linux-kernel - Re: [PATCH 02/10] media: uapi: h264: Further clarify scaling lists order

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Date:   Thu, 16 Jul 2020 09:23:14 +0200
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Ezequiel Garcia <ezequiel@...labora.com>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Tomasz Figa <tfiga@...omium.org>, kernel@...labora.com,
        Jonas Karlman <jonas@...boo.se>,
        Alexandre Courbot <acourbot@...omium.org>,
        Jeffrey Kardatzke <jkardatzke@...omium.org>,
        Nicolas Dufresne <nicolas.dufresne@...labora.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Maxime Ripard <mripard@...nel.org>,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>
Subject: Re: [PATCH 02/10] media: uapi: h264: Further clarify scaling lists
 order

On 15/07/2020 22:22, Ezequiel Garcia wrote:
> Commit 0b0393d59eb4a ("media: uapi: h264: clarify
> expected scaling_list_4x4/8x8 order") improved the
> documentation on H264 scaling lists order.
> 
> This commit improves the documentation by clarifying
> that the lists themselves are expected in matrix order.

"matrix order" is not a well defined term. Especially since different
programming languages lay out matrices differently (e.g. fortran uses
column-major order). Perhaps something like this is more unambiguous:

"The values on each scaling list are in row-major order."

BTW, why not be explicit and use:

__u8 scaling_list_4x4[6][4][4];
__u8 scaling_list_8x8[6][8][8];

That makes it explicit and the order is just that of what the C language
uses.

Regards,

	Hans

> 
> Signed-off-by: Ezequiel Garcia <ezequiel@...labora.com>
> ---
>  Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
> index c2e17c02f77e..16bfc98ab2f6 100644
> --- a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
> +++ b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
> @@ -1725,12 +1725,14 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
>        - ``scaling_list_4x4[6][16]``
>        - Scaling matrix after applying the inverse scanning process.
>          Expected list order is Intra Y, Intra Cb, Intra Cr, Inter Y,
> -        Inter Cb, Inter Cr.
> +        Inter Cb, Inter Cr. The values on each scaling list are
> +        expected in matrix order.
>      * - __u8
>        - ``scaling_list_8x8[6][64]``
>        - Scaling matrix after applying the inverse scanning process.
>          Expected list order is Intra Y, Inter Y, Intra Cb, Inter Cb,
> -        Intra Cr, Inter Cr.
> +        Intra Cr, Inter Cr. The values on each scaling list are
> +        expected in matrix order.
>  
>  ``V4L2_CID_MPEG_VIDEO_H264_SLICE_PARAMS (struct)``
>      Specifies the slice parameters (as extracted from the bitstream)
>