Re: [Linaro-mm-sig] [PATCHv6 00/13] Integration of videobuf2 with dmabuf

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

Hi Hans,

On Wednesday 06 June 2012 10:17:03 Hans Verkuil wrote:
> On Wed 6 June 2012 05:46:34 Laurent Pinchart wrote:
> > On Monday 04 June 2012 12:34:23 Rebecca Schultz Zavin wrote:
> > > I have a system where the data is planar, but the kernel drivers
> > > expect to get one allocation with offsets for the planes.  I can't
> > > figure out how to do that with the current dma_buf implementation.  I
> > > thought I could pass the same dma_buf several times and use the
> > > data_offset field of the v4l2_plane struct but it looks like that's
> > > only for output.  Am I missing something?  Is this supported?
> > 
> > data_offset is indeed for video output only at the moment, and doesn't
> > seem to be used by any driver in mainline for now.
> 
> Actually, data_offset may be set by capture drivers. For output buffers it
> is set by userspace, for capture buffers it is set by the driver. This
> data_offset typically contains meta data.

Is that documented somewhere ? I wasn't aware of this use case.

> > I can't really see a reason why data_offset couldn't be used for video
> > capture devices as well.
> > 
> > Sanity checks are currently missing. For output devices we should check
> > that data_offset + bytesused < length in the vb2 core. For input devices
> > the check will have to be performed by drivers. Taking data_offset into
> > account automatically would also be useful. I think most of that should
> > be possible to implement in the allocators.
> 
> See this proposal of how to solve this:
> 
> http://www.spinics.net/lists/linux-media/msg40376.html

This requires more discussions regarding how the app_offset and data_offset 
fields should be used for the different memory types we support.

For instance app_offset would not make that much sense for the USERPTR memory 
type, as we can include the offset in the user pointer already (using 
app_offset there would only make the code more complex without any added 
benefit).

For the MMAP memory type adding an app_offset would require allocating buffers 
large enough to accomodate the offset, and would thus only be useful with 
CREATE_BUFS. I'm also wondering whether the main use case (passing the buffer 
to another device that requires that app_offset) wouldn't be better addressed 
by the DMABUF memory type anyway.

Comments are welcome.

-- 
Regards,

Laurent Pinchart