[Bf-cycles] Regarding the state of cycles OpenCL

[Martijn Berger]

Currently OpenCL support is hidden behind an environment variable called
‘CYCLES_OPENCL_TEST’. I believe this should change for 2.69 for the
following reasons.

1) It discourages use of the OpenCL backend. While it is true the
experience of using opencl might be less good than using cuda on some
platforms and fail totally on some others (AMD/ATI GPU’s) it is not Blender
fault as much as it is the vendors providing not the best possible OpenCL
support. Users actually complaining to them would help us.

2) It discourages development. If no one uses it no one cares and no one
will invest time to keep OpenCL working or make it work more great.

3) It discourages industry backing the project. They could perceive the
current state of our OpenCL support as not enabled or even non existent.

All of this enforces the chicken - egg problem we are having with regards
to OpenCL

I would like to propose that we change this and enable OpenCL before
releasing 2.69. Given how close we are to the blender 2.7x development
changes and the special status that will give 2.69x the change should
really happen this cycles to have any impact.

Maybe we could add a warning in some cases for platforms where we know
using it is not recommended.

Further I think we should communicate very clearly with the vendors of GPU
and GPU like platforms that we might need something like Nvidia’s
maxrregcount setting as we do not want cycles complexity to eat up all to
available registers and thereby decrease the amount of threads to much. For
performance on those platforms we need a lot of threads even though that
implies spilling and for the best performance we will most likely need to
be able to influence how much threads we want to use.

As a last point I would like to express how stunning it is to me that
specifically AMD seems to have such a hard time getting these features in
place. I think they need them to compete in the GPU space and they need to
compete their to make the their APU strategy work.
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