Subject: [PATCH 03/10] drm/i915: Use preempt_disable/enable_rt() where recommended
From: Mike Galbraith <[email-masked]>
Date: Sat, 27 Feb 2016 08:09:11 +0100

Mario Kleiner suggest in commit
  ad3543ede630f ("drm/intel: Push get_scanout_position() timestamping into kms driver.")

a spots where preemption should be disabled on PREEMPT_RT. The
difference is that on PREEMPT_RT the intel_uncore::lock disables neither
preemption nor interrupts and so region remains preemptible.

The area covers only register reads and writes. The part that worries me
is:
- __intel_get_crtc_scanline() the worst case is 100us if no match is
  found.

- intel_crtc_scanlines_since_frame_timestamp() not sure how long this
  may take in the worst case.

It was in the RT queue for a while and nobody complained.
Disable preemption on PREEPMPT_RT during timestamping.

[bigeasy: patch description.]

Cc: Mario Kleiner <[email-masked]>
Signed-off-by: Mike Galbraith <[email-masked]>
Signed-off-by: Thomas Gleixner <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 drivers/gpu/drm/i915/display/intel_vblank.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

Index: linux-6.3.0-rt11/drivers/gpu/drm/i915/display/intel_vblank.c
===================================================================
--- linux-6.3.0-rt11.orig/drivers/gpu/drm/i915/display/intel_vblank.c
+++ linux-6.3.0-rt11/drivers/gpu/drm/i915/display/intel_vblank.c
@@ -293,7 +293,8 @@ static bool i915_get_crtc_scanoutpos(str
 	 */
 	spin_lock_irqsave(&dev_priv->uncore.lock, irqflags);
 
-	/* preempt_disable_rt() should go right here in PREEMPT_RT patchset. */
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_disable();
 
 	/* Get optional system timestamp before query. */
 	if (stime)
@@ -358,7 +359,8 @@ static bool i915_get_crtc_scanoutpos(str
 	if (etime)
 		*etime = ktime_get();
 
-	/* preempt_enable_rt() should go right here in PREEMPT_RT patchset. */
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_enable();
 
 	spin_unlock_irqrestore(&dev_priv->uncore.lock, irqflags);