i965/chv: Check that compute threads are above threshold

The way we are organizing this code, the statically configured max_cs_threads
should always be the minimum value we actually support (ie. are aware of). As a
result, we can fall back to that if we get invalid numbers from the kernel (ie.
when the query succeeds, but the result is lower than expected).

I was originally planning to use an assert, but there is no reason to be so
mean.

Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com

src/mesa/drivers/dri/i965/brw_context.c

@@ -934,6 +934,10 @@ brwCreateContext(gl_api api,
        screen->subslice_total > 0 && screen->eu_total > 0) {
       /* Logical CS threads = EUs per subslice * 7 threads per EU */
       brw->max_cs_threads = screen->eu_total / screen->subslice_total * 7;
+
+      /* Fuse configurations may give more threads than expected, never less. */
+      if (brw->max_cs_threads < devinfo->max_cs_threads)
+         brw->max_cs_threads = devinfo->max_cs_threads;
    } else {
       brw->max_cs_threads = devinfo->max_cs_threads;
    }

src/mesa/drivers/dri/i965/brw_device_info.h

@@ -71,6 +71,11 @@ struct brw_device_info
    /**
     * Total number of slices present on the device whether or not they've been
     * fused off.
+    *
+    * XXX: CS thread counts are limited by the inability to do cross subslice
+    * communication. It is the effectively the number of logical threads which
+    * can be executed in a subslice. Fuse configurations may cause this number
+    * to change, so we program @max_cs_threads as the lower maximum.
     */
    unsigned num_slices;
    unsigned max_vs_threads;