[FFmpeg-devel] Re: [PATCH] libavcodec/v4l2_buffers: fixing buffer alignment issue

[Zhao Zhili via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

> On Nov 13, 2025, at 11:29, Wenjie Yin via ffmpeg-devel <ffmpeg-devel@ffmpeg.org> wrote:
> 
> V4l2 drivers may return buffer width & height with aligned values, which
> may be different from frame width & height. Bytesperlines and strides
> should be check for aligned buffers when copying pixel data. When width
> aligned, copy with planar; otherwise, copy line-by-line to dest buffer.
> 
> Co-authored-by: Renjiang Han <renjiang.han@oss.qualcomm.com>
> Signed-off-by: Wenjie.Yin <quic_wenjyin@quicinc.com>
> Signed-off-by: Wenjie Yin <wenjyin@qti.qualcomm.com>
> ---
> libavcodec/v4l2_buffers.c | 68 +++++++++++++++++++++++++++++++++++----
> 1 file changed, 62 insertions(+), 6 deletions(-)
> 
> diff --git a/libavcodec/v4l2_buffers.c b/libavcodec/v4l2_buffers.c
> index d869ce9e77..5f5a8aefd5 100644
> --- a/libavcodec/v4l2_buffers.c
> +++ b/libavcodec/v4l2_buffers.c
> @@ -356,6 +356,33 @@ static int v4l2_buffer_buf_to_swframe(AVFrame *frame, V4L2Buffer *avbuf)
>     return 0;
> }
> 
> +static int v4l2_plane_to_plane_align(V4L2Buffer *out, int plane, const uint8_t* data, int size, int offset, int width, int height, int stride)
> +{
> +    unsigned int bytesused, length;
> +
> +    if (plane >= out->num_planes)
> +        return AVERROR(EINVAL);
> +
> +    length = out->plane_info[plane].length;
> +    bytesused = FFMIN(size+offset, length);
> +
> +    for (int i = 0; i < height; i++) {
> +        memcpy((uint8_t*)out->plane_info[plane].mm_addr + offset, data, width);
> +        offset += stride;
> +        data += width;
> +    }
> +
> +    if (V4L2_TYPE_IS_MULTIPLANAR(out->buf.type)) {
> +        out->planes[plane].bytesused = bytesused;
> +        out->planes[plane].length = length;
> +    } else {
> +        out->buf.bytesused = bytesused;
> +        out->buf.length = length;
> +    }
> +
> +    return 0;
> +}
> +
> static int v4l2_buffer_swframe_to_buf(const AVFrame *frame, V4L2Buffer *out)
> {
>     int i, ret;
> @@ -364,7 +391,11 @@ static int v4l2_buffer_swframe_to_buf(const AVFrame *frame, V4L2Buffer *out)
>                        fmt.fmt.pix_mp.pixelformat : fmt.fmt.pix.pixelformat;
>     int height       = V4L2_TYPE_IS_MULTIPLANAR(fmt.type) ?
>                        fmt.fmt.pix_mp.height : fmt.fmt.pix.height;
> -    int is_planar_format = 0;
> +    int bytesperline = V4L2_TYPE_IS_MULTIPLANAR(fmt.type) ?
> +                       (fmt.fmt.pix_mp.num_planes ? fmt.fmt.pix_mp.plane_fmt[0].bytesperline: 0) : fmt.fmt.pix.bytesperline;

The doc says bytesperline can be zero
 __u32           bytesperline;   /* for padding, zero if unused */

> +    int sizeimage    = V4L2_TYPE_IS_MULTIPLANAR(fmt.type) ?
> +                       (fmt.fmt.pix_mp.num_planes ? fmt.fmt.pix_mp.plane_fmt[0].sizeimage: 0) : fmt.fmt.pix.sizeimage;
> +    int is_planar_format = 0, scanline_y = 0, scanline_uv = 0;
> 
>     switch (pixel_format) {
>     case V4L2_PIX_FMT_YUV420M:
> @@ -398,16 +429,41 @@ static int v4l2_buffer_swframe_to_buf(const AVFrame *frame, V4L2Buffer *out)
>         for (i = 0; i < desc->nb_components; i++)
>             planes_nb = FFMAX(planes_nb, desc->comp[i].plane + 1);
> 
> +        switch (pixel_format)
> +        {
> +            /* Currently only NV12 & NV21 are addressed for buffer alignment issue */
> +        case V4L2_PIX_FMT_NV12:
> +        case V4L2_PIX_FMT_NV21:
> +            scanline_y = sizeimage / bytesperline * 2 / 3;
> +            scanline_uv = scanline_y / 2;
> +            break;
> +        default:
> +            /* By default, no buffer alignment issues are expected */
> +            scanline_y = height;
> +            scanline_uv = AV_CEIL_RSHIFT(height, desc->log2_chroma_h);
> +            bytesperline = frame->linesize[0];
> +            break;
> +        }

I think you can use the information in const AVPixFmtDescriptor *desc to handle
all formats than switch case for each one.

Please send a PR to https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls

> +
>         for (i = 0; i < planes_nb; i++) {
>             int size, h = height;
>             if (i == 1 || i == 2) {
>                 h = AV_CEIL_RSHIFT(h, desc->log2_chroma_h);
>             }
> -            size = frame->linesize[i] * h;
> -            ret = v4l2_bufref_to_buf(out, 0, frame->data[i], size, offset);
> -            if (ret)
> -                return ret;
> -            offset += size;
> +
> +            if (bytesperline == frame->linesize[0]) {
> +                size = frame->linesize[i] * h;
> +                ret = v4l2_bufref_to_buf(out, 0, frame->data[i], size, offset);
> +                if (ret)
> +                    return ret;
> +                offset += frame->linesize[i] * (i ? scanline_uv : scanline_y);
> +            } else {
> +                size = bytesperline * (i ? scanline_uv : scanline_y);
> +                ret = v4l2_plane_to_plane_align(out, 0, frame->data[i], size, offset, frame->linesize[i], h, bytesperline);
> +                if (ret)
> +                    return ret;
> +                offset += size;
> +            }
>         }
>         return 0;
>     }
> -- 
> 2.34.1
> 
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