[PATCH 20/92] drm/i915: Spin after waking up for an interrupt

[Chris Wilson]

When waiting for an interrupt (waiting for the GPU to complete some
work), we know we are the single waiter for the GPU. We also know when
the GPU has nearly completed our request (or at least started processing
it), so after being woken and we detect that the GPU is almost finished,
allow the bottom-half to spin for a very short while to reduce client
latencies.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index e8658c815371..0cadacba29e4 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1155,9 +1155,9 @@ static bool busywait_stop(unsigned long timeout, unsigned cpu)
 	return this_cpu != cpu;
 }
 
-static bool __i915_spin_request(struct drm_i915_gem_request *req, int state)
+static bool __i915_spin_request(struct drm_i915_gem_request *req,
+				int state, unsigned long timeout_us)
 {
-	unsigned long timeout;
 	unsigned cpu;
 
 	/* When waiting for high frequency requests, e.g. during synchronous
@@ -1170,11 +1170,7 @@ static bool __i915_spin_request(struct drm_i915_gem_request *req, int state)
 	 * takes to sleep on a request, on the order of a microsecond.
 	 */
 
-	/* Only spin if we know the GPU is processing this request */
-	if (!i915_gem_request_started(req, true))
-		return false;
-
-	timeout = local_clock_us(&cpu) + 5;
+	timeout_us += local_clock_us(&cpu);
 	do {
 		if (i915_gem_request_completed(req, true))
 			return true;
@@ -1182,7 +1178,7 @@ static bool __i915_spin_request(struct drm_i915_gem_request *req, int state)
 		if (signal_pending_state(state, current))
 			break;
 
-		if (busywait_stop(timeout, cpu))
+		if (busywait_stop(timeout_us, cpu))
 			break;
 
 		cpu_relax_lowlatency();
@@ -1249,7 +1245,8 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
 		gen6_rps_boost(req->i915, rps, req->emitted_jiffies);
 
 	/* Optimistic spin for the next ~jiffie before touching IRQs */
-	if (__i915_spin_request(req, state))
+	if (i915_gem_request_started(req, true) &&
+	    __i915_spin_request(req, state, 5))
 		goto complete;
 
 	intel_wait_init(&wait, req->seqno);
@@ -1290,6 +1287,11 @@ wakeup:
 		 */
 		if (__i915_request_irq_complete(req))
 			break;
+
+		/* Only spin if we know the GPU is processing this request */
+		if (i915_gem_request_started(req, true) &&
+		    __i915_spin_request(req, state, 2))
+			break;
 	}
 	remove_wait_queue(&req->i915->gpu_error.wait_queue, &reset);
 
-- 
2.8.1