drm/i915/frontbuffer: Split fb_tracking.lock into two

Our fb_tracking.lock is serving a double duty:
- protects fb_tracking.busy_bits
- provides the write-side protection for obj->frontbuffer

Split obj->frontbuffer role into a separate lock so that
we can clean up the current mess with the frontbuffer lifetime
management.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20251016185408.22735-7-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>

4 files changed, 11 insertions, 6 deletions

diff --git a/drivers/gpu/drm/i915/display/intel_display_core.h b/drivers/gpu/drm/i915/display/intel_display_core.h
index 2eb742ab5222..9e53ae599cf0 100644
--- a/drivers/gpu/drm/i915/display/intel_display_core.h
+++ b/drivers/gpu/drm/i915/display/intel_display_core.h
@@ -142,6 +142,10 @@ struct intel_dpll_global {
 };
 
 struct intel_frontbuffer_tracking {
+	/* protects obj->frontbuffer (write-side) */
+	spinlock_t frontbuffer_lock;
+
+	/* protects busy_bits */
 	spinlock_t lock;
 
 	/*
diff --git a/drivers/gpu/drm/i915/display/intel_display_driver.c b/drivers/gpu/drm/i915/display/intel_display_driver.c
index eb0727b9a0f6..c7a74b0df25f 100644
--- a/drivers/gpu/drm/i915/display/intel_display_driver.c
+++ b/drivers/gpu/drm/i915/display/intel_display_driver.c
@@ -186,6 +186,7 @@ void intel_display_driver_early_probe(struct intel_display *display)
 	if (!HAS_DISPLAY(display))
 		return;
 
+	spin_lock_init(&display->fb_tracking.frontbuffer_lock);
 	spin_lock_init(&display->fb_tracking.lock);
 	mutex_init(&display->backlight.lock);
 	mutex_init(&display->audio.mutex);
diff --git a/drivers/gpu/drm/i915/display/intel_frontbuffer.c b/drivers/gpu/drm/i915/display/intel_frontbuffer.c
index bdf8bfa7deb9..02b06dfb9fee 100644
--- a/drivers/gpu/drm/i915/display/intel_frontbuffer.c
+++ b/drivers/gpu/drm/i915/display/intel_frontbuffer.c
@@ -210,7 +210,7 @@ static void frontbuffer_retire(struct i915_active *ref)
 }
 
 static void frontbuffer_release(struct kref *ref)
-	__releases(&to_intel_display(front->obj->dev)->fb_tracking.lock)
+	__releases(&to_intel_display(front->obj->dev)->fb_tracking.frontbuffer_lock)
 {
 	struct intel_frontbuffer *ret, *front =
 		container_of(ref, typeof(*front), ref);
@@ -223,7 +223,7 @@ static void frontbuffer_release(struct kref *ref)
 
 	ret = intel_bo_set_frontbuffer(obj, NULL);
 	drm_WARN_ON(display->drm, ret);
-	spin_unlock(&display->fb_tracking.lock);
+	spin_unlock(&display->fb_tracking.frontbuffer_lock);
 
 	i915_active_fini(&front->write);
 
@@ -256,9 +256,9 @@ intel_frontbuffer_get(struct drm_gem_object *obj)
 			 I915_ACTIVE_RETIRE_SLEEPS);
 	INIT_WORK(&front->flush_work, intel_frontbuffer_flush_work);
 
-	spin_lock(&display->fb_tracking.lock);
+	spin_lock(&display->fb_tracking.frontbuffer_lock);
 	cur = intel_bo_set_frontbuffer(obj, front);
-	spin_unlock(&display->fb_tracking.lock);
+	spin_unlock(&display->fb_tracking.frontbuffer_lock);
 
 	if (cur != front) {
 		drm_gem_object_put(obj);
@@ -272,7 +272,7 @@ void intel_frontbuffer_put(struct intel_frontbuffer *front)
 {
 	kref_put_lock(&front->ref,
 		      frontbuffer_release,
-		      &to_intel_display(front->obj->dev)->fb_tracking.lock);
+		      &to_intel_display(front->obj->dev)->fb_tracking.frontbuffer_lock);
 }
 
 /**
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object_frontbuffer.h b/drivers/gpu/drm/i915/gem/i915_gem_object_frontbuffer.h
index b682969e3a29..1ec382c43aee 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_object_frontbuffer.h
+++ b/drivers/gpu/drm/i915/gem/i915_gem_object_frontbuffer.h
@@ -77,7 +77,7 @@ i915_gem_object_get_frontbuffer(const struct drm_i915_gem_object *obj)
  * Set object's frontbuffer pointer. If frontbuffer is already set for the
  * object keep it and return it's pointer to the caller. Please note that RCU
  * mechanism is used to handle e.g. ongoing removal of frontbuffer pointer. This
- * function is protected by i915->display->fb_tracking.lock
+ * function is protected by i915->display->fb_tracking.frontbuffer_lock
  *
  * Return: pointer to frontbuffer which was set.
  */