Re: [Linaro-mm-sig] [RFCv2 PATCH 4/9] v4l: add buffer exporting via dmabuf

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

Hi Subash,

On Thursday 22 March 2012 19:27:01 Subash Patel wrote:
> On 03/22/2012 04:46 PM, Laurent Pinchart wrote:
> > On Tuesday 13 March 2012 11:17:02 Tomasz Stanislawski wrote:

[snip]

> >> 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 ?
> 
> +1 to Laurent. Any particular need for reserved0 and reserved[13] below?
> I think one void user pointer is sufficient even for future need.

I'd rather have more than one void user pointer, just in case. A couple of 
bytes won't be expensive, and they will save lots of hassle in the future if 
we need to add a couple of fields. I was just wondering why there was a 
reserved field between fd and mem_offset.

> >> +	__u32		mem_offset;
> >> +	__u32		reserved[13];
> >> +};
> >> +
> 
> Also, what is the reason for returning the fd through this structure? To
> keep it aligned with other v4l2 calls? I liked(or now hate making change
> in the app) how it was being returned through the ioctl in your last patch.

Probably to be consistent with the V4L2 API, yes. It won't make a big 
difference for applications, I would favor consistency in this case.

-- 
Regards,

Laurent Pinchart