Re: DRM/V4L2 buffer sharing

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

Hi Mauro,

On Friday 17 August 2012 19:03:47 Mauro Carvalho Chehab wrote:
> Em 17-08-2012 18:01, Sylwester Nawrocki escreveu:
> > On 08/15/2012 11:09 PM, Laurent Pinchart wrote:
> >> On Wednesday 15 August 2012 18:13:19 Sylwester Nawrocki wrote:
> >>> On 08/15/2012 12:06 AM, Laurent Pinchart wrote:
> >>>> On Tuesday 14 August 2012 18:37:23 Sylwester Nawrocki wrote:
> >>>>> On 08/14/2012 03:04 PM, Mauro Carvalho Chehab wrote:
> >>>>>> This one requires more testing:
> >>>>>> 
> >>>>>> May,15 2012: [GIT,PULL,FOR,3.5] DMABUF importer feature in V4L2 API
> >>>>>> 
> >>>>>>            http://patchwork.linuxtv.org/patch/11268  Sylwester
> >>>>>>            Nawrocki
> >>>>>> 
> >>>>>> <s.nawrocki@samsung.com>
> >>>>> 
> >>>>> Hmm, this is not valid any more. Tomasz just posted a new patch series
> >>>>> that adds DMABUF importer and exporter feature altogether.
> >>>>> 
> >>>>> [PATCHv8 00/26] Integration of videobuf2 with DMABUF
> >>>>> 
> >>>>> I guess we need someone else to submit test patches for other H/W than
> >>>>> just Samsung SoCs. I'm not sure if we've got enough resources to port
> >>>>> this to other hardware. We have been using these features internally
> >>>>> for some time already. It's been 2 kernel releases and I can see only
> >>>>> Ack tags from Laurent on Tomasz's patch series, hence it seems there
> >>>>> is no wide interest in DMABUF support in V4L2 and this patch series is
> >>>>> probably going to stay in a fridge for another few kernel releases.
> >>>> 
> >>>> What would be required to push it to v3.7 ?
> >>> 
> >>> Mauro requested more test coverage on that, which is understood since
> >>> this is a fairly important API enhancement and the V4L2 video overlay
> >>> API replacement.
> >>> 
> >>> We need DMABUF support added at some webcam driver and a DRM driver with
> >>> prime support (or some V4L2 output driver), I guess it would be best to
> >>> have that in a PC environment. It looks like i915/radeon/nouveau drivers
> >>> already have prime support.
> >> 
> >> uvcvideo has recently been moved to videobuf2, using vb2_vmalloc. I can
> >> easily test that, except that I have no idea how to export buffers on
> >> the i915 side when X is running. Have you looked into that ?
> > 
> > All right. Yes, I'm also not sure yet how to do it. I tried it on a laptop
> > with i915 driver, but in the running system drmModeGetResources() just
> > fails with EPERM. I've CCed dri-devel, so hopefully someone can shed some
> > light on this.
> 
> Likely, you need to run with root permission to use it, or to write an Xorg
> driver.
> 
> It is probably easier to get the V4L driver there, that uses the
> VIDIOC_OVERLAY stuff, and make it work via DMABUF:
> 	http://cgit.freedesktop.org/xorg/driver/xf86-video-v4l/

That won't really help for our test cases. I want to capture from a UVC device 
using DMABUF import directly to the i915 DRM device using DRM export. In order 
to do so I will need to get hold of GEM objects that I can use to display the 
data, possibly through the OpenGL API. I'm looking for help on that last 
point, I can easily handle the UVC capture code myself.

> In order to test it, xawtv has already the code needed to talk with the v4l
> plugin.
> 
> What the plugin does is to export the video board as a XV extension,
> accessible via xawtv. It currently talks with the display card also via XV,
> but I believe it won't be hard to port it to work with DMABUF.
> 
> As the interface between xawtv and the v4l plugin is just Xv, changing the
> code there from VIDIOC_OVERLAY to DMABUF should be trivial.

-- 
Regards,

Laurent Pinchart