[PATCH v2 2/5] drm/i915: Use preempt_disable/enable_rt() where recommended

[Maarten Lankhorst]

From: Mike Galbraith <umgwanakikbuti at 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 at gmail.com>
Signed-off-by: Mike Galbraith <umgwanakikbuti at gmail.com>
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
Link: https://lore.kernel.org/r/20250714153954.629393-2-bigeasy@linutronix.de
Link: https://lore.kernel.org/r/20250715101636.906092-7-dev@lankhorst.se
Signed-off-by: Maarten Lankhorst <dev at lankhorst.se>
---
 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 70ba7aa26bf49..b82e9fc51af01 100644
--- a/drivers/gpu/drm/i915/display/intel_vblank.c
+++ b/drivers/gpu/drm/i915/display/intel_vblank.c
@@ -312,6 +312,20 @@ static void intel_vblank_section_exit(struct intel_display *display)
 	struct drm_i915_private *i915 = to_i915(display->drm);
 	spin_unlock(&i915->uncore.lock);
 }
+
+static void intel_vblank_section_enter_irqf(struct intel_display *display, unsigned long *flags)
+	__acquires(i915->uncore.lock)
+{
+	struct drm_i915_private *i915 = to_i915(display->drm);
+	spin_lock_irqsave(&i915->uncore.lock, *flags);
+}
+
+static void intel_vblank_section_exit_irqf(struct intel_display *display, unsigned long flags)
+	__releases(i915->uncore.lock)
+{
+	struct drm_i915_private *i915 = to_i915(display->drm);
+	spin_unlock_irqrestore(&i915->uncore.lock, flags);
+}
 #else
 static void intel_vblank_section_enter(struct intel_display *display)
 {
@@ -320,6 +334,17 @@ 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,
@@ -356,10 +381,10 @@ 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)
@@ -423,10 +448,10 @@ 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
@@ -464,13 +489,11 @@ 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;
 }
-- 
2.45.2