drm/i915: compile-time consistency check on __EXEC_OBJECT flags

Two different sets of flag bits are stored in the 'flags' member of a
'struct drm_i915_gem_exec_object2', and they're defined in two different
source files, increasing the risk of an accidental clash.

Some flags in this field are supplied by the user; these are defined in
i915_drm.h, and they start from the LSB and work up.

Other flags are defined in i915_gem_execbuffer, for internal use within
that file only; they start from the MSB and work down.

So here we add a compile-time check that the two sets of flags do not
overlap, which would cause all sorts of confusion.
Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1468504324-12690-1-git-send-email-david.s.gordon@intel.com

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -38,6 +38,7 @@
 #define  __EXEC_OBJECT_HAS_FENCE	(1<<30)
 #define  __EXEC_OBJECT_NEEDS_MAP	(1<<29)
 #define  __EXEC_OBJECT_NEEDS_BIAS	(1<<28)
+#define  __EXEC_OBJECT_INTERNAL_FLAGS (0xf<<28) /* all of the above */
 
 #define BATCH_OFFSET_BIAS (256*1024)
 
@@ -1007,6 +1008,9 @@ validate_exec_list(struct drm_device *dev,
 	unsigned invalid_flags;
 	int i;
 
+	/* INTERNAL flags must not overlap with external ones */
+	BUILD_BUG_ON(__EXEC_OBJECT_INTERNAL_FLAGS & ~__EXEC_OBJECT_UNKNOWN_FLAGS);
+
 	invalid_flags = __EXEC_OBJECT_UNKNOWN_FLAGS;
 	if (USES_FULL_PPGTT(dev))
 		invalid_flags |= EXEC_OBJECT_NEEDS_GTT;

include/uapi/drm/i915_drm.h

@@ -703,7 +703,8 @@ struct drm_i915_gem_exec_object2 {
 #define EXEC_OBJECT_WRITE		 (1<<2)
 #define EXEC_OBJECT_SUPPORTS_48B_ADDRESS (1<<3)
 #define EXEC_OBJECT_PINNED		 (1<<4)
-#define __EXEC_OBJECT_UNKNOWN_FLAGS -(EXEC_OBJECT_PINNED<<1)
+/* All remaining bits are MBZ and RESERVED FOR FUTURE USE */
+#define __EXEC_OBJECT_UNKNOWN_FLAGS	(-(EXEC_OBJECT_PINNED<<1))
 	__u64 flags;
 
 	__u64 rsvd1;