[arch-dev-public] Bringing libcl 1.2 into Arch

[Lukas Jirkovsky]

Hello,
I've been toying with the idea of bumping libcl from version 1.1 to
1.2. Bellow I will sum up my findings.

Current State
-------------------
The opencl support is split into three different packages:

  * opencl-headers - provides header files necessary for compilation,
currently at version 1.1

  * libcl - provides libCL.so which is an ICD loader, ie. this
libraryis used to the actual OpenCL implementation depending on the
user needs. In arch we use proprietary libcl from the nVidia drivers.
Because nVidia supports only OpenCL 1.1 our library does, too.

  * opencl-nvidia - an OpenCL implementation. There are packages in
AUR that provide the implementation for other vendors, too, such as
intel-opencl-sdk

There are two main reasons why our OpenCL support is stuck at 1.1:
  * when the OpenCL support was introduced, libcl was only provided by
nVidia and AMD as part of their proprietary drivers, but AMD binary
drivers are not supported
  * nvidia supports only OpenCL 1.1

Proposed Changes
----------------------------
The time has changed and now there are two opensource ICD loaders available:
  * ocl-icd (https://aur.archlinux.org/packages/ocl-icd/)
  * opencl-icd (https://aur.archlinux.org/packages/opencl-icd/)
Both ICD loaders provide libCL.so in version 1.2. Both ICD loaders are
drop-in replacements for the current libcl, ie. no rebuild is needed.

I propose the following changes:
  1. Update the opencl-headers to 1.2 (again). There is a package
opencl-headers12 that grabs them from svn.
  2. replace libcl with either ocl-icd or opencl-icd

Reasons for change:
  * replace proprietary libCL.so with an open source one
  * support for OpenCL 1.2. While nVidia doesn't support this, it will
allow using OpenCL 1.2 features on devices that support it (Intel
CPU's, AMD GPU's) which couldn't have been used before as they were
not provided by libCL

Possible Problems
---------------------------
So far I was able to find only one possible problem. Applications
using OpenCL has to check for the supported OpenCL version on runtime.
However, if an application does the check only on compile time, it
will crash. IIRC this used to be a problem with luxmark, but I tested
it and it works correctly now. I have also tested imagemagick and
blender and both work fine. Well, blender is broken for me, but it was
before, too. At least it doesn't crash.

In more detail, the reason is that you can compile OpenCL 1.2
application without problems even on OpenCL 1.1 system as the
libraries and headers will now provide everything needed. However, if
the application tries to call an OpenCL 1.2 function, the driver will
not provide it, in which case it will crash.

In any case if such issue arises, it needs to be fixed upstream.

The BIG Question
--------------------------
We once had problem that there were no suitable ICD loaders part the
one from nVidia. Now we have two. The question is which one to use.

  * opencl-icd is the reference ICD loader from Khronos. It uses a
custom license (or at least a license I don't know) which seems to
forbid distributing of sources, but which explicitly allows
redistribution of the compiled binaries

  * ocl-icd is an independent project under 2-clause BSD license. I
think from the two this one has longer development

Lukas