>  	__u8 colour_plane_id;
>  	__u8 redundant_pic_cnt;
> @@ -191,22 +331,59 @@ struct v4l2_ctrl_h264_slice_params {
>  #define V4L2_H264_DPB_ENTRY_FLAG_LONG_TERM	0x04
>  #define V4L2_H264_DPB_ENTRY_FLAG_FIELD		0x08
>  
> +/**
> + * struct v4l2_h264_dpb_entry - H264 decoded picture buffer entry
> + *
> + * @reference_ts: timestamp of the V4L2 capture buffer to use as reference.
> + * The timestamp refers to the timestamp field in struct v4l2_buffer.
> + * Use v4l2_timeval_to_ns() to convert the struct timeval to a __u64.
> + * @pic_num: matches PicNum variable assigned during the reference
> + * picture lists construction process.
> + * @frame_num: frame identifier which matches frame_num syntax element.
> + * @fields: indicates how the DPB entry is referenced. Valid values are
> + * V4L2_H264_{}_REF.
> + * @reserved: padding field. Should be zeroed by applications.
> + * @top_field_order_cnt: matches TopFieldOrderCnt picture value.
> + * @bottom_field_order_cnt: matches BottomFieldOrderCnt picture value.
> + * Note that picture field is indicated by v4l2_buffer.field.
> + * @flags: see V4L2_H264_DPB_ENTRY_FLAG_{}.
> + */
>  struct v4l2_h264_dpb_entry {
>  	__u64 reference_ts;
>  	__u32 pic_num;
>  	__u16 frame_num;
>  	__u8 fields;
>  	__u8 reserved[5];
> -	/* Note that field is indicated by v4l2_buffer.field */
>  	__s32 top_field_order_cnt;
>  	__s32 bottom_field_order_cnt;
> -	__u32 flags; /* V4L2_H264_DPB_ENTRY_FLAG_* */
> +	__u32 flags;
>  };
>  
>  #define V4L2_H264_DECODE_PARAM_FLAG_IDR_PIC		0x01
>  #define V4L2_H264_DECODE_PARAM_FLAG_FIELD_PIC		0x02
>  #define V4L2_H264_DECODE_PARAM_FLAG_BOTTOM_FIELD	0x04
>  
> +/**
> + * struct v4l2_ctrl_h264_decode_params - H264 decoding parameters
> + *
> + * @dpb: decoded picture buffer.
> + * @nal_ref_idc: slice header syntax element.
> + * @frame_num: slice header syntax element.
> + * @top_field_order_cnt: matches TopFieldOrderCnt picture value.
> + * @bottom_field_order_cnt: matches BottomFieldOrderCnt picture value.
> + * Note that picture field is indicated by v4l2_buffer.field.
> + * @idr_pic_id: slice header syntax element.
> + * @pic_order_cnt_lsb: slice header syntax element.
> + * @delta_pic_order_cnt_bottom: slice header syntax element.
> + * @delta_pic_order_cnt0: slice header syntax element.
> + * @delta_pic_order_cnt1: slice header syntax element.
> + * @dec_ref_pic_marking_bit_size: size in bits of dec_ref_pic_marking()
> + * syntax element.
> + * @pic_order_cnt_bit_size: size in bits of pic order count syntax.
> + * @slice_group_change_cycle: slice header syntax element.
> + * @reserved: padding field. Should be zeroed by applications.
> + * @flags: see V4L2_H264_DECODE_PARAM_FLAG_{}.
> + */
>  struct v4l2_ctrl_h264_decode_params {
>  	struct v4l2_h264_dpb_entry dpb[V4L2_H264_NUM_DPB_ENTRIES];
>  	__u16 nal_ref_idc;
> @@ -218,14 +395,12 @@ struct v4l2_ctrl_h264_decode_params {
>  	__s32 delta_pic_order_cnt_bottom;
>  	__s32 delta_pic_order_cnt0;
>  	__s32 delta_pic_order_cnt1;
> -	/* Size in bits of dec_ref_pic_marking() syntax element. */
>  	__u32 dec_ref_pic_marking_bit_size;
> -	/* Size in bits of pic order count syntax. */
>  	__u32 pic_order_cnt_bit_size;
>  	__u32 slice_group_change_cycle;
>  
>  	__u32 reserved;
> -	__u32 flags; /* V4L2_H264_DECODE_PARAM_FLAG_* */
> +	__u32 flags;
>  };
>  
>  #endif

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