[Intel-gfx] [PATCH] drm/i915/execlists: Skip a lite-restore immediately prior to a context-completion

[Chris Wilson]

If we are about to do another context-switch in the near future skip
doing performing a lite-restore now. (Forcing a lite-restore just before
a context-switch effectively doubles the cost of that context-switch, so
long as we can handle the interrupt and resubmit before the GPU powers
down, which under normal conditions is expected.)

Coincidentally, skipping a lite-restore just before idling also prevents
(not entirely, but the window for occurrence now depends on the next
context completing within the same period and we have already noted that
we can resubmit faster than the GPU can context switch to make this
worthwhile) an issue on bxt/glk where it may become confused in an attempt
to lite-restore an already idle context, after which nothing is quite the
same again.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102035
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/intel_lrc.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 5a22bdffd387..5272eb23d98a 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -584,6 +584,25 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 			if (port_count(&port[1]))
 				goto unlock;
 
+			/*
+			 * If we are about to do another context-switch in
+			 * the near future skip doing performing a lite-restore
+			 * now. (Forcing a lite-restore just before a
+			 * context-switch effectively doubles the cost of that
+			 * context-switch, so long as we can handle the
+			 * interrupt and resubmit before the GPU powers down,
+			 * which under normal conditions is expected.)
+			 *
+			 * Skipping a lite-restore just before idling also
+			 * prevents an issue on bxt/glk where it may become
+			 * confused in an attempt to lite-restore an already
+			 * idle context, after which nothing is quite the same
+			 * again.
+			 */
+			if (i915_seqno_passed(intel_engine_get_seqno(engine),
+					      last->global_seqno - 1))
+				goto unlock;
+
 			/* WaIdleLiteRestore:bdw,skl
 			 * Apply the wa NOOPs to prevent
 			 * ring:HEAD == req:TAIL as we resubmit the
-- 
2.15.0