Hi Laurent, > -----Original Message----- > From: Laurent Pinchart [mailto:laurent.pinchart@xxxxxxxxxxxxxxxx] > Sent: Monday, April 30, 2012 3:51 PM > 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 Thursday 26 April 2012 13:23:59 Bhupesh SHARMA wrote: > > 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.. > > [snip] > > > 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? > > You need USERPTR support on one side only. In practice many (all?) soc- > camera > drivers require physically contiguous memory, so you will need to use > MMAP on > the soc-camera side and USERPTR on the UVC gadget side. DMABUF, when > merged in > the kernel, will be a better solution (but will require all drivers to > use > vb2). Perfect. So, I plan now to add vb2 support for uvc-gadget and leave soc-camera side to use the mmap stuff. Now, waiting for your pointers for managing the race-conditions in the UVC gadget and also avoiding the memcpy that is happening in the QBUF call on the UVC gadget, before I start the actual work. Thanks for your help. Regards, Bhupesh > > - 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? > > Please see above :-) > > > - 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. > > That's easy. Request the same number of buffers on both sides with > REQBUFS, > mmap() them to userspace on the soc-camera side, and then use the user > pointer > to queue them with QBUF on the UVC side. You just need to ensure that a > buffer > is never enqueued to two drivers at the same time. Wait for buffers to > be > ready on both sides with select(), and when a buffer is ready dequeue > it and > requeue it on the other side. > > > - 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?). > > DMA to userspace is quite hackish. You should use the MMAP method on > the soc- > camera side. > > -- > Regards, > > Laurent Pinchart -- 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
