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/6.8/older/patches-6.8.2-rt11.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 | 38 ++++++++++++++++++++-------- 1 file changed, 28 insertions(+), 10 deletions(-) --- a/drivers/gpu/drm/i915/display/intel_vblank.c +++ b/drivers/gpu/drm/i915/display/intel_vblank.c @@ -275,6 +275,26 @@ int intel_crtc_scanline_to_hw(struct int * all register accesses to the same cacheline to be serialized, * otherwise they may hang. */ +static void intel_vblank_section_enter_irqsave(struct drm_i915_private *i915, unsigned long *flags) + __acquires(i915->uncore.lock) +{ +#ifdef I915 + spin_lock_irqsave(&i915->uncore.lock, *flags); +#else + *flags = 0; +#endif +} + +static void intel_vblank_section_exit_irqrestore(struct drm_i915_private *i915, unsigned long flags) + __releases(i915->uncore.lock) +{ +#ifdef I915 + spin_unlock_irqrestore(&i915->uncore.lock, flags); +#else + if (flags) + return; +#endif +} static void intel_vblank_section_enter(struct drm_i915_private *i915) __acquires(i915->uncore.lock) { @@ -332,10 +352,10 @@ static bool i915_get_crtc_scanoutpos(str * timing critical raw register reads, potentially with * preemption disabled, so the following code must not block. */ - local_irq_save(irqflags); - intel_vblank_section_enter(dev_priv); + intel_vblank_section_enter_irqsave(dev_priv, &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) @@ -399,10 +419,10 @@ 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(); - intel_vblank_section_exit(dev_priv); - local_irq_restore(irqflags); + intel_vblank_section_exit_irqrestore(dev_priv, irqflags); /* * While in vblank, position will be negative @@ -440,13 +460,11 @@ int intel_get_crtc_scanline(struct intel unsigned long irqflags; int position; - local_irq_save(irqflags); - intel_vblank_section_enter(dev_priv); + intel_vblank_section_enter_irqsave(dev_priv, &irqflags); position = __intel_get_crtc_scanline(crtc); - intel_vblank_section_exit(dev_priv); - local_irq_restore(irqflags); + intel_vblank_section_exit_irqrestore(dev_priv, irqflags); return position; }