Re: [Intel-gfx] [PATCH v3] drm/i915: handle uncore spinlock when not available

[Tvrtko Ursulin]

On 23/10/2023 11:33, Luca Coelho wrote:
The uncore code may not always be available (e.g. when we build the
display code with Xe), so we can't always rely on having the uncore's
spinlock.

To handle this, split the spin_lock/unlock_irqsave/restore() into
spin_lock/unlock() followed by a call to local_irq_save/restore() and
create wrapper functions for locking and unlocking the uncore's
spinlock.  In these functions, we have a condition check and only
actually try to lock/unlock the spinlock when I915 is defined, and
thus uncore is available.

This keeps the ifdefs contained in these new functions and all such
logic inside the display code.

Signed-off-by: Luca Coelho <luciano.coe...@intel.com>
---

In v2:

    * Renamed uncore_spin_*() to intel_spin_*()
    * Corrected the order: save, lock, unlock, restore

In v3:

    * Undid the change to pass drm_i915_private instead of the lock
      itself, since we would have to include i915_drv.h and that pulls
      in a truckload of other includes.

  drivers/gpu/drm/i915/display/intel_display.h | 20 ++++++++++++++++++++
  drivers/gpu/drm/i915/display/intel_vblank.c  | 19 ++++++++++++-------
  2 files changed, 32 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_display.h 
b/drivers/gpu/drm/i915/display/intel_display.h
index 0e5dffe8f018..2a33fcc8ce68 100644
--- a/drivers/gpu/drm/i915/display/intel_display.h
+++ b/drivers/gpu/drm/i915/display/intel_display.h
@@ -559,4 +559,24 @@ bool assert_port_valid(struct drm_i915_private *i915, enum 
port port);
  
  bool intel_scanout_needs_vtd_wa(struct drm_i915_private *i915);
  
+/*
+ * The uncore version of the spin lock functions is used to decide
+ * whether we need to lock the uncore lock or not.  This is only
+ * needed in i915, not in Xe.  Keep the decision-making centralized
+ * here.
+ */
+static inline void intel_spin_lock(spinlock_t *lock)
+{
+#ifdef I915
+       spin_lock(lock);
+#endif
+}
+
+static inline void intel_spin_unlock(spinlock_t *lock)
+{
+#ifdef I915
+       spin_unlock(lock);
+#endif
+}
+
  #endif
diff --git a/drivers/gpu/drm/i915/display/intel_vblank.c 
b/drivers/gpu/drm/i915/display/intel_vblank.c
index 2cec2abf9746..9b482d648762 100644
--- a/drivers/gpu/drm/i915/display/intel_vblank.c
+++ b/drivers/gpu/drm/i915/display/intel_vblank.c
@@ -306,7 +306,8 @@ static bool i915_get_crtc_scanoutpos(struct drm_crtc *_crtc,
         * register reads, potentially with preemption disabled, so the
         * following code must not block on uncore.lock.
         */
-       spin_lock_irqsave(&dev_priv->uncore.lock, irqflags);
+       local_irq_save(irqflags);

Does Xe needs interrupts off?

+       intel_spin_lock(&dev_priv->uncore.lock);

My 2p/c is that intel_spin_lock as a name does not work when it is 
specifically about the single and specific (uncore) lock. One cannot 
call intel_spin_lock(some->other->lock) etc.

Perhaps call it i915_uncore_lock_irqsave(i915, flags) so it is clear it 
is only for i915.

Regards,

Tvrtko

        /* preempt_disable_rt() should go right here in PREEMPT_RT patchset. */
  
@@ -374,7 +375,8 @@ static bool i915_get_crtc_scanoutpos(struct drm_crtc *_crtc,
  
  	/* preempt_enable_rt() should go right here in PREEMPT_RT patchset. */
  
-	spin_unlock_irqrestore(&dev_priv->uncore.lock, irqflags);
+       intel_spin_unlock(&dev_priv->uncore.lock);
+       local_irq_restore(irqflags);
  
  	/*
         * While in vblank, position will be negative
@@ -412,9 +414,13 @@ int intel_get_crtc_scanline(struct intel_crtc *crtc)
        unsigned long irqflags;
        int position;
  
-	spin_lock_irqsave(&dev_priv->uncore.lock, irqflags);
+       local_irq_save(irqflags);
+       intel_spin_lock(&dev_priv->uncore.lock);
+
        position = __intel_get_crtc_scanline(crtc);
-       spin_unlock_irqrestore(&dev_priv->uncore.lock, irqflags);
+
+       intel_spin_unlock(&dev_priv->uncore.lock);
+       local_irq_restore(irqflags);
  
  	return position;
  }
@@ -537,7 +543,7 @@ void intel_crtc_update_active_timings(const struct 
intel_crtc_state *crtc_state,
         * Need to audit everything to make sure it's safe.
         */
        spin_lock_irqsave(&i915->drm.vblank_time_lock, irqflags);
-       spin_lock(&i915->uncore.lock);
+       intel_spin_lock(&i915->uncore.lock);
  
  	drm_calc_timestamping_constants(&crtc->base, &adjusted_mode);
  
@@ -546,7 +552,6 @@ void intel_crtc_update_active_timings(const struct intel_crtc_state *crtc_state,
        crtc->mode_flags = mode_flags;
  
  	crtc->scanline_offset = intel_crtc_scanline_offset(crtc_state);
-
-       spin_unlock(&i915->uncore.lock);
+       intel_spin_unlock(&i915->uncore.lock);
        spin_unlock_irqrestore(&i915->drm.vblank_time_lock, irqflags);
  }