title: OpenCL clGetDeviceIDs returns -1 when GPU and 0 when CPU
body: <p>I was running my OpenCL/C++ code on Ubuntu 14.04.2 LTS (NVIDIA Corporation GM204 [GeForce GTX 980]).
  It's working correctly on CPU but <code>clGetDeviceIDs</code> method returned -1 when I changed <code>CL_DEVICE_TYPE_CPU</code> to <code>CL_DEVICE_TYPE_GPU</code>.</p>
  
  <p>The code in question:</p>
  
  <pre><code>ret = clGetDeviceIDs(platform_id, CL_DEVICE_TYPE_GPU, 1, &amp;device_id, &amp;ret_num_devices);
  cout &lt;&lt; ret;
  checkError(ret, "clGetDeviceIDs");
  </code></pre>
  
  <p>Outputs -1.</p>
  
  <p>But the following:</p>
  
  <pre><code>ret = clGetDeviceIDs(platform_id, CL_DEVICE_TYPE_CPU, 1, &amp;device_id, &amp;ret_num_devices);
  cout &lt;&lt; ret;
  checkError(ret, "clGetDeviceIDs");
  </code></pre>
  
  <p>Outputs 0.</p>
  
  <p>Why?</p>