Bug#889287: opencl-headers 2.0 conflict with nvidia-libopencl1

[Vincent]

Package: nvidia-opencl-dev
Version: 7.5.18-4~bpo8+1

On a Debian8 machine:
- fresh installed from the ec2 debian8 image
- with jessie-backports repository

installing ‘ nvidia-opencl-dev’ from jessie-backports triggers the install of (among other packages) of:
- nvidia-libopencl1 amd64 375.82-1~deb9u1~bpo8+1
- opencl-headers 2.0~svn31815-2~bpo8+1  

	The issue is that nvidia-libopencl1 provides an ICD loader in libOpenCL.so which only supports OpenCL 1.2, whereas opencl-headers 2.0 advertises OpenCL version 2, which is inconsistent and leads to compilation errors (see the exchange on https://lists.tiker.net/pipermail/pyopencl/2018-February/002340.html <https://lists.tiker.net/pipermail/pyopencl/2018-February/002340.html>).

#####################
	Exact script from a base debian8 image to reproduce:

echo "deb http://httpredir.debian.org/debian/ jessie contrib non-free" >> jessie-contrib-nonfree-backports.list
echo "deb http://httpredir.debian.org/debian/ jessie-backports main contrib non-free" >> jessie-contrib-nonfree-backports.list
sudo mv jessie-contrib-nonfree-backports.list  /etc/apt/sources.list.d/
sudo apt-get update
sudo apt-get -t jessie-backports install -y nvidia-opencl-dev

.. and then /usr/lib/x86_64-linux-gnu/libOpenCL.so does not have OpenCL 2.0 symbols (e.g. clSVMFree) 

but /usr/include/CL/cl.h advertises OpenCL 2.0:

/usr/include/CL/cl.h:198:#define CL_VERSION_2_0                              1

#####################

	Possible solutions (I did not look at the detailed decency tree..) may be:
- nvidia-libopencl1 375.82 to request opencl-headers version <=1.2 (which is in jessie)
- do not install nvidia-libopencl1 but rather the generic ICD loader ocl-icd-opencl-dev

	For those who meet this, an immediate workaround is to first install opencl-headers from jessie repository (without the ‘-t jessie-backports’), and only then install nvidia-opencl-dev / nvidia-cuda-toolkit) from jessie-backport.

	Thanks,
-- 
Vincent Favre-Nicolin

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