i965: Reserve batchbuffer space for a closing MI_REPORT_PERF_COUNT.

In order to use the Observability Architecture effectively, we'll need
to take snapshots of the OA counters via MI_REPORT_PERF_COUNT at the
start and end of each batch.

Experimentation reveals that we need to flush before and after each
MI_REPORT_PERF_COUNT to get working values.  For simplicitly, I chose to
use intel_batchbuffer_emit_mi_flush(), which unfortunately expands to
triple pipe controls on Sandybridge.

We may want to start computing per-generation reserved batch space to
avoid the insanity of Sandybridge's PIPE_CONTROL cost.  That said, much
of this cost existed before I rewrote the query object support to use
hardware contexts, so it's at least not entirely new.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>

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

@@ -20,8 +20,14 @@ extern "C" {
  * - Any state emitted by vtbl->finish_batch():
  *   - Gen4-5 record ending occlusion query values (4 * 4 = 16 bytes)
  *   - Disabling OA counters on Gen6+ (3 DWords = 12 bytes)
+ *   - Ending MI_REPORT_PERF_COUNT on Gen5+, plus associated PIPE_CONTROLs:
+ *     - Two sets of PIPE_CONTROLs, which become 3 PIPE_CONTROLs each on SNB,
+ *       which are 4 DWords each ==> 2 * 3 * 4 * 4 = 96 bytes
+ *     - 3 DWords for MI_REPORT_PERF_COUNT itself on Gen6+.  ==> 12 bytes.
+ *       On Ironlake, it's 6 DWords, but we have some slack due to the lack of
+ *       Sandybridge PIPE_CONTROL madness.
  */
-#define BATCH_RESERVED 36
+#define BATCH_RESERVED 146
 
 struct intel_batchbuffer;