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

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Date:   Thu, 16 Jul 2020 08:43:22 -0300
From:   Ezequiel Garcia <ezequiel@...labora.com>
To:     Hans Verkuil <hverkuil@...all.nl>, 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 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?

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)
> >