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

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Date:   Thu, 16 Jul 2020 13:44:58 +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 16/07/2020 13:43, Ezequiel Garcia wrote:
> On Thu, 2020-07-16 at 09:23 +0200, Hans Verkuil wrote:
>> 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:
>>
> 
> Agreed, "matrix order" is perhaps not a proper choice of words.
> 
>> "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.
>>
> 
> I am not sure if that'll go in clearer direction.
> 
> I'm thinking we just need to clarify the coefficients
> are in raster scan order, as opposed to a zig-zag scan
> order, which is a part of the decoding process.
> 
> How about we simply say "raster scan order" and keep the patch as is?

That works for me.

Regards,

	Hans

> 
> Thanks,
> Ezequiel
> 
> 
>> 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)
>>>
> 
>