Re: [PATCH v3 6/9] media: videobuf2: Allow exporting of a struct dmabuf

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

(CC'ing Hans)

Tomasz, Hans, could you review this patch ? It is the only one in the
series that touches the V4L2 core.

On Fri, Jul 17, 2026 at 04:34:21PM +0530, Jai Luthra wrote:
> From: Dave Stevenson <dave.stevenson@raspberrypi.org>
> 
> videobuf2 only allowed exporting a dmabuf as a file descriptor,
> but there are instances where having the struct dma_buf is
> useful within the kernel.
> 
> Split the current implementation into two, one step which
> exports a struct dma_buf, and the second which converts that
> into an fd.
> 
> Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>
> Signed-off-by: Jai Luthra <jai.luthra@ideasonboard.com>
> ---
>  drivers/media/common/videobuf2/videobuf2-core.c | 21 ++++++++++++++++++---
>  include/media/videobuf2-core.h                  | 15 +++++++++++++++
>  2 files changed, 33 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> index b0a6084f1757..0e62f3d97194 100644
> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> @@ -2419,11 +2419,11 @@ static int __find_plane_by_offset(struct vb2_queue *q, unsigned long offset,
>  	return 0;
>  }
>  
> -int vb2_core_expbuf(struct vb2_queue *q, int *fd, unsigned int type,
> -		    struct vb2_buffer *vb, unsigned int plane, unsigned int flags)
> +int vb2_core_expbuf_dmabuf(struct vb2_queue *q, unsigned int type,
> +			   struct vb2_buffer *vb, unsigned int plane,
> +			   unsigned int flags, struct dma_buf **dmabuf)
>  {
>  	struct vb2_plane *vb_plane;
> -	int ret;
>  	struct dma_buf *dbuf;
>  
>  	if (q->memory != VB2_MEMORY_MMAP) {
> @@ -2468,6 +2468,21 @@ int vb2_core_expbuf(struct vb2_queue *q, int *fd, unsigned int type,
>  		return -EINVAL;
>  	}
>  
> +	*dmabuf = dbuf;
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(vb2_core_expbuf_dmabuf);
> +
> +int vb2_core_expbuf(struct vb2_queue *q, int *fd, unsigned int type,
> +		    struct vb2_buffer *vb, unsigned int plane, unsigned int flags)
> +{
> +	struct dma_buf *dbuf;
> +	int ret;
> +
> +	ret = vb2_core_expbuf_dmabuf(q, type, vb, plane, flags, &dbuf);
> +	if (ret)
> +		return ret;
> +
>  	ret = dma_buf_fd(dbuf, flags & ~O_ACCMODE);
>  	if (ret < 0) {
>  		dprintk(q, 3, "buffer %d, plane %d failed to export (%d)\n",
> diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
> index 4b4f4c15c53a..15aeaaeaf8ad 100644
> --- a/include/media/videobuf2-core.h
> +++ b/include/media/videobuf2-core.h
> @@ -954,6 +954,21 @@ int vb2_core_streamon(struct vb2_queue *q, unsigned int type);
>   */
>  int vb2_core_streamoff(struct vb2_queue *q, unsigned int type);
>  
> +/**
> + * vb2_core_expbuf_dmabuf() - Export a buffer as a dma_buf structure
> + * @q:         videobuf2 queue
> + * @type:      buffer type

The type must be identical to the queue type. I would remove the
argument from the function, and move the type check to
vb2_core_expbuf().

A separate patch could then push the check out of vb2_core_expbuf() as
well, as in most cases the type received from userspace in the
v4l2_exportbuffer structure is used to get the corresponding queue.

> + * @index:     id number of the buffer

This isn't correct, the function takes a pointer to a vb2_buffer.

> + * @plane:     index of the plane to be exported, 0 for single plane queues
> + * @flags:     flags for newly created file, currently only O_CLOEXEC is
> + *             supported, refer to manual of open syscall for more details
> + * @dmabuf:    Returns the dmabuf pointer

Is there a reason not to return the dmabuf directly from the function,
with an error pointer for errors ?

> + *
> + */
> +int vb2_core_expbuf_dmabuf(struct vb2_queue *q, unsigned int type,
> +			   struct vb2_buffer *vb, unsigned int plane,
> +			   unsigned int flags, struct dma_buf **dmabuf);
> +
>  /**
>   * vb2_core_expbuf() - Export a buffer as a file descriptor.
>   * @q:		pointer to &struct vb2_queue with videobuf2 queue.

-- 
Regards,

Laurent Pinchart