[PATCH] drm: Defer disabling the vblank IRQ until the next interrupt (for instant-off)

[Chris Wilson]

On vblank instant-off systems, we can get into a situation where the cost
of enabling and disabling the vblank IRQ around a drmWaitVblank query
dominates. However, we know that if the user wants the current vblank
counter, they are also very likely to immediately queue a vblank wait
and so we can keep the interrupt around and only turn it off if we have
no further vblank requests in the interrupt interval.

After vblank event delivery there is a shadow of one vblank where the
interrupt is kept alive for the user to query and queue another vblank
event. Similarly, if the user is using blocking drmWaitVblanks, the
interrupt will be disabled on the IRQ following the wait completion.
However, if the user is simply querying the current vblank counter and
timestamp, the interrupt will be disabled after every IRQ and the user
will enabled it again on the first query following the IRQ.

Testcase: igt/kms_vblank
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Daniel Vetter <daniel at ffwll.ch>
Cc: Michel Dänzer <michel at daenzer.net>
Cc: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
Cc: Dave Airlie <airlied at redhat.com>,
Cc: Mario Kleiner <mario.kleiner.de at gmail.com>
---
 drivers/gpu/drm/drm_irq.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index c8a34476570a..6f5dc18779e2 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -1091,9 +1091,9 @@ void drm_vblank_put(struct drm_device *dev, int crtc)
 	if (atomic_dec_and_test(&vblank->refcount)) {
 		if (drm_vblank_offdelay == 0)
 			return;
-		else if (dev->vblank_disable_immediate || drm_vblank_offdelay < 0)
+		else if (drm_vblank_offdelay < 0)
 			vblank_disable_fn((unsigned long)vblank);
-		else
+		else if (!dev->vblank_disable_immediate)
 			mod_timer(&vblank->disable_timer,
 				  jiffies + ((drm_vblank_offdelay * HZ)/1000));
 	}
@@ -1697,6 +1697,17 @@ bool drm_handle_vblank(struct drm_device *dev, int crtc)
 
 	spin_lock_irqsave(&dev->event_lock, irqflags);
 
+	if (dev->vblank_disable_immediate && !atomic_read(&vblank->refcount)) {
+		unsigned long vbl_lock_irqflags;
+
+		spin_lock_irqsave(&dev->vbl_lock, vbl_lock_irqflags);
+		if (atomic_read(&vblank->refcount) == 0 && vblank->enabled) {
+			DRM_DEBUG("disabling vblank on crtc %d\n", crtc);
+			vblank_disable_and_save(dev, crtc);
+		}
+		spin_unlock_irqrestore(&dev->vbl_lock, vbl_lock_irqflags);
+	}
+
 	/* Need timestamp lock to prevent concurrent execution with
 	 * vblank enable/disable, as this would cause inconsistent
 	 * or corrupted timestamps and vblank counts.
-- 
2.1.4