drm/i915: Remove HW semaphores for gen7 inter-engine synchronisation

The writing is on the wall for the existence of a single execution queue
along each engine, and as a consequence we will not be able to track
dependencies along the HW queue itself, i.e. we will not be able to use
HW semaphores on gen7 as they use a global set of registers (and unlike
gen8+ we can not effectively target memory to keep per-context seqno and
dependencies).

On the positive side, when we implement request reordering for gen7 we
also can not presume a simple execution queue and would also require
removing the current semaphore generation code. So this bring us another
step closer to request reordering for ringbuffer submission!

The negative side is that using interrupts to drive inter-engine
synchronisation is much slower (4us -> 15us to do a nop on each of the 3
engines on ivb). This is much better than it was at the time of introducing
the HW semaphores and equally important userspace weaned itself off
intermixing dependent BLT/RENDER operations (the prime culprit was glyph
rendering in UXA). So while we regress the microbenchmarks, it should not
impact the user.

References: https://bugs.freedesktop.org/show_bug.cgi?id=108888
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181228140736.32606-2-chris@chris-wilson.co.uk

1 files changed, 0 insertions, 8 deletions

diff --git a/drivers/gpu/drm/i915/i915_timeline.h b/drivers/gpu/drm/i915/i915_timeline.h
index ebd71b487220..38c1e15e927a 100644
--- a/drivers/gpu/drm/i915/i915_timeline.h
+++ b/drivers/gpu/drm/i915/i915_timeline.h
@@ -63,14 +63,6 @@ struct i915_timeline {
 	 * redundant and we can discard it without loss of generality.
 	 */
 	struct i915_syncmap *sync;
-	/**
-	 * Separately to the inter-context seqno map above, we track the last
-	 * barrier (e.g. semaphore wait) to the global engine timelines. Note
-	 * that this tracks global_seqno rather than the context.seqno, and
-	 * so it is subject to the limitations of hw wraparound and that we
-	 * may need to revoke global_seqno (on pre-emption).
-	 */
-	u32 global_sync[I915_NUM_ENGINES];
 
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