[Intel-gfx] [PATCH v2] drm/i915: Queue page flip work via a low latency, unbound workqueue

[Imre Deak]

While user space has control over the scheduling priority of its page
flipping thread, the corresponding work the driver schedules for MMIO
flips always runs from the generic system workqueue which has some
scheduling overhead due it being CPU bound. This would hinder an
application that wants more stringent guarantees over flip timing (to
avoid missing a flip at the next frame count).

Fix this by scheduling the work from a dedicated, unbound workqueue
which provides for minimal scheduling latency.

v2:
- Use an unbound workqueue instead of a high-prio one. (Tvrtko, Chris)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97775
Testcase: igt/kms_cursor_legacy
CC: Chris Wilson <chris at chris-wilson.co.uk>
CC: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
CC: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Signed-off-by: Imre Deak <imre.deak at intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com> (v1)
---
 drivers/gpu/drm/i915/i915_drv.c      | 7 +++++++
 drivers/gpu/drm/i915/i915_drv.h      | 4 ++++
 drivers/gpu/drm/i915/intel_display.c | 2 +-
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
index 7f4e8ad..b5445a1 100644
--- a/drivers/gpu/drm/i915/i915_drv.c
+++ b/drivers/gpu/drm/i915/i915_drv.c
@@ -755,8 +755,14 @@ static int i915_workqueues_init(struct drm_i915_private *dev_priv)
 	if (dev_priv->hotplug.dp_wq == NULL)
 		goto out_free_wq;
 
+	dev_priv->flip_wq = alloc_workqueue("i915-flip", WQ_UNBOUND, 0);
+	if (dev_priv->flip_wq == NULL)
+		goto out_free_dp_wq;
+
 	return 0;
 
+out_free_dp_wq:
+	destroy_workqueue(dev_priv->hotplug.dp_wq);
 out_free_wq:
 	destroy_workqueue(dev_priv->wq);
 out_err:
@@ -767,6 +773,7 @@ out_err:
 
 static void i915_workqueues_cleanup(struct drm_i915_private *dev_priv)
 {
+	destroy_workqueue(dev_priv->flip_wq);
 	destroy_workqueue(dev_priv->hotplug.dp_wq);
 	destroy_workqueue(dev_priv->wq);
 }
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index d60ade9..cdd835b 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1859,6 +1859,10 @@ struct drm_i915_private {
 	 * result in deadlocks.
 	 */
 	struct workqueue_struct *wq;
+	/**
+	 * flip_wq - Low latency, unordered flip workqueue.
+	 */
+	struct workqueue_struct *flip_wq;
 
 	/* Display functions */
 	struct drm_i915_display_funcs display;
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 7bb7874..7728c18 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -12266,7 +12266,7 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
 
 		work->flip_queued_req = i915_gem_active_get(&obj->last_write,
 							    &obj->base.dev->struct_mutex);
-		schedule_work(&work->mmio_work);
+		queue_work(dev_priv->flip_wq, &work->mmio_work);
 	} else {
 		request = i915_gem_request_alloc(engine, engine->last_context);
 		if (IS_ERR(request)) {
-- 
2.5.0