Re: [RFCv2 PATCH 4/9] v4l: add buffer exporting via dmabuf

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

Hi Tomasz,

Thanks for the patch.

On Tuesday 13 March 2012 11:17:02 Tomasz Stanislawski wrote:
> This patch adds extension to V4L2 api. It allow to export a mmap buffer as
> file descriptor. New ioctl VIDIOC_EXPBUF is added. It takes a buffer offset
> used by mmap and return a file descriptor on success.

I know code is more fun to write than documentation, but 
Documentation/DocBook/media/v4l will be sad if this patch is merged as-is ;-)

> Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
> Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
> ---
>  drivers/media/video/v4l2-compat-ioctl32.c |    1 +
>  drivers/media/video/v4l2-ioctl.c          |   11 +++++++++++
>  include/linux/videodev2.h                 |   20 ++++++++++++++++++++
>  include/media/v4l2-ioctl.h                |    2 ++
>  4 files changed, 34 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/media/video/v4l2-compat-ioctl32.c
> b/drivers/media/video/v4l2-compat-ioctl32.c index e6f67aa..fd157cb 100644
> --- a/drivers/media/video/v4l2-compat-ioctl32.c
> +++ b/drivers/media/video/v4l2-compat-ioctl32.c
> @@ -954,6 +954,7 @@ long v4l2_compat_ioctl32(struct file *file, unsigned int
> cmd, unsigned long arg) case VIDIOC_S_FBUF32:
>  	case VIDIOC_OVERLAY32:
>  	case VIDIOC_QBUF32:
> +	case VIDIOC_EXPBUF:
>  	case VIDIOC_DQBUF32:
>  	case VIDIOC_STREAMON32:
>  	case VIDIOC_STREAMOFF32:
> diff --git a/drivers/media/video/v4l2-ioctl.c
> b/drivers/media/video/v4l2-ioctl.c index 74cab51..a125016 100644
> --- a/drivers/media/video/v4l2-ioctl.c
> +++ b/drivers/media/video/v4l2-ioctl.c
> @@ -207,6 +207,7 @@ static const char *v4l2_ioctls[] = {
>  	[_IOC_NR(VIDIOC_S_FBUF)]           = "VIDIOC_S_FBUF",
>  	[_IOC_NR(VIDIOC_OVERLAY)]          = "VIDIOC_OVERLAY",
>  	[_IOC_NR(VIDIOC_QBUF)]             = "VIDIOC_QBUF",
> +	[_IOC_NR(VIDIOC_EXPBUF)]           = "VIDIOC_EXPBUF",
>  	[_IOC_NR(VIDIOC_DQBUF)]            = "VIDIOC_DQBUF",
>  	[_IOC_NR(VIDIOC_STREAMON)]         = "VIDIOC_STREAMON",
>  	[_IOC_NR(VIDIOC_STREAMOFF)]        = "VIDIOC_STREAMOFF",
> @@ -938,6 +939,16 @@ static long __video_do_ioctl(struct file *file,
>  			dbgbuf(cmd, vfd, p);
>  		break;
>  	}
> +	case VIDIOC_EXPBUF:
> +	{
> +		struct v4l2_exportbuffer *p = arg;
> +
> +		if (!ops->vidioc_expbuf)
> +			break;
> +
> +		ret = ops->vidioc_expbuf(file, fh, p);

You can pass arg to ops->vidioc_expbuf() directly, there's no need to create a 
struct v4l2_exportbuffer *p variable.

> +		break;
> +	}
>  	case VIDIOC_DQBUF:
>  	{
>  		struct v4l2_buffer *p = arg;
> diff --git a/include/linux/videodev2.h b/include/linux/videodev2.h
> index bb6844e..e71c787 100644
> --- a/include/linux/videodev2.h
> +++ b/include/linux/videodev2.h
> @@ -680,6 +680,25 @@ struct v4l2_buffer {
>  #define V4L2_BUF_FLAG_NO_CACHE_INVALIDATE	0x0800
>  #define V4L2_BUF_FLAG_NO_CACHE_CLEAN		0x1000
> 
> +/**
> + * struct v4l2_exportbuffer - export of video buffer as DMABUF file
> descriptor
> + *
> + * @fd:		file descriptor associated with DMABUF (set by driver)
> + * @mem_offset:	for non-multiplanar buffers with memory ==
> V4L2_MEMORY_MMAP;

I don't think we will ever support exporting anything else than 
V4L2_MEMORY_MMAP buffers. What will happen for multiplanar buffers ?

> + *		offset from the start of the device memory for this plane,
> + *		(or a "cookie" that should be passed to mmap() as offset)
> + *

Shouldn't the mem_offset field always be set to the mmap cookie value ? I'm a 
bit confused by the "or" part, it seems to have been copied from the 
v4l2_buffer documentation directly. We should clarify that.

> + * Contains data used for exporting a video buffer as DMABUF file
> + * descriptor. Uses the same 'cookie' as mmap() syscall. All reserved
> fields
> + * must be set to zero.
> + */
> +struct v4l2_exportbuffer {
> +	__u32		fd;
> +	__u32		reserved0;

Why is there a reserved field here ?

> +	__u32		mem_offset;
> +	__u32		reserved[13];
> +};
> +
>  /*
>   *	O V E R L A Y   P R E V I E W
>   */
> @@ -2303,6 +2322,7 @@ struct v4l2_create_buffers {
>  #define VIDIOC_S_FBUF		 _IOW('V', 11, struct v4l2_framebuffer)
>  #define VIDIOC_OVERLAY		 _IOW('V', 14, int)
>  #define VIDIOC_QBUF		_IOWR('V', 15, struct v4l2_buffer)
> +#define VIDIOC_EXPBUF		_IOWR('V', 16, struct v4l2_exportbuffer)
>  #define VIDIOC_DQBUF		_IOWR('V', 17, struct v4l2_buffer)
>  #define VIDIOC_STREAMON		 _IOW('V', 18, int)
>  #define VIDIOC_STREAMOFF	 _IOW('V', 19, int)
> diff --git a/include/media/v4l2-ioctl.h b/include/media/v4l2-ioctl.h
> index 4df031a..d8716c6f 100644
> --- a/include/media/v4l2-ioctl.h
> +++ b/include/media/v4l2-ioctl.h
> @@ -120,6 +120,8 @@ struct v4l2_ioctl_ops {
>  	int (*vidioc_reqbufs) (struct file *file, void *fh, struct
> v4l2_requestbuffers *b); int (*vidioc_querybuf)(struct file *file, void
> *fh, struct v4l2_buffer *b); int (*vidioc_qbuf)    (struct file *file, void
> *fh, struct v4l2_buffer *b);
> +	int (*vidioc_expbuf)  (struct file *file, void *fh,
> +				struct v4l2_exportbuffer *e);
>  	int (*vidioc_dqbuf)   (struct file *file, void *fh, struct v4l2_buffer
> *b);
> 
>  	int (*vidioc_create_bufs)(struct file *file, void *fh, struct
> v4l2_create_buffers *b);

-- 
Regards,

Laurent Pinchart