[PATCH 1/2] drm/i915: Delegate our irq handler to a thread

[Chris Wilson]

Moving our primary irq handler to a RT thread incurs an extra 1us delay
in process interrupts. This is most notice in waking up client threads,
where it adds about 20% of extra latency. It also imposes a delay in
feeding the GPU, an extra 1us before signaling secondary engines and
extra latency in resubmitting work to keep the GPU busy. The latter case
is insignificant as the latency hidden by the active GPU, and
preempt-to-busy ensures that no extra latency is incurred for
preemption.

The benefit is that we reduced the impact on the rest of the system, the
cycletest shows a reduction from 5us mean latency to 2us, with the
maximum observed latency (in a 2 minute window) reduced by over 160us.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Clark Williams <williams at redhat.com>
Cc: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
---
 drivers/gpu/drm/i915/gt/intel_engine_cs.c |  2 --
 drivers/gpu/drm/i915/i915_irq.c           | 10 ++++++++--
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_engine_cs.c b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
index f451d5076bde..6d85306d5afc 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine_cs.c
+++ b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
@@ -1058,8 +1058,6 @@ bool intel_engine_is_idle(struct intel_engine_cs *engine)
 	if (execlists_active(&engine->execlists)) {
 		struct tasklet_struct *t = &engine->execlists.tasklet;
 
-		synchronize_hardirq(engine->i915->drm.pdev->irq);
-
 		local_bh_disable();
 		if (tasklet_trylock(t)) {
 			/* Must wait for any GPU reset in progress. */
diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index bc83f094065a..6e34f48699ec 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -4478,6 +4478,7 @@ static void intel_irq_postinstall(struct drm_i915_private *dev_priv)
 int intel_irq_install(struct drm_i915_private *dev_priv)
 {
 	int irq = dev_priv->drm.pdev->irq;
+	irq_handler_t fn, thread_fn;
 	int ret;
 
 	/*
@@ -4491,8 +4492,13 @@ int intel_irq_install(struct drm_i915_private *dev_priv)
 
 	intel_irq_reset(dev_priv);
 
-	ret = request_irq(irq, intel_irq_handler(dev_priv),
-			  IRQF_SHARED, DRIVER_NAME, dev_priv);
+	fn = intel_irq_handler(dev_priv);
+	thread_fn = NULL;
+	if (dev_priv->drm.pdev->msi_enabled)
+		swap(fn, thread_fn);
+
+	ret = request_threaded_irq(irq, fn, thread_fn,
+				   IRQF_SHARED, DRIVER_NAME, dev_priv);
 	if (ret < 0) {
 		dev_priv->drm.irq_enabled = false;
 		return ret;
-- 
2.23.0