Hi Laurent, > -----Original Message----- > From: Bhupesh SHARMA > Sent: Thursday, April 26, 2012 10:54 AM > To: 'Laurent Pinchart' > Cc: 'linux-usb@xxxxxxxxxxxxxxx'; 'linux-media@xxxxxxxxxxxxxxx'; > 'balbi@xxxxxx'; 'g.liakhovetski@xxxxxx' > Subject: RE: Using UVC webcam gadget with a real v4l2 device > > Hi Laurent, > > Sorry to jump-in before your reply on my previous mail, > but as I was studying the USERPTR stuff in more detail, I have a few > more > queries which I believe you can include in your reply as well.. > > > -----Original Message----- > > From: Bhupesh SHARMA > > Sent: Wednesday, April 25, 2012 8:37 PM > > To: 'Laurent Pinchart' > > Cc: linux-usb@xxxxxxxxxxxxxxx; linux-media@xxxxxxxxxxxxxxx; > > balbi@xxxxxx; g.liakhovetski@xxxxxx > > Subject: RE: Using UVC webcam gadget with a real v4l2 device > > > > Hi Laurent, > > > > > -----Original Message----- > > > From: Laurent Pinchart [mailto:laurent.pinchart@xxxxxxxxxxxxxxxx] > > > Sent: Tuesday, April 24, 2012 2:26 AM > > > To: Bhupesh SHARMA > > > Cc: linux-usb@xxxxxxxxxxxxxxx; linux-media@xxxxxxxxxxxxxxx; > > > balbi@xxxxxx; g.liakhovetski@xxxxxx > > > Subject: 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 am now a bit confused on how the entire system will work now: > - Does USERPTR method needs to be supported both in UVC gadget > and soc-camera side, > or one can still support the MMAP method and the other can now > be changed to support USERPTR method > and we can achieve a ZERO buffer copy operation using this > method? > > - More specifically, I would like to keep the soc-camera still > using MMAP (and hence still using video-buf) > and make changes at the UVC gadget side to support USERPTR and > videobuf2. Will this work? > > - At the application side how should we design the flow in case > both support USERPTR, i.e. the buffer needs > to be protected from simultaneous access from the UVC gadget > driver and soc-camera driver (to ensure that > a single buffer can be shared across them). Also in case we > keep soc-camera still using MMAP and UVC gadget > side supporting USERPTR, how can we share a common buffer > across the UVC gadget and soc-camera driver. > > - In case of USERPTR method the camera capture hardware should be > able to DMA the received data to the user > space buffers. Are there any specific requirements on the DMA > capability of these use-space buffers > (scatter-gather or contiguous?). Can you help me with these queries. Thanks, Bhupesh > Regards, > Bhupesh > > > > > > 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. > > > > It's a pity. You are the best person to do it as you have in-depth > > know > > -how of both v4l2 and UVC webcam gadget. But I can give it a try if > you > > can provide me 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. > > > > That's what I was afraid of. But can you let me know where the gadget > > driver > > performs a second copy internally, so that I can also start exploring > > the > > USERPTR method using the pointers provided by you.. > > > > Regards, > > Bhupesh -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
