[Intel-gfx] [PATCH 4/8] drm/i915: Use preempt_disable/enable_rt() where recommended

Sebastian Andrzej Siewior
2021-12-14T14:03:26Z

From: Mike Galbraith <umgwanakikbuti@gmail.com>

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 <mario.kleiner.de@gmail.com>
Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
drivers/gpu/drm/i915/i915_irq.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 038a9ec563c10..8e9ff0bcbc7e4 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -916,7 +916,8 @@ static bool i915_get_crtc_scanoutpos(struct drm_crtc *_crtc,
*/
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)
@@ -980,7 +981,8 @@ static bool i915_get_crtc_scanoutpos(struct drm_crtc *_crtc,
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);

--
2.34.1