drm/i915: Use atomics to manipulate obj->frontbuffer_bits

The individual bits inside obj->frontbuffer_bits are protected by each
plane->mutex, but the whole bitfield may be accessed by multiple KMS
operations simultaneously and so the RMW need to be under atomics.
However, for updating the single field we do not need to mandate that it
be under the struct_mutex, one more step towards its removal as the de
facto BKL.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470324762-2545-21-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_debugfs.c

@@ -138,6 +138,7 @@ describe_obj(struct seq_file *m, struct drm_i915_gem_object *obj)
 	struct drm_i915_private *dev_priv = to_i915(obj->base.dev);
 	struct intel_engine_cs *engine;
 	struct i915_vma *vma;
+	unsigned int frontbuffer_bits;
 	int pin_count = 0;
 	enum intel_engine_id id;
 
@@ -204,8 +205,9 @@ describe_obj(struct seq_file *m, struct drm_i915_gem_object *obj)
 	if (engine)
 		seq_printf(m, " (%s)", engine->name);
 
-	if (obj->frontbuffer_bits)
-		seq_printf(m, " (frontbuffer: 0x%03x)", obj->frontbuffer_bits);
+	frontbuffer_bits = atomic_read(&obj->frontbuffer_bits);
+	if (frontbuffer_bits)
+		seq_printf(m, " (frontbuffer: 0x%03x)", frontbuffer_bits);
 }
 
 static int i915_gem_object_list_info(struct seq_file *m, void *data)

drivers/gpu/drm/i915/i915_drv.h

@@ -2127,8 +2127,6 @@ struct drm_i915_gem_object_ops {
  */
 #define INTEL_MAX_SPRITE_BITS_PER_PIPE 5
 #define INTEL_FRONTBUFFER_BITS_PER_PIPE 8
-#define INTEL_FRONTBUFFER_BITS \
-	(INTEL_FRONTBUFFER_BITS_PER_PIPE * I915_MAX_PIPES)
 #define INTEL_FRONTBUFFER_PRIMARY(pipe) \
 	(1 << (INTEL_FRONTBUFFER_BITS_PER_PIPE * (pipe)))
 #define INTEL_FRONTBUFFER_CURSOR(pipe) \
@@ -2216,7 +2214,7 @@ struct drm_i915_gem_object {
 	unsigned int cache_level:3;
 	unsigned int cache_dirty:1;
 
-	unsigned int frontbuffer_bits:INTEL_FRONTBUFFER_BITS;
+	atomic_t frontbuffer_bits;
 
 	unsigned int has_wc_mmap;
 	/** Count of VMA actually bound by this object */

drivers/gpu/drm/i915/i915_gem.c

@@ -4040,7 +4040,7 @@ void i915_gem_free_object(struct drm_gem_object *gem_obj)
 	if (obj->stolen)
 		i915_gem_object_unpin_pages(obj);
 
-	WARN_ON(obj->frontbuffer_bits);
+	WARN_ON(atomic_read(&obj->frontbuffer_bits));
 
 	if (obj->pages && obj->madv == I915_MADV_WILLNEED &&
 	    dev_priv->quirks & QUIRK_PIN_SWIZZLED_PAGES &&
@@ -4557,16 +4557,23 @@ void i915_gem_track_fb(struct drm_i915_gem_object *old,
 		       struct drm_i915_gem_object *new,
 		       unsigned frontbuffer_bits)
 {
+	/* Control of individual bits within the mask are guarded by
+	 * the owning plane->mutex, i.e. we can never see concurrent
+	 * manipulation of individual bits. But since the bitfield as a whole
+	 * is updated using RMW, we need to use atomics in order to update
+	 * the bits.
+	 */
+	BUILD_BUG_ON(INTEL_FRONTBUFFER_BITS_PER_PIPE * I915_MAX_PIPES >
+		     sizeof(atomic_t) * BITS_PER_BYTE);
+
 	if (old) {
-		WARN_ON(!mutex_is_locked(&old->base.dev->struct_mutex));
-		WARN_ON(!(old->frontbuffer_bits & frontbuffer_bits));
-		old->frontbuffer_bits &= ~frontbuffer_bits;
+		WARN_ON(!(atomic_read(&old->frontbuffer_bits) & frontbuffer_bits));
+		atomic_andnot(frontbuffer_bits, &old->frontbuffer_bits);
 	}
 
 	if (new) {
-		WARN_ON(!mutex_is_locked(&new->base.dev->struct_mutex));
-		WARN_ON(new->frontbuffer_bits & frontbuffer_bits);
-		new->frontbuffer_bits |= frontbuffer_bits;
+		WARN_ON(atomic_read(&new->frontbuffer_bits) & frontbuffer_bits);
+		atomic_or(frontbuffer_bits, &new->frontbuffer_bits);
 	}
 }
 

drivers/gpu/drm/i915/intel_display.c

@@ -2601,7 +2601,8 @@ valid_fb:
 	primary->fb = primary->state->fb = fb;
 	primary->crtc = primary->state->crtc = &intel_crtc->base;
 	intel_crtc->base.state->plane_mask |= (1 << drm_plane_index(primary));
-	obj->frontbuffer_bits |= to_intel_plane(primary)->frontbuffer_bit;
+	atomic_or(to_intel_plane(primary)->frontbuffer_bit,
+		  &obj->frontbuffer_bits);
 }
 
 static void i9xx_update_primary_plane(struct drm_plane *primary,
@@ -13810,19 +13811,12 @@ static void intel_atomic_track_fbs(struct drm_atomic_state *state)
 {
 	struct drm_plane_state *old_plane_state;
 	struct drm_plane *plane;
-	struct drm_i915_gem_object *obj, *old_obj;
-	struct intel_plane *intel_plane;
 	int i;
 
-	mutex_lock(&state->dev->struct_mutex);
-	for_each_plane_in_state(state, plane, old_plane_state, i) {
-		obj = intel_fb_obj(plane->state->fb);
-		old_obj = intel_fb_obj(old_plane_state->fb);
-		intel_plane = to_intel_plane(plane);
-
-		i915_gem_track_fb(old_obj, obj, intel_plane->frontbuffer_bit);
-	}
-	mutex_unlock(&state->dev->struct_mutex);
+	for_each_plane_in_state(state, plane, old_plane_state, i)
+		i915_gem_track_fb(intel_fb_obj(old_plane_state->fb),
+				  intel_fb_obj(plane->state->fb),
+				  to_intel_plane(plane)->frontbuffer_bit);
 }
 
 /**

drivers/gpu/drm/i915/intel_frontbuffer.c

@@ -67,23 +67,22 @@
 #include "i915_drv.h"
 
 void __intel_fb_obj_invalidate(struct drm_i915_gem_object *obj,
-			       enum fb_op_origin origin)
+			       enum fb_op_origin origin,
+			       unsigned int frontbuffer_bits)
 {
 	struct drm_device *dev = obj->base.dev;
 	struct drm_i915_private *dev_priv = to_i915(dev);
 
-	WARN_ON(!mutex_is_locked(&dev->struct_mutex));
-
 	if (origin == ORIGIN_CS) {
 		spin_lock(&dev_priv->fb_tracking.lock);
-		dev_priv->fb_tracking.busy_bits |= obj->frontbuffer_bits;
-		dev_priv->fb_tracking.flip_bits &= ~obj->frontbuffer_bits;
+		dev_priv->fb_tracking.busy_bits |= frontbuffer_bits;
+		dev_priv->fb_tracking.flip_bits &= ~frontbuffer_bits;
 		spin_unlock(&dev_priv->fb_tracking.lock);
 	}
 
-	intel_psr_invalidate(dev, obj->frontbuffer_bits);
-	intel_edp_drrs_invalidate(dev, obj->frontbuffer_bits);
-	intel_fbc_invalidate(dev_priv, obj->frontbuffer_bits, origin);
+	intel_psr_invalidate(dev, frontbuffer_bits);
+	intel_edp_drrs_invalidate(dev, frontbuffer_bits);
+	intel_fbc_invalidate(dev_priv, frontbuffer_bits, origin);
 }
 
 /**
@@ -119,15 +118,11 @@ static void intel_frontbuffer_flush(struct drm_device *dev,
 
 void __intel_fb_obj_flush(struct drm_i915_gem_object *obj,
 			  bool retire,
-			  enum fb_op_origin origin)
+			  enum fb_op_origin origin,
+			  unsigned int frontbuffer_bits)
 {
 	struct drm_device *dev = obj->base.dev;
 	struct drm_i915_private *dev_priv = to_i915(dev);
-	unsigned frontbuffer_bits;
-
-	WARN_ON(!mutex_is_locked(&dev->struct_mutex));
-
-	frontbuffer_bits = obj->frontbuffer_bits;
 
 	if (retire) {
 		spin_lock(&dev_priv->fb_tracking.lock);

drivers/gpu/drm/i915/intel_frontbuffer.h

@@ -36,10 +36,12 @@ void intel_frontbuffer_flip(struct drm_device *dev,
 			    unsigned frontbuffer_bits);
 
 void __intel_fb_obj_invalidate(struct drm_i915_gem_object *obj,
-			       enum fb_op_origin origin);
+			       enum fb_op_origin origin,
+			       unsigned int frontbuffer_bits);
 void __intel_fb_obj_flush(struct drm_i915_gem_object *obj,
 			  bool retire,
-			  enum fb_op_origin origin);
+			  enum fb_op_origin origin,
+			  unsigned int frontbuffer_bits);
 
 /**
  * intel_fb_obj_invalidate - invalidate frontbuffer object
@@ -55,10 +57,13 @@ void __intel_fb_obj_flush(struct drm_i915_gem_object *obj,
 static inline void intel_fb_obj_invalidate(struct drm_i915_gem_object *obj,
 					   enum fb_op_origin origin)
 {
-	if (!obj->frontbuffer_bits)
+	unsigned int frontbuffer_bits;
+
+	frontbuffer_bits = atomic_read(&obj->frontbuffer_bits);
+	if (!frontbuffer_bits)
 		return;
 
-	__intel_fb_obj_invalidate(obj, origin);
+	__intel_fb_obj_invalidate(obj, origin, frontbuffer_bits);
 }
 
 /**
@@ -75,10 +80,13 @@ static inline void intel_fb_obj_flush(struct drm_i915_gem_object *obj,
 				      bool retire,
 				      enum fb_op_origin origin)
 {
-	if (!obj->frontbuffer_bits)
+	unsigned int frontbuffer_bits;
+
+	frontbuffer_bits = atomic_read(&obj->frontbuffer_bits);
+	if (!frontbuffer_bits)
 		return;
 
-	__intel_fb_obj_flush(obj, retire, origin);
+	__intel_fb_obj_flush(obj, retire, origin, frontbuffer_bits);
 }
 
 #endif /* __INTEL_FRONTBUFFER_H__ */