drm: rcar-du: Fix race condition in hardware plane allocator

The plane allocator has been inherently racy since the beginning of the
transition to atomic updates, as the allocator lock is released between
free plane check (at .atomic_check() time) and the reservation (at
.atomic_update() time).

To fix it, create a new allocator solely based on the atomic plane
states without keeping any external state and perform allocation in the
.atomic_check() handler. The core idea is to replace the free planes
bitmask with a collective knowledge based on the allocated hardware
plane(s) for each KMS plane. The allocator then loops over all plane
states to compute the free planes bitmask, allocates hardware planes
based on that bitmask, and stores the result back in the plane states.

For this to work we need to access the current state of planes not
touched by the atomic update. To ensure that it won't be modified, we
need to lock all planes using drm_atomic_get_plane_state(). This
effectively serializes atomic updates from .atomic_check() up to
completion, either when swapping the states if the check step has
succeeded, or when freeing the states if the check step has failed.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>

1 files changed, 6 insertions, 5 deletions

diff --git a/drivers/gpu/drm/rcar-du/rcar_du_plane.h b/drivers/gpu/drm/rcar-du/rcar_du_plane.h
index 0941c4830edd..abff0ebeb195 100644
--- a/drivers/gpu/drm/rcar-du/rcar_du_plane.h
+++ b/drivers/gpu/drm/rcar-du/rcar_du_plane.h
@@ -14,8 +14,6 @@
 #ifndef __RCAR_DU_PLANE_H__
 #define __RCAR_DU_PLANE_H__
 
-#include <linux/mutex.h>
-
 #include <drm/drmP.h>
 #include <drm/drm_crtc.h>
 
@@ -33,13 +31,15 @@ struct rcar_du_group;
 struct rcar_du_plane {
 	struct drm_plane plane;
 	struct rcar_du_group *group;
-	int hwindex;		/* 0-based, -1 means unused */
 };
 
+static inline struct rcar_du_plane *to_rcar_plane(struct drm_plane *plane)
+{
+	return container_of(plane, struct rcar_du_plane, plane);
+}
+
 struct rcar_du_planes {
 	struct rcar_du_plane planes[RCAR_DU_NUM_KMS_PLANES];
-	unsigned int free;
-	struct mutex lock;
 
 	struct drm_property *alpha;
 	struct drm_property *colorkey;
@@ -50,6 +50,7 @@ struct rcar_du_plane_state {
 	struct drm_plane_state state;
 
 	const struct rcar_du_format_info *format;
+	int hwindex;		/* 0-based, -1 means unused */
 
 	unsigned int alpha;
 	unsigned int colorkey;