[PATCH 2/2] drm/i915: Keep the forcewake timer alive for 1ms past the most recent use

[Chris Wilson]

Currently the timer is armed for 1ms after the first use and is killed
immediately, dropping the forcewake as early as possible. However, for
very frequent operations the forcewake dance has a large impact on
latency and keeping the timer alive until we are idle is preferred. To
achieve this, if we call intel_uncore_forcewake_get whilst the timer is
alive (repeated use), then set a flag to restart the timer on expiry
rather than drop the forcewake usage count. The timer is racy, the
consequence of the race is to expire the timer earlier than is now
desired but does not impact on correct behaviour. The offset the race
slightly, we set the active flag again on intel_uncore_forcewake_put.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala at intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/intel_uncore.c | 15 ++++++++++++---
 drivers/gpu/drm/i915/intel_uncore.h |  1 +
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_uncore.c b/drivers/gpu/drm/i915/intel_uncore.c
index 7eaa592aed26..174870e19539 100644
--- a/drivers/gpu/drm/i915/intel_uncore.c
+++ b/drivers/gpu/drm/i915/intel_uncore.c
@@ -216,6 +216,9 @@ intel_uncore_fw_release_timer(struct hrtimer *timer)
 
 	assert_rpm_device_not_suspended(dev_priv);
 
+	if (xchg(&domain->active, false))
+		return HRTIMER_RESTART;
+
 	spin_lock_irqsave(&dev_priv->uncore.lock, irqflags);
 	if (WARN_ON(domain->wake_count == 0))
 		domain->wake_count++;
@@ -246,6 +249,7 @@ static void intel_uncore_forcewake_reset(struct drm_i915_private *dev_priv,
 		active_domains = 0;
 
 		for_each_fw_domain(domain, dev_priv, tmp) {
+			xchg(&domain->active, false);
 			if (hrtimer_cancel(&domain->timer) == 0)
 				continue;
 
@@ -453,9 +457,12 @@ static void __intel_uncore_forcewake_get(struct drm_i915_private *dev_priv,
 
 	fw_domains &= dev_priv->uncore.fw_domains;
 
-	for_each_fw_domain_masked(domain, fw_domains, dev_priv, tmp)
-		if (domain->wake_count++)
+	for_each_fw_domain_masked(domain, fw_domains, dev_priv, tmp) {
+		if (domain->wake_count++) {
 			fw_domains &= ~domain->mask;
+			domain->active = true;
+		}
+	}
 
 	if (fw_domains)
 		dev_priv->uncore.funcs.force_wake_get(dev_priv, fw_domains);
@@ -520,8 +527,10 @@ static void __intel_uncore_forcewake_put(struct drm_i915_private *dev_priv,
 		if (WARN_ON(domain->wake_count == 0))
 			continue;
 
-		if (--domain->wake_count)
+		if (--domain->wake_count) {
+			domain->active = true;
 			continue;
+		}
 
 		fw_domain_arm_timer(domain);
 	}
diff --git a/drivers/gpu/drm/i915/intel_uncore.h b/drivers/gpu/drm/i915/intel_uncore.h
index 5fec5fd4346c..dfead585835c 100644
--- a/drivers/gpu/drm/i915/intel_uncore.h
+++ b/drivers/gpu/drm/i915/intel_uncore.h
@@ -95,6 +95,7 @@ struct intel_uncore {
 	struct intel_uncore_forcewake_domain {
 		unsigned int mask;
 		unsigned int wake_count;
+		bool active;
 		struct hrtimer timer;
 		i915_reg_t reg_set;
 		i915_reg_t reg_ack;
-- 
2.11.0