Re: [PATCH v3 3/4] media: v4l2-common: Add kernel-doc for v4l2_fill_pixfmt_mp_aligned()

[Sven Püschel]

Hi Jacopo,

On 7/9/26 11:51 AM, Jacopo Mondi wrote:
> Hi Tommaso
>
> On Wed, Jul 08, 2026 at 06:14:04PM +0200, Tommaso Merciai wrote:
> > Replace the bare placeholder comment with a full kernel-doc block
> > documenting all parameters, the function behaviour for both single
> > memory plane (mem_planes == 1) and multiple memory plane (mem_planes > 1)
> > formats, and the return value.
> >
> > Signed-off-by: Tommaso Merciai <tommaso.merciai.xr@xxxxxxxxxxxxxx>
> > ---
> > v2->v3:
> >   - Moved to PATCH 3/4
> >   - Fixed documentation as suggested by Sven Püschel
> >
> > v1->v2:
> >   - New patch
> >
> >   include/media/v4l2-common.h | 28 +++++++++++++++++++++++++++-
> >   1 file changed, 27 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/media/v4l2-common.h b/include/media/v4l2-common.h
> > index be4dd9762196..f2b0c336ac81 100644
> > --- a/include/media/v4l2-common.h
> > +++ b/include/media/v4l2-common.h
> > @@ -591,7 +591,33 @@ static inline int v4l2_fill_pixfmt(struct v4l2_pix_format *pixfmt,
> >   	return v4l2_fill_pixfmt_aligned(pixfmt, pixelformat, width, height, 1);
> >   }
> >
> > -/* @stride_alignment is a power of 2 value in bytes */
> > +/**
> > + * v4l2_fill_pixfmt_mp_aligned - Fill in a &struct v4l2_pix_format_mplane with
> > + *	stride alignment requirements.
> > + *
> > + * @pixfmt: pointer to the &struct v4l2_pix_format_mplane to be filled
> > + * @pixelformat: the V4L2 pixel format (V4L2_PIX_FMT_*)
> > + * @width: image width in pixels
> > + * @height: image height in pixels
> > + * @stride_alignment: stride alignment in bytes; must be a power of 2
> > + *
> > + * Fills all fields of @pixfmt for the given pixel format, dimensions, and
> > + * stride alignment.
> > + *
> > + * For formats stored in a single memory plane (mem_planes == 1), the
> > + * behaviour matches v4l2_fill_pixfmt_aligned(): plane_fmt[0].bytesperline
> > + * is set to the primary plane stride. The strides of all components are
> > + * aligned to the @stride_alignment. To keep the chroma strides consistently
> > + * derivable from the luma stride, strides may be aligned to a multiple of
> > + * the @stride_alignment instead. plane_fmt[0].sizeimage covers all
>
> I guess this
>
> "To keep the chroma strides consistently derivable from the luma
> stride, strides may be aligned to a multiple of the @stride_alignment
> instead."
>
> comes from teh v4l2_format_plane_stride() implementation.
>
> I admit is not 100% clear to me why the chroma strides is multiplied
> and to which format this applies. But this is not on this patch...

When not using multi-planar formats, we only have the stride value for 
the Y component and the other stride values are derived from it. This is 
the cause of this whole scaling.

E.g. for YUV420 4x2px picture, we have 4 bytes stride in the y plane and 
2 byte in the cb and cr plane. If we align the stride to 4 bytes (in all 
planes), we want both values to be a multiple of 4. As the cb/cr stride 
is derived from the y stride, we have to set the y stride to 8 bytes to 
get the desired 4 bytes stride in the cb/cr planes.

The rare case for scaling the component stride is NV24/42 (at least this 
is the only one I currently know of), where we actually have 4:4:4 
sub-sampling and have the cb/cr parts interleaved. So for a 1x2px 
picture we have 1 bytes in the y plane and 2 bytes in the c plane. To 
align to 4 bytes we need to set the c plane stride to 8 to be able to 
set the y plane stride to 4.


For multi-planar formats we have a separate stride for each component, 
so we just align all component strides to the given alignment.


Sincerely
    Sven