OpenCL driver: new environment variable STARPU_OPENCL_ONLY_ON_CPUS to only enable OpenCL CPU devices

ChangeLog

@@ -61,6 +61,9 @@ New features:
   * Add support for OpenCL simulators (which provide simulated execution time)
   * Add support for Temanejo, a task graph debugger
   * Theoretical bound lp output now includes data transfer time.
+  * Update OpenCL driver to only enable CPU devices (the environment
+        variable STARPU_OPENCL_ONLY_ON_CPUS must be set to a positive
+	value when executing an application)
 
 Changes:
   * Fix the block filter functions.

doc/chapters/configuration.texi

@@ -348,6 +348,13 @@ devices. By setting the environment variable
 CPU devices.
 @end defvr
 
+@defvr {Environment variable} STARPU_OPENCL_ONLY_ON_CPUS
+By default, the OpenCL driver enables GPU and accelerator
+devices. By setting the environment variable
+@code{STARPU_OPENCL_ONLY_ON_CPUS} to 1, the OpenCL driver will ONLY enable
+CPU devices.
+@end defvr
+
 @defvr {Environment variable} STARPU_WORKERS_NOBIND
 Setting it to non-zero will prevent StarPU from binding its threads to
 CPUs. This is for instance useful when running the testsuite in parallel.

src/drivers/opencl/driver_opencl.c

@@ -345,6 +345,8 @@ void _starpu_opencl_init(void)
                 // Get Platforms
 		if (starpu_get_env_number("STARPU_OPENCL_ON_CPUS") > 0)
 		     device_type |= CL_DEVICE_TYPE_CPU;
+		if (starpu_get_env_number("STARPU_OPENCL_ONLY_ON_CPUS") > 0)
+		     device_type = CL_DEVICE_TYPE_CPU;
                 err = clGetPlatformIDs(_STARPU_OPENCL_PLATFORM_MAX, platform_id, &nb_platforms);
                 if (STARPU_UNLIKELY(err != CL_SUCCESS)) nb_platforms=0;
                 _STARPU_DEBUG("Platforms detected: %u\n", nb_platforms);