drm/i915: Convert i915_ggtt_view to use an anonymous union

Reading the ggtt_views is much more pleasant without the extra
characters from specifying the union (i.e. ggtt_view.partial rather than
ggtt_view.params.partial). To make this work inside i915_vma_compare()
with only a single memcmp requires us to ensure that there are no
uninitialised bytes within each branch of the union (we make sure the
structs are packed) and we need to store the size of each branch.

v4: Rewrite changelog and add comments explaining the assert.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170114002827.31315-5-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/drivers/gpu/drm/i915/i915_vma.h b/drivers/gpu/drm/i915/i915_vma.h
index fdbacc036080..86b60fb4e954 100644
--- a/drivers/gpu/drm/i915/i915_vma.h
+++ b/drivers/gpu/drm/i915/i915_vma.h
@@ -212,10 +212,17 @@ i915_vma_compare(struct i915_vma *vma,
 	 * different views using it as a "type" and also use a compact (no
 	 * accessing of uninitialised padding bytes) memcmp without storing
 	 * an extra parameter or adding more code.
+	 *
+	 * To ensure that the memcmp is valid for all branches of the union,
+	 * even though the code looks like it is just comparing one branch,
+	 * we assert above that all branches have the same address, and that
+	 * each branch has a unique type/size.
 	 */
 	BUILD_BUG_ON(I915_GGTT_VIEW_NORMAL >= I915_GGTT_VIEW_PARTIAL);
 	BUILD_BUG_ON(I915_GGTT_VIEW_PARTIAL >= I915_GGTT_VIEW_ROTATED);
-	return memcmp(&vma->ggtt_view.params, &view->params, view->type);
+	BUILD_BUG_ON(offsetof(typeof(*view), rotated) !=
+		     offsetof(typeof(*view), partial));
+	return memcmp(&vma->ggtt_view.partial, &view->partial, view->type);
 }
 
 int i915_vma_bind(struct i915_vma *vma, enum i915_cache_level cache_level,