OpenCL How to use both GPUs in battery saving mode

Question

Hello everyone,
Currently I'm developing a software using OpenCL under OS X 10.6.4.
The galaxies (n-body) application provided by Apple is able to query and use both the 9400M and 9600M GT of my Macbook Pro. The interesting thing is, it can even do so in battery saving mode i.e. when the 9400M is the display device and the 9600 should be turned off. Looking at the XCode project and source code it just does:
return_value = clGetDeviceIDs(NULL, CLDEVICE_TYPEGPU, 4, mdeviceid, &count);
To get a device count of 2.
My Makefile based project also links with -framework OpenCL. Executing the same line of code yields just the 9400M though.
Since the NVidia samples show the same behaviour I guess there is a difference in the way the XCode project is set up.
Thanks in advance for any hints!
Kai

Answer 1

Just found out the following:
It is possible to utilise the 9600 in battery saving mode for calculation purposes.
All you need to do is to create an OpenGL context and iterate over the virtual screens like so:
GLint i = 0;
CGLContextObj ctx = CGLGetCurrentContext();
while(CGLSetVirtualScreen(ctx, i) == kCGLNoError) {
i++;
After doing that, a call to clGetDeviceIDs yields two gpu devices:
unsigned int count = 0;
cldeviceid mdeviceid[32];
clGetDeviceIDs(NULL, CLDEVICE_TYPEGPU, 32, mdeviceid, &count);
printf("Found %d GPU devices...\n", count);
Test environment:
Macbook Pro (first unibody, w. 9400M + 9600M GT).
OS X 10.6.4
Explicitly settings the pixel format attribute to allow for offline renderers wasn't necessary in my case.
Unfortunately this solution sounds rather like a bug than feature, so Apple might 'fix' it in the future.
What is more, being required to carry around an OpenGL context might is not possible in every case.
Would be nice if we could just decide which card is used for display purposes in high performance mode, then there would be no reason to fallback to these kind of workarounds...
Kai