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Re: Using UVC webcam gadget with a real v4l2 device

Hi Bhupesh,

On Tuesday 24 April 2012 02:46:22 Bhupesh SHARMA wrote:
> On Monday, April 23, 2012 7:47 PM Laurent Pinchart wrote:
> > On Monday 23 April 2012 02:24:53 Bhupesh SHARMA wrote:
> > > Hi Laurent,
> > > 
> > > I have been doing some experimentation with the UVC webcam gadget along
> > > with the UVC user-space application which you have written.
> > > 
> > > The UVC webcam gadget works fine with the user space application
> > > handling the CONTROL events and providing DATA events. Now, I wish to
> > > interface a real v4l2 device, for e.g. VIVI or more particularly a
> > > soc_camera based host and subdev pair.
> > > 
> > > Now, I see that I can achieve this by opening the UVC and V4L2 devices
> > > and doing MMAP -> REQBUF -> QBUF -> DQBUF calls on both the devices per
> > > the UVC control event received. But this will involve copying the video
> > > buffer in the user-space application from v4l2 (_CAPTURE) to uvc
> > > (_OUTPUT) domains, which will significantly reduce the video capture
> > > performance.
> > > 
> > > Is there a better solution to this issue? Maybe doing something like a
> > > RNDIS gadget does with the help of u_ether.c like helper routines. But
> > > if I remember well it also requires the BRCTL (Bridge Control Utility)
> > > in userspace to route data arriving on usb0 to eth0 and vice-versa. Not
> > > sure though, if it does copying of a skb buffer from ethernet to usb
> > > domain and vice-versa.
> > 
> > To avoid copying data between the two devices you should use USERPTR
> > instead of MMAP on at least one of the two V4L2 devices. The UVC gadget
> > driver doesn't support USERPTR yet though. This shouldn't be too difficult
> > to fix, we need toreplace the custom buffers queue implementation with
> > videobuf2, as has been done in the uvcvideo driver.
> I was thinking of using the USERPTR method too, but I realized that
> currently neither UVC webcam gadget nor soc-camera subsystem supports this
> IO method. They support only MMAP IO as of now :(

Both soc-camera and the UVC gadget driver should be ported to videobuf2 to fix 
the problem.

> > I'll try to implement this. Would you then be able to test patches ?
> For sure, I can test your patches on my setup.

I had a quick look, but there's a bit more work than expected. The UVC gadget 
driver locking scheme needs to be revisited. I unfortunately won't have time 
to work on that in the next couple of weeks, and very probably not before end 
of June. Sorry.

If you want to give it a try, I can provide you with some pointers.

> BTW, I was exploring GSTREAMER to use the data arriving from soc-camera
> (v4l2) capture device '/dev/video1' via 'v4l2src' plugin and routing the
> same to the UVC gadget '/dev/video0' via the 'v4l2sink' plugin.
> Don't know if this can work cleanly in my setup and whether GSTREAMER
> actually performs a buffer copy internally. But I will at-least give it a
> try :)

There will definitely be a buffer copy (and actually two copies, as the UVC 
gadget driver performs a second copy internally) if you don't use USERPTR.


Laurent Pinchart

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