drm/i915/frontbuffer: Move bo refcounting intel_frontbuffer_{get,release}()

Currently xe's intel_frontbuffer implementation forgets to
hold a reference on the bo. This makes the entire thing
extremely fragile as the cleanup order now depends on bo
references held by other things
(namely intel_fb_bo_framebuffer_fini()).

Move the bo refcounting to intel_frontbuffer_{get,release}()
so that both i915 and xe do this the same way.

I first tried to fix this by having xe do the refcounting
from its intel_bo_set_frontbuffer() implementation
(which is what i915 does currently), but turns out xe's
drm_gem_object_free() can sleep and thus drm_gem_object_put()
isn't safe to call while we hold fb_tracking.lock.

Fixes: 10690b8a49bc ("drm/i915/display: Add intel_fb_bo_framebuffer_fini")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20251003145734.7634-2-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/drivers/gpu/drm/i915/display/intel_frontbuffer.c b/drivers/gpu/drm/i915/display/intel_frontbuffer.c
index 43be5377ddc1..73ed28ac9573 100644
--- a/drivers/gpu/drm/i915/display/intel_frontbuffer.c
+++ b/drivers/gpu/drm/i915/display/intel_frontbuffer.c
@@ -270,6 +270,8 @@ static void frontbuffer_release(struct kref *ref)
 	spin_unlock(&display->fb_tracking.lock);
 
 	i915_active_fini(&front->write);
+
+	drm_gem_object_put(obj);
 	kfree_rcu(front, rcu);
 }
 
@@ -287,6 +289,8 @@ intel_frontbuffer_get(struct drm_gem_object *obj)
 	if (!front)
 		return NULL;
 
+	drm_gem_object_get(obj);
+
 	front->obj = obj;
 	kref_init(&front->ref);
 	atomic_set(&front->bits, 0);
@@ -299,8 +303,12 @@ intel_frontbuffer_get(struct drm_gem_object *obj)
 	spin_lock(&display->fb_tracking.lock);
 	cur = intel_bo_set_frontbuffer(obj, front);
 	spin_unlock(&display->fb_tracking.lock);
-	if (cur != front)
+
+	if (cur != front) {
+		drm_gem_object_put(obj);
 		kfree(front);
+	}
+
 	return cur;
 }