From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: Bhupesh SHARMA <bhupesh.sharma@st.com>
Cc: "linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
"linux-media@vger.kernel.org" <linux-media@vger.kernel.org>,
"balbi@ti.com" <balbi@ti.com>,
"g.liakhovetski@gmx.de" <g.liakhovetski@gmx.de>
Subject: Re: Using UVC webcam gadget with a real v4l2 device
Date: Mon, 30 Apr 2012 12:20:47 +0200 [thread overview]
Message-ID: <1649797.NTzsYukYS5@avalon> (raw)
In-Reply-To: <D5ECB3C7A6F99444980976A8C6D896384FA4445DA8@EAPEX1MAIL1.st.com>
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).
> - 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
next prev parent reply other threads:[~2012-04-30 10:20 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-04-22 18:24 Using UVC webcam gadget with a real v4l2 device Bhupesh SHARMA
2012-04-23 14:17 ` Laurent Pinchart
2012-04-23 18:46 ` Bhupesh SHARMA
2012-04-23 20:55 ` Laurent Pinchart
2012-04-25 15:06 ` Bhupesh SHARMA
2012-04-26 5:23 ` Bhupesh SHARMA
2012-04-30 10:20 ` Laurent Pinchart [this message]
2012-04-30 10:47 ` Bhupesh SHARMA
2012-05-02 11:55 ` Laurent Pinchart
2012-04-30 3:34 ` Bhupesh SHARMA
2012-04-24 0:36 ` Peter Chen
2012-04-24 9:04 ` Laurent Pinchart
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