drm/i915/ttm: Embed a ttm buffer object in the i915 gem object

Embed a struct ttm_buffer_object into the i915 gem object, making sure
we alias the gem object part. It's a bit unfortunate that the
struct ttm_buffer_ojbect embeds a gem object since we otherwise could
make the TTM part private to the TTM backend, and use the usual
i915 gem object for the other backends.
To make this a bit more storage efficient for the other backends,
we'd have to use a pointer for the gem object which would require
a lot of changes in the driver. We postpone that for later.
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210602083818.241793-3-thomas.hellstrom@linux.intel.com

drivers/gpu/drm/i915/gem/i915_gem_object.c

@@ -62,6 +62,13 @@ void i915_gem_object_init(struct drm_i915_gem_object *obj,
 			  const struct drm_i915_gem_object_ops *ops,
 			  struct lock_class_key *key, unsigned flags)
 {
+	/*
+	 * A gem object is embedded both in a struct ttm_buffer_object :/ and
+	 * in a drm_i915_gem_object. Make sure they are aliased.
+	 */
+	BUILD_BUG_ON(offsetof(typeof(*obj), base) !=
+		     offsetof(typeof(*obj), __do_not_access.base));
+
 	spin_lock_init(&obj->vma.lock);
 	INIT_LIST_HEAD(&obj->vma.list);
 

drivers/gpu/drm/i915/gem/i915_gem_object_types.h

@@ -10,6 +10,7 @@
 #include <linux/mmu_notifier.h>
 
 #include <drm/drm_gem.h>
+#include <drm/ttm/ttm_bo_api.h>
 #include <uapi/drm/i915_drm.h>
 
 #include "i915_active.h"
@@ -99,7 +100,16 @@ struct i915_gem_object_page_iter {
 };
 
 struct drm_i915_gem_object {
+	/*
+	 * We might have reason to revisit the below since it wastes
+	 * a lot of space for non-ttm gem objects.
+	 * In any case, always use the accessors for the ttm_buffer_object
+	 * when accessing it.
+	 */
+	union {
 		struct drm_gem_object base;
+		struct ttm_buffer_object __do_not_access;
+	};
 
 	const struct drm_i915_gem_object_ops *ops;