From: Mike Galbraith Date: Sat, 27 Feb 2016 08:09:11 +0100 Subject: [PATCH 03/10] drm/i915: Use preempt_disable/enable_rt() where recommended Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.7/older/patches-6.7-rt6.tar.xz 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 Signed-off-by: Mike Galbraith Signed-off-by: Thomas Gleixner Signed-off-by: Sebastian Andrzej Siewior --- drivers/gpu/drm/i915/display/intel_vblank.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) --- a/drivers/gpu/drm/i915/display/intel_vblank.c +++ b/drivers/gpu/drm/i915/display/intel_vblank.c @@ -308,7 +308,8 @@ 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) @@ -372,7 +373,8 @@ 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);