Re: [PATCH v3 2/3] media: uapi: Add VP9 stateless decoder controls

[Hans Verkuil]

On 08/05/2020 14:28, Ezequiel Garcia wrote:
> Hi Hans,
> 
> On Fri, 2020-05-08 at 12:27 +0200, Hans Verkuil wrote:
>> On 05/05/2020 15:41, Ezequiel Garcia wrote:
>>> From: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
>>>
>>> Add the VP9 stateless decoder controls plus the documentation that goes
>>> with it.
>>>
>>> Signed-off-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
>>> Signed-off-by: Ezequiel Garcia <ezequiel@xxxxxxxxxxxxx>
>>> ---
>>>  .../userspace-api/media/v4l/biblio.rst        |  10 +
>>>  .../media/v4l/ext-ctrls-codec.rst             | 581 ++++++++++++++++++
>>>  drivers/media/v4l2-core/v4l2-ctrls.c          | 242 ++++++++
>>>  drivers/media/v4l2-core/v4l2-ioctl.c          |   1 +
>>>  include/media/v4l2-ctrls.h                    |   1 +
>>>  include/media/vp9-ctrls.h                     | 510 +++++++++++++++
>>>  6 files changed, 1345 insertions(+)
>>>  create mode 100644 include/media/vp9-ctrls.h
>>>
>>> diff --git a/Documentation/userspace-api/media/v4l/biblio.rst b/Documentation/userspace-api/media/v4l/biblio.rst
>>> index 3c9634173e82..e09102e572fd 100644
>>> --- a/Documentation/userspace-api/media/v4l/biblio.rst
>>> +++ b/Documentation/userspace-api/media/v4l/biblio.rst
>>> @@ -414,3 +414,13 @@ VP8
>>>  :title:     RFC 6386: "VP8 Data Format and Decoding Guide"
>>>  
>>>  :author:    J. Bankoski et al.
>>> +
>>> +.. _vp9:
>>> +
>>> +VP9
>>> +===
>>> +
>>> +
>>> +:title:     VP9 Bitstream & Decoding Process Specification
>>> +
>>> +:author:    Adrian Grange (Google), Peter de Rivaz (Argon Design), Jonathan Hunt (Argon Design)
>>> diff --git a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
>>> index d0d506a444b1..fb657197b372 100644
>>> --- a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
>>> +++ b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
>>> @@ -2668,6 +2668,587 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
>>>        - ``padding[3]``
>>>        - Applications and drivers must set this to zero.
>>>  
>>> +.. _v4l2-mpeg-vp9:
>>> +
>>> +``V4L2_CID_MPEG_VIDEO_VP9_FRAME_CONTEXT(0..3) (struct)``
>>> +    Stores VP9 probabilities attached to a specific frame context. The VP9
>>> +    specification allows using a maximum of 4 contexts. Each frame being
>>> +    decoded refers to one of those context. See section '7.1.2 Refresh
>>> +    probs semantics' section of :ref:`vp9` for more details about these
>>> +    contexts.
>>> +
>>> +    This control is bi-directional:
>>> +
>>> +    * all 4 contexts must be initialized by userspace just after the
>>> +      stream is started and before the first decoding request is submitted
>>
>> Missing . at the end.
>>
> 
> Oops, thanks for catching this.
> 
>>> +    * the referenced context might be read by the kernel when a decoding
>>> +      request is submitted, and will be updated after the decoder is done
>>> +      decoding the frame if the `V4L2_VP9_FRAME_FLAG_REFRESH_FRAME_CTX` flag
>>> +      is set.
>>> +    * contexts will be read back by user space before each decoding request
>>> +      to retrieve the updated probabilities.
>>> +    * userspace will re-initialize the context to their default values when
>>> +      a reset context is required.
>>> +
>>> +    .. note::
>>> +
>>> +       This compound control is not yet part of the public kernel API and
>>> +       it is expected to change.
>>> +
>>> +.. c:type:: v4l2_ctrl_vp9_frame_ctx
>>> +
>>> +.. cssclass:: longtable
>>> +
>>> +.. tabularcolumns:: |p{5.8cm}|p{4.8cm}|p{6.6cm}|
>>> +
>>> +.. flat-table:: struct v4l2_ctrl_vp9_frame_ctx
>>> +    :header-rows:  0
>>> +    :stub-columns: 0
>>> +    :widths:       1 1 2
>>> +
>>> +    * - struct :c:type:`v4l2_vp9_probabilities`
>>> +      - ``probs``
>>> +      - Structure with VP9 probabilities attached to the context.
>>> +
>>> +.. c:type:: v4l2_vp9_probabilities
>>> +
>>> +.. cssclass:: longtable
>>> +
>>> +.. tabularcolumns:: |p{1.5cm}|p{6.3cm}|p{9.4cm}|
>>> +
>>> +.. flat-table:: struct v4l2_vp9_probabilities
>>> +    :header-rows:  0
>>> +    :stub-columns: 0
>>> +    :widths:       1 1 2
>>> +
>>> +    * - __u8
>>> +      - ``tx8[2][1]``
>>> +      - TX 8x8 probabilities.
>>> +    * - __u8
>>> +      - ``tx16[2][2]``
>>> +      - TX 16x16 probabilities.
>>> +    * - __u8
>>> +      - ``tx32[2][3]``
>>> +      - TX 32x32 probabilities.
>>> +    * - __u8
>>> +      - ``coef[4][2][2][6][6][3]``
>>> +      - Coefficient probabilities.
>>> +    * - __u8
>>> +      - ``skip[3]``
>>> +      - Skip probabilities.
>>> +    * - __u8
>>> +      - ``inter_mode[7][3]``
>>> +      - Inter prediction mode probabilities.
>>> +    * - __u8
>>> +      - ``interp_filter[4][2]``
>>> +      - Interpolation filter probabilities.
>>> +    * - __u8
>>> +      - ``is_inter[4]``
>>> +      - Is inter-block probabilities.
>>> +    * - __u8
>>> +      - ``comp_mode[5]``
>>> +      - Compound prediction mode probabilities.
>>> +    * - __u8
>>> +      - ``single_ref[5][2]``
>>> +      - Single reference probabilities.
>>> +    * - __u8
>>> +      - ``comp_mode[5]``
>>> +      - Compound reference probabilities.
>>> +    * - __u8
>>> +      - ``y_mode[4][9]``
>>> +      - Y prediction mode probabilities.
>>> +    * - __u8
>>> +      - ``uv_mode[10][9]``
>>> +      - UV prediction mode probabilities.
>>> +    * - __u8
>>> +      - ``partition[16][3]``
>>> +      - Partition probabilities.
>>> +    * - __u8
>>> +      - ``mv.joint[3]``
>>> +      - Motion vector joint probabilities.
>>> +    * - __u8
>>> +      - ``mv.sign[2]``
>>> +      - Motion vector sign probabilities.
>>> +    * - __u8
>>> +      - ``mv.class[2][10]``
>>> +      - Motion vector class probabilities.
>>> +    * - __u8
>>> +      - ``mv.class0_bit[2]``
>>> +      - Motion vector class0 bit probabilities.
>>> +    * - __u8
>>> +      - ``mv.bits[2][10]``
>>> +      - Motion vector bits probabilities.
>>> +    * - __u8
>>> +      - ``mv.class0_fr[2][2][3]``
>>> +      - Motion vector class0 fractional bit probabilities.
>>> +    * - __u8
>>> +      - ``mv.fr[2][3]``
>>> +      - Motion vector fractional bit probabilities.
>>> +    * - __u8
>>> +      - ``mv.class0_hp[2]``
>>> +      - Motion vector class0 high precision fractional bit probabilities.
>>> +    * - __u8
>>> +      - ``mv.hp[2]``
>>> +      - Motion vector high precision fractional bit probabilities.
>>> +
>>> +``V4L2_CID_MPEG_VIDEO_VP9_FRAME_DECODE_PARAMS (struct)``
>>> +    Specifies the frame parameters for the associated VP9 frame decode request.
>>> +    This includes the necessary parameters for configuring a stateless hardware
>>> +    decoding pipeline for VP9. The bitstream parameters are defined according
>>> +    to :ref:`vp9`.
>>> +
>>> +    .. note::
>>> +
>>> +       This compound control is not yet part of the public kernel API and
>>> +       it is expected to change.
>>> +
>>> +.. c:type:: v4l2_ctrl_vp9_frame_decode_params
>>> +
>>> +.. cssclass:: longtable
>>> +
>>> +.. tabularcolumns:: |p{1.5cm}|p{6.3cm}|p{9.4cm}|
>>> +
>>> +.. flat-table:: struct v4l2_ctrl_vp9_frame_decode_params
>>> +    :header-rows:  0
>>> +    :stub-columns: 0
>>> +    :widths:       1 1 2
>>> +
>>> +    * - __u32
>>> +      - ``flags``
>>> +      - Combination of V4L2_VP9_FRAME_FLAG_* flags. See
>>> +        :c:type:`v4l2_vp9_frame_flags`.
>>> +    * - __u16
>>> +      - ``compressed_header_size``
>>> +      - Compressed header size in bytes.
>>> +    * - __u16
>>> +      - ``uncompressed_header_size``
>>> +      - Uncompressed header size in bytes.
>>> +    * - __u8
>>> +      - ``profile``
>>> +      - VP9 profile. Can be 0, 1, 2 or 3.
>>> +    * - __u8
>>> +      - ``reset_frame_context``
>>> +      - Frame context that should be used/updated when decoding the frame.
>>> +    * - __u8
>>> +      - ``bit_depth``
>>> +      - Component depth in bits. Must be 8 for profile 0 and 1. Must 10 or 12
>>> +        for profile 2 and 3.
>>> +    * - __u8
>>> +      - ``color_space``
>>> +      - Specifies the color space of the stream. See V4L2_VP9_COLOR_SPACE_*
>>> +        values. See :c:type:`v4l2_vp9_color_space`
>>
>> Ditto.
>>
>>> +    * - __u8
>>> +      - ``interpolation_filter``
>>> +      - Specifies the filter selection used for performing inter prediction. See
>>> +        :c:type:`v4l2_vp9_interpolation_filter`.
>>> +    * - __u8
>>> +      - ``tile_cols_log2``
>>> +      - Specifies the base 2 logarithm of the width of each tile (where the
>>> +        width is measured in units of 8x8 blocks). Shall be less than or equal
>>> +        to 6.
>>> +    * - __u8
>>> +      - ``tile_rows_log2``
>>> +      - Specifies the base 2 logarithm of the height of each tile (where the
>>> +        height is measured in units of 8x8 blocks)
>>
>> Ditto
>>
>>> +    * - __u8
>>> +      - ``tx_mode``
>>> +      - Specifies the TX mode. See :c:type:`v4l2_vp9_tx_mode`.
>>> +    * - __u8
>>> +      - ``reference_mode``
>>> +      - Specifies the type of inter prediction to be used. See
>>> +        :c:type:`v4l2_vp9_reference_mode`.
>>> +    * - __u8
>>> +      - ``padding``
>>> +      - Needed to make this struct 64 bit aligned. Shall be filled with zeros.
>>> +    * - __u16
>>> +      - ``frame_width_minus_1``
>>> +      - Add 1 to get the frame width expressed in pixels.
>>> +    * - __u16
>>> +      - ``frame_height_minus_1``
>>> +      - Add 1 to to get the frame height expressed in pixels.
>>> +    * - __u16
>>> +      - ``frame_width_minus_1``
>>> +      - Add 1 to to get the expected render width expressed in pixels. This is
>>> +        not used during the decoding process but might be used by HW scalers to
>>> +        prepare a frame that's ready for scanout.
>>> +    * - __u16
>>> +      - frame_height_minus_1
>>> +      - Add 1 to get the expected render height expressed in pixels. This is
>>> +        not used during the decoding process but might be used by HW scalers to
>>> +        prepare a frame that's ready for scanout.
>>> +    * - __u64
>>> +      - ``refs[3]``
>>> +      - Array of reference frame timestamps.
>>> +    * - struct :c:type:`v4l2_vp9_loop_filter`
>>> +      - ``lf``
>>> +      - Loop filter parameters. See struct :c:type:`v4l2_vp9_loop_filter`.
>>> +    * - struct :c:type:`v4l2_vp9_quantization`
>>> +      - ``quant``
>>> +      - Quantization parameters. See :c:type:`v4l2_vp9_quantization`.
>>> +    * - struct :c:type:`v4l2_vp9_segmentation`
>>> +      - ``seg``
>>> +      - Segmentation parameters. See :c:type:`v4l2_vp9_segmentation`.
>>> +    * - struct :c:type:`v4l2_vp9_probabilities`
>>> +      - ``probs``
>>> +      - Probabilities. See :c:type:`v4l2_vp9_probabilities`.
>>> +
>>> +.. c:type:: v4l2_vp9_frame_flags
>>> +
>>> +.. cssclass:: longtable
>>> +
>>> +.. tabularcolumns:: |p{1.5cm}|p{6.3cm}|p{9.4cm}|
>>> +
>>> +.. flat-table:: enum v4l2_vp9_frame_flags
>>> +    :header-rows:  0
>>> +    :stub-columns: 0
>>> +    :widths:       1 2
>>> +
>>> +    * - ``V4L2_VP9_FRAME_FLAG_KEY_FRAME``
>>> +      - The frame is a key frame.
>>> +    * - ``V4L2_VP9_FRAME_FLAG_SHOW_FRAME``
>>> +      - The frame should be displayed.
>>> +    * - ``V4L2_VP9_FRAME_FLAG_ERROR_RESILIENT``
>>> +      - The decoding should be error resilient.
>>> +    * - ``V4L2_VP9_FRAME_FLAG_INTRA_ONLY``
>>> +      - The frame does not reference other frames.
>>> +    * - ``V4L2_VP9_FRAME_FLAG_ALLOW_HIGH_PREC_MV``
>>> +      - the frame might can high precision motion vectors.
>>> +    * - ``V4L2_VP9_FRAME_FLAG_REFRESH_FRAME_CTX``
>>> +      - Frame context should be updated after decoding.
>>> +    * - ``V4L2_VP9_FRAME_FLAG_PARALLEL_DEC_MODE``
>>> +      - Parallel decoding is used.
>>> +    * - ``V4L2_VP9_FRAME_FLAG_X_SUBSAMPLING``
>>> +      - Vertical subsampling is enabled.
>>> +    * - ``V4L2_VP9_FRAME_FLAG_Y_SUBSAMPLING``
>>> +      - Horizontal subsampling is enabled.
>>> +    * - ``V4L2_VP9_FRAME_FLAG_COLOR_RANGE_FULL_SWING``
>>> +      - The full UV range is used.
>>> +
>>> +.. c:type:: v4l2_vp9_ref_id
>>> +
>>> +.. cssclass:: longtable
>>> +
>>> +.. tabularcolumns:: |p{1.5cm}|p{6.3cm}|p{9.4cm}|
>>> +
>>> +.. flat-table:: enum v4l2_vp9_ref_id
>>> +    :header-rows:  0
>>> +    :stub-columns: 0
>>> +    :widths:       1 2
>>> +
>>> +    * - ``V4L2_REF_ID_LAST``
>>> +      - Last reference frame.
>>> +    * - ``V4L2_REF_ID_GOLDEN``
>>> +      - Golden reference frame.
>>> +    * - ``V4L2_REF_ID_ALTREF``
>>> +      - Alternative reference frame.
>>> +    * - ``V4L2_REF_ID_CNT``
>>> +      - Number of reference frames.
>>> +
>>> +.. c:type:: v4l2_vp9_tx_mode
>>> +
>>> +.. cssclass:: longtable
>>> +
>>> +.. tabularcolumns:: |p{1.5cm}|p{6.3cm}|p{9.4cm}|
>>> +
>>> +.. flat-table:: enum v4l2_vp9_tx_mode
>>> +    :header-rows:  0
>>> +    :stub-columns: 0
>>> +    :widths:       1 2
>>> +
>>> +    * - ``V4L2_VP9_TX_MODE_ONLY_4X4``
>>> +      - Transform size is 4x4.
>>> +    * - ``V4L2_VP9_TX_MODE_ALLOW_8X8``
>>> +      - Transform size can be up to 8x8.
>>> +    * - ``V4L2_VP9_TX_MODE_ALLOW_16X16``
>>> +      - Transform size can be up to 16x16.
>>> +    * - ``V4L2_VP9_TX_MODE_ALLOW_32X32``
>>> +      - transform size can be up to 32x32.
>>> +    * - ``V4L2_VP9_TX_MODE_SELECT``
>>> +      - Bitstream contains transform size for each block.
>>> +
>>> +.. c:type:: v4l2_vp9_reference_mode
>>> +
>>> +.. cssclass:: longtable
>>> +
>>> +.. tabularcolumns:: |p{1.5cm}|p{6.3cm}|p{9.4cm}|
>>> +
>>> +.. flat-table:: enum v4l2_vp9_reference_mode
>>> +    :header-rows:  0
>>> +    :stub-columns: 0
>>> +    :widths:       1 2
>>> +
>>> +    * - ``V4L2_VP9_REF_MODE_SINGLE``
>>> +      - Indicates that all the inter blocks use only a single reference frame
>>> +        to generate motion compensated prediction.
>>> +    * - ``V4L2_VP9_REF_MODE_COMPOUND``
>>> +      - Requires all the inter blocks to use compound mode. Single reference
>>> +        frame prediction is not allowed.
>>> +    * - ``V4L2_VP9_REF_MODE_SELECT``
>>> +      - Allows each individual inter block to select between single and
>>> +        compound prediction modes.
>>> +
>>> +.. c:type:: v4l2_vp9_interpolation_filter
>>> +
>>> +.. cssclass:: longtable
>>> +
>>> +.. tabularcolumns:: |p{1.5cm}|p{6.3cm}|p{9.4cm}|
>>> +
>>> +.. flat-table:: enum v4l2_vp9_interpolation_filter
>>> +    :header-rows:  0
>>> +    :stub-columns: 0
>>> +    :widths:       1 2
>>> +
>>> +    * - ``V4L2_VP9_INTERP_FILTER_8TAP``
>>> +      - Height tap filter.
>>> +    * - ``V4L2_VP9_INTERP_FILTER_8TAP_SMOOTH``
>>> +      - Height tap smooth filter.
>>> +    * - ``V4L2_VP9_INTERP_FILTER_8TAP_SHARP``
>>> +      - Height tap sharp filter.
>>> +    * - ``V4L2_VP9_INTERP_FILTER_BILINEAR``
>>> +      - Bilinear filter.
>>> +    * - ``V4L2_VP9_INTERP_FILTER_SWITCHABLE``
>>> +      - Filter selection is signaled at the block level.
>>> +
>>> +.. c:type:: v4l2_vp9_color_space
>>> +
>>> +.. cssclass:: longtable
>>> +
>>> +.. tabularcolumns:: |p{1.5cm}|p{6.3cm}|p{9.4cm}|
>>> +
>>> +.. flat-table:: enum v4l2_vp9_color_space
>>> +    :header-rows:  0
>>> +    :stub-columns: 0
>>> +    :widths:       1 2
>>> +
>>> +    * - ``V4L2_VP9_COLOR_SPACE_UNKNOWN``
>>> +      - Unknown color space. In this case the color space must be signaled
>>> +        outside the VP9 bitstream.
>>> +    * - ``V4L2_VP9_COLOR_SPACE_BT_601``
>>> +      - Rec. ITU-R BT.601-7 color space.
>>> +    * - ``V4L2_VP9_COLOR_SPACE_BT_709``
>>> +      - Rec. ITU-R BT.709-6 color space.
>>> +    * - ``V4L2_VP9_COLOR_SPACE_SMPTE_170``
>>> +      - SMPTE-170 color space.
>>> +    * - ``V4L2_VP9_COLOR_SPACE_SMPTE_240``
>>> +      - SMPTE-240 color space.
>>> +    * - ``V4L2_VP9_COLOR_SPACE_BT_2020``
>>> +      - Rec. ITU-R BT.2020-2 color space.
>>> +    * - ``V4L2_VP9_COLOR_SPACE_RESERVED``
>>> +      - Reserved. This value shall never be passed.
>>> +    * - ``V4L2_VP9_COLOR_SPACE_SRGB``
>>> +      - sRGB (IEC 61966-2-1) color space.
>>
>> Should this be here? We already have colorspace information in the pix format structs.
>>
>> It is also unclear from reading just this bit whether e.g. V4L2_VP9_COLOR_SPACE_SRGB
>> just describes the sRGB color primaries or also the transfer function and quantization
>> range.
>>
>> I'd like to see some more about how this is used. None of the other codec headers
>> have this.
>>
> 
> I think this comes directly from the spec. The Rockchip VDEC doesn't
> seem to use it.
> 
> Do you think we can drop it from here, and rely on the V4L2 colorspace
> passed in the format negotiation?

That would be preferred, yes. Otherwise you would have two places where you can
define colorspaces, and that's confusing.

>  
>>> +
>>> +.. c:type:: v4l2_vp9_reset_frame_context
>>> +
>>> +.. cssclass:: longtable
>>> +
>>> +.. tabularcolumns:: |p{1.5cm}|p{6.3cm}|p{9.4cm}|
>>> +
>>> +.. flat-table:: enum v4l2_vp9_reset_frame_context
>>> +    :header-rows:  0
>>> +    :stub-columns: 0
>>> +    :widths:       1 2
>>> +
>>> +    * - ``V4L2_VP9_RESET_FRAME_CTX_NONE``
>>> +      - Do not reset any frame context.
>>> +    * - ``V4L2_VP9_RESET_FRAME_CTX_SPEC``
>>> +      - Reset the frame context pointed by
>>> +        :c:type:`v4l2_ctrl_vp9_frame_decode_params`.frame_context_idx.
>>> +    * - ``V4L2_VP9_RESET_FRAME_CTX_ALL``
>>> +      - Reset all frame contexts.
>>> +
>>> +.. c:type:: v4l2_vp9_intra_prediction_mode
>>> +
>>> +.. cssclass:: longtable
>>> +
>>> +.. tabularcolumns:: |p{1.5cm}|p{6.3cm}|p{9.4cm}|
>>> +
>>> +.. flat-table:: enum v4l2_vp9_intra_prediction_mode
>>> +    :header-rows:  0
>>> +    :stub-columns: 0
>>> +    :widths:       1 2
>>> +
>>> +    * - ``V4L2_VP9_INTRA_PRED_DC``
>>> +      - DC intra prediction.
>>> +    * - ``V4L2_VP9_INTRA_PRED_MODE_V``
>>> +      - Vertical intra prediction.
>>> +    * - ``V4L2_VP9_INTRA_PRED_MODE_H``
>>> +      - Horizontal intra prediction.
>>> +    * - ``V4L2_VP9_INTRA_PRED_MODE_D45``
>>> +      - D45 intra prediction.
>>> +    * - ``V4L2_VP9_INTRA_PRED_MODE_D135``
>>> +      - D135 intra prediction.
>>> +    * - ``V4L2_VP9_INTRA_PRED_MODE_D117``
>>> +      - D117 intra prediction.
>>> +    * - ``V4L2_VP9_INTRA_PRED_MODE_D153``
>>> +      - D153 intra prediction.
>>> +    * - ``V4L2_VP9_INTRA_PRED_MODE_D207``
>>> +      - D207 intra prediction.
>>> +    * - ``V4L2_VP9_INTRA_PRED_MODE_D63``
>>> +      - D63 intra prediction.
>>> +    * - ``V4L2_VP9_INTRA_PRED_MODE_TM``
>>> +      - True motion intra prediction.
>>> +
>>> +.. c:type:: v4l2_vp9_segmentation
>>> +
>>> +.. cssclass:: longtable
>>> +
>>> +.. tabularcolumns:: |p{1.5cm}|p{6.3cm}|p{9.4cm}|
>>> +
>>> +.. flat-table:: struct v4l2_vp9_segmentation
>>> +    :header-rows:  0
>>> +    :stub-columns: 0
>>> +    :widths:       1 1 2
>>> +
>>> +    * - __u8
>>> +      - ``flags``
>>> +      - Combination of V4L2_VP9_SEGMENTATION_FLAG_* flags. See
>>> +        :c:type:`v4l2_vp9_segmentation_flags`.
>>> +    * - __u8
>>> +      - ``tree_probs[7]``
>>> +      - Specifies the probability values to be used when decoding segment_id.
>>
>> segment_id? Does this refer to a term defined in the VP9 spec?
>>
> 
> Yes, this is all strictly spec compliant. segment_id and seg_id_predicted
> are VP9 syntax elements.
> 
> In particular that description is copy-pasted from the spec, but
> I agree it may seem confusing in this context. How about:
> 
> "to be used when decoding a Segment-ID"

That would be better, perhaps with a reference to the spec where this is
described.

> 
>>> +    * - __u8
>>> +      - ``pred_prob[3]``
>>> +      - Specifies the probability values to be used when decoding
>>> +        seg_id_predicte.
>>
>> seg_id_predicte: typo? I don't see any field called seg_id_predicte.
>>
>> It looks like segment_id and seg_id_predicte are C field names, but I suspect
>> they are not. I think this should be clarified.
>>
> 
> Ditto,
> 
> "to be used when decoding a Predicted-Segment-ID"
> 
> Maybe this will be clearer ?

Yes, and also with a reference if possible.

> 
> 
>>> +    * - __u8
>>> +      - ``padding[5]``
>>> +      - Used to align this struct on 64 bit. Shall be filled with zeros.
>>
>> zeroes.
>>
> 
> Oops, more typos :-)
> 
> Thanks for reviewing,
> Ezequiel
> 

Regards,

	Hans