[PATCH v7 08/26] drm/i915: Use preempt_disable/enable_rt() where recommended

Maarten Lankhorst
2026-03-10T11:57:39Z

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>
Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
---
drivers/gpu/drm/i915/display/intel_vblank.c | 43 ++++++++++++++++-----
1 file changed, 33 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_vblank.c b/drivers/gpu/drm/i915/display/intel_vblank.c
index 3eb6418723a84..8263cefaa0fb6 100644
--- a/drivers/gpu/drm/i915/display/intel_vblank.c
+++ b/drivers/gpu/drm/i915/display/intel_vblank.c
@@ -316,6 +316,20 @@ static void intel_vblank_section_exit(struct intel_display *display)
struct intel_uncore *uncore = to_intel_uncore(display->drm);
spin_unlock(&uncore->lock);
}
+
+static void intel_vblank_section_enter_irqf(struct intel_display *display, unsigned long *flags)
+ __acquires(i915->uncore.lock)
+{
+ struct intel_uncore *uncore = to_intel_uncore(display->drm);
+ spin_lock_irqsave(&uncore->lock, *flags);
+}
+
+static void intel_vblank_section_exit_irqf(struct intel_display *display, unsigned long flags)
+ __releases(i915->uncore.lock)
+{
+ struct intel_uncore *uncore = to_intel_uncore(display->drm);
+ spin_unlock_irqrestore(&uncore->lock, flags);
+}
#else
static void intel_vblank_section_enter(struct intel_display *display)
{
@@ -324,6 +338,17 @@ static void intel_vblank_section_enter(struct intel_display *display)
static void intel_vblank_section_exit(struct intel_display *display)
{
}
+
+static void intel_vblank_section_enter_irqf(struct intel_display *display, unsigned long *flags)
+{
+ *flags = 0;
+}
+
+static void intel_vblank_section_exit_irqf(struct intel_display *display, unsigned long flags)
+{
+ if (flags)
+ return;
+}
#endif

static bool i915_get_crtc_scanoutpos(struct drm_crtc *_crtc,
@@ -360,10 +385,10 @@ static bool i915_get_crtc_scanoutpos(struct drm_crtc *_crtc,
* timing critical raw register reads, potentially with
* preemption disabled, so the following code must not block.
*/
- local_irq_save(irqflags);
- intel_vblank_section_enter(display);
+ intel_vblank_section_enter_irqf(display, &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)
@@ -427,10 +452,10 @@ 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();

- intel_vblank_section_exit(display);
- local_irq_restore(irqflags);
+ intel_vblank_section_exit_irqf(display, irqflags);

/*
* While in vblank, position will be negative
@@ -468,13 +493,11 @@ int intel_get_crtc_scanline(struct intel_crtc *crtc)
unsigned long irqflags;
int position;

- local_irq_save(irqflags);
- intel_vblank_section_enter(display);
+ intel_vblank_section_enter_irqf(display, &irqflags);

position = __intel_get_crtc_scanline(crtc);

- intel_vblank_section_exit(display);
- local_irq_restore(irqflags);
+ intel_vblank_section_exit_irqf(display, irqflags);

return position;
}
--
2.51.0