On 2023-12-01 12:00:32 [+0200], Luca Coelho wrote:
> To handle this, split the spin_lock/unlock_irqsave/restore() into
> spin_lock/unlock() followed by a call to local_irq_save/restore() and
On PREEMPT_RT spinlock_t becomes a sleeping lock so this split is not
working. See Documentation/locking/locktypes.rst
What I don't understand: do you have to keep the interrupts disabled
for some reasons or is it just to avoid using _irqsave() twice?
I do have more i915 related patches in the PREEMPT_RT queue and I can
post them if you folks have time for it. For now, let me show what I did
here:
----------->8-------------
From: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
Date: Sat, 27 Feb 2016 08:09:11 +0100
Subject: [PATCH 03/10] drm/i915: Use preempt_disable/enable_rt() where
recommended
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@xxxxxxxxx>
Signed-off-by: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
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 = NULL;
+#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)
+ ;
+#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;
}
