[Beignet] [PATCH v2 3/3] Add document to describe the detials of v4l2 buffer sharing.

[Zhigang Gong]

One major comment is we should not introduce such an extension
without considering the queue. The correct APIs should be something
like clEnqueueAcquireMemObjectFdIntel/clEnqueueReleaseMemObjectFdIntel.
And the corresponding queue must be the parameters. Without that,
the user even has no idea how to make the external application synchronized
with opencl.

Another more generic comment for all of these internal extensions
which are not going to be submitted to the standard is as below:
It seems better to add some beignet specified suffix after Intel
which indicates this is a beignet only extension.

Let's us leave the INTEL suffix as a official name for those Intel
specific extensions which have been accepted by Khronos.

And for the internal libva sharing extension, as there is already an officially
accepted extension. It's time to just apply that offical extension rather
than the current unofficial one.


Thanks,
Zhigang Gong.

On Sat, Mar 28, 2015 at 12:34:17AM +0800, Chuanbo Weng wrote:
> This document includes the steps of using DMABUF buffer sharing between
> v4l2 and Beignet. Also steps to run corresponding example.
> 
> Signed-off-by: Chuanbo Weng <chuanbo.weng at intel.com>
> ---
>  docs/Beignet.mdwn                         |  1 +
>  docs/howto/v4l2-buffer-sharing-howto.mdwn | 67 +++++++++++++++++++++++++++++++
>  2 files changed, 68 insertions(+)
>  create mode 100644 docs/howto/v4l2-buffer-sharing-howto.mdwn
> 
> diff --git a/docs/Beignet.mdwn b/docs/Beignet.mdwn
> index aacd7d2..31c0d9a 100644
> --- a/docs/Beignet.mdwn
> +++ b/docs/Beignet.mdwn
> @@ -254,6 +254,7 @@ Documents for OpenCL application developers
>  - [[Work with old system without c++11|Beignet/howto/oldgcc-howto]]
>  - [[Kernel Optimization Guide|Beignet/optimization-guide]]
>  - [[Libva Buffer Sharing|Beignet/howto/libva-buffer-sharing-howto]]
> +- [[V4l2 Buffer Sharing|Beignet/howto/v4l2-buffer-sharing-howto]]
>  
>  The wiki URL is as below:
>  [http://www.freedesktop.org/wiki/Software/Beignet/](http://www.freedesktop.org/wiki/Software/Beignet/)
> diff --git a/docs/howto/v4l2-buffer-sharing-howto.mdwn b/docs/howto/v4l2-buffer-sharing-howto.mdwn
> new file mode 100644
> index 0000000..d5a9b56
> --- /dev/null
> +++ b/docs/howto/v4l2-buffer-sharing-howto.mdwn
> @@ -0,0 +1,67 @@
> +V4l2 Buffer Sharing HowTo
> +=========================
> +
> +Beignet has extensions (clGetMemObjectFdIntel/clCloseMemObjectFdIntel) to share gpu
> +buffer object with v4l2. So users can utilize OpenCL to do processing on input/ouput
> +buffers of v4l2 device without buffer copy.
> +
> +Prerequisite
> +------------
> +
> +Linux kernel supports DMABUF buffer sharing for v4l2 from version 3.8. DMABUF buffer
> +sharing runs well for V4L2_PIX_FMT_MJPEG format on this version, but there is a bug
> +for V4L2_PIX_FMT_YUYV format. Linux kernel 3.19.0-rc1 fix this bug, so please use kernel
> +version 3.19.0-rc1 at least if you want to utilize this feature for V4L2_PIX_FMT_YUYV
> +format.
> +
> +Steps
> +-----
> +
> +The below official v4l2 document describes the details of sharing DMA buffers between
> +v4l devices and other devices using v4l2 as a DMABUF importer:
> +[http://linuxtv.org/downloads/v4l-dvb-apis/dmabuf.html](http://linuxtv.org/downloads/v4l-dvb-apis/dmabuf.html)
> +Beignet has added extensions(clGetMemObjectFdIntel/clCloseMemObjectFdIntel) to support
> +this mechanism. Please follow the steps as below to utilize DMABUF buffer sharing between
> +v4l devices and Beignet:
> +
> +- Get the address of this extension by the function:
> +  clGetExtensionFunctionAddress("clGetMemObjectFdIntel")
> +  and clGetExtensionFunctionAddress("clCloseMemObjectFdIntel")
> +
> +- Create a number of cl buffer objects, invoke clGetMemObjectFdIntel to get these buffer
> +  objects' file descriptors.
> +
> +- Initiating streaming I/O with DMABUF buffer sharing by calling the VIDIOC_REQBUFS v4l2 ioctl.
> +
> +- Enqueue these buffers by calling the VIDIOC_QBUF, dequeue a buffer by calling VIDIOC_DQBUF,
> +  use OpenCL to do processing on this buffer and re-enqueue...
> +
> +- Close file descriptors of these buffers by clCloseMemObjectFdIntel if your program doesn't
> +  need DMABUF buffer sharing anymore.
> +
> +Sample code
> +-----------
> +
> +We have developed an example showing how to share DMA buffers between webcam and Beignet in
> +examples/v4l2_buffer_sharing directory. The webcam directly captures V4L2_PIX_FMT_YUYV frames
> +into cl buffer objects by the way of DMABUF buffer sharing, then frames are got mirror effect
> +by OpenCL kernel, and finally show on screen by libva.
> +
> +Steps to build and run this example:
> +
> +- Update your linux kernel to at least 3.19.0-rc1.
> +
> +- Make sure there is a webcam connected to your pc.
> +
> +- Add option -DBUILD_EXAMPLES=ON to enable building examples when running cmake, such as:
> +  `> mkdir build`
> +  `> cd build`
> +  `> cmake -DBUILD_EXAMPLES=ON ../`
> +
> +- Build source code:
> +  `> make`
> +
> +- Run:
> +  `> cd examples`
> +  `> . ../utests/setenv.sh`
> +  `> ./example-v4l2_buffer_sharing`
> -- 
> 1.9.1
> 
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