Re: [PATCH v3 2/4] media: v4l2-common: Add v4l2_fill_pixfmt_aligned() helper

Hi Jacopo,

On 7/9/26 11:35 AM, Jacopo Mondi wrote:
> Hi Tommaso
>
> On Wed, Jul 08, 2026 at 06:14:03PM +0200, Tommaso Merciai wrote:
>
>> + *
>> + * @pixfmt: pointer to the &struct v4l2_pix_format 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. Only formats stored in a single memory plane are
>> + * supported; returns -EINVAL for multi-memory-plane formats.
>> + *
>> + * @pixfmt->bytesperline is set to the stride of the primary (plane 0) plane,
>> + * rounded up to a multiple of @stride_alignment. For formats that store
>> + * multiple component planes in a single memory buffer (e.g. NV12), the
>> + * alignment applied to each component plane's stride is scaled relative to
>> + * @stride_alignment so that the chroma stride remains consistently derivable
> Does this rather mean that
>
> "For formats that store multiple component planes in a single memory
> buffer (e.g. NV12), the alignment applied to each component plane is
> the first plane @stride_alignment scaled by the plane's sub-sampling
> ratio" or have I mis-read this ?

No, for the example of NV12, no stride will get scaled (although the 
sub-sampling of 4:2:0, resulting in a vdiv and hdiv of 2).

This is due to the fact, that while we have a hdiv of 2 we also 
interleave the cb and cr parts in a single plane, which results in the 
stride being the same number of bytes as for the y plane (and vdiv isn't 
relevant for the stride).

Therefore the stride scaling also respects the bits per plane (bpp) 
value to determine the scaling.

@Tommaso : While the sentence looks ok, the NV12 example is misguided. 
The intention is that for non-mp (not ending with M) formats we might do 
the scaling (e.g. YUV420 will have it's Y component stride alignment 
scaled to not break the u and v stride alignments, but YUV420M not)

Sincerely
     Sven