drm: fix potential dangling else problems in for_each_ macros

We have serious dangling else bugs waiting to happen in our for_each_
style macros with ifs. Consider, for example,

 #define drm_for_each_plane_mask(plane, dev, plane_mask) \
         list_for_each_entry((plane), &(dev)->mode_config.plane_list, head) \
                 if ((plane_mask) & (1 << drm_plane_index(plane)))

If this is used in context:

	if (condition)
		drm_for_each_plane_mask(plane, dev, plane_mask);
	else
		foo();

foo() will be called for each plane *not* in plane_mask, if condition
holds, and not at all if condition doesn't hold.

Fix this by reversing the conditions in the macros, and adding an else
branch for the "for each" block, so that other if/else blocks can't
interfere. Provide a "for_each_if" helper macro to make it easier to get
this right.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448392916-2281-1-git-send-email-jani.nikula@intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

include/drm/drmP.h

@@ -1111,4 +1111,7 @@ static __inline__ bool drm_can_sleep(void)
 	return true;
 }
 
+/* helper for handling conditionals in various for_each macros */
+#define for_each_if(condition) if (!(condition)) {} else
+
 #endif

include/drm/drm_atomic.h

@@ -149,7 +149,7 @@ int __must_check drm_atomic_async_commit(struct drm_atomic_state *state);
 	     ((connector) = (state)->connectors[__i],			\
 	     (connector_state) = (state)->connector_states[__i], 1); 	\
 	     (__i)++)							\
-		if (connector)
+		for_each_if (connector)
 
 #define for_each_crtc_in_state(state, crtc, crtc_state, __i)	\
 	for ((__i) = 0;						\
@@ -157,7 +157,7 @@ int __must_check drm_atomic_async_commit(struct drm_atomic_state *state);
 	     ((crtc) = (state)->crtcs[__i],			\
 	     (crtc_state) = (state)->crtc_states[__i], 1);	\
 	     (__i)++)						\
-		if (crtc_state)
+		for_each_if (crtc_state)
 
 #define for_each_plane_in_state(state, plane, plane_state, __i)		\
 	for ((__i) = 0;							\
@@ -165,7 +165,7 @@ int __must_check drm_atomic_async_commit(struct drm_atomic_state *state);
 	     ((plane) = (state)->planes[__i],				\
 	     (plane_state) = (state)->plane_states[__i], 1);		\
 	     (__i)++)							\
-		if (plane_state)
+		for_each_if (plane_state)
 static inline bool
 drm_atomic_crtc_needs_modeset(struct drm_crtc_state *state)
 {

include/drm/drm_crtc.h

@@ -1170,7 +1170,7 @@ struct drm_mode_config {
  */
 #define drm_for_each_plane_mask(plane, dev, plane_mask) \
 	list_for_each_entry((plane), &(dev)->mode_config.plane_list, head) \
-		if ((plane_mask) & (1 << drm_plane_index(plane)))
+		for_each_if ((plane_mask) & (1 << drm_plane_index(plane)))
 
 
 #define obj_to_crtc(x) container_of(x, struct drm_crtc, base)
@@ -1547,7 +1547,7 @@ static inline struct drm_property *drm_property_find(struct drm_device *dev,
 /* Plane list iterator for legacy (overlay only) planes. */
 #define drm_for_each_legacy_plane(plane, dev) \
 	list_for_each_entry(plane, &(dev)->mode_config.plane_list, head) \
-		if (plane->type == DRM_PLANE_TYPE_OVERLAY)
+		for_each_if (plane->type == DRM_PLANE_TYPE_OVERLAY)
 
 #define drm_for_each_plane(plane, dev) \
 	list_for_each_entry(plane, &(dev)->mode_config.plane_list, head)