drm/ttm: Explain why ttm_bo_add_move_fence uses a shared slot

Motivated because I got confused and Christian confirmed why this
works. I think this is non-obvious enough that it merits a slightly
longer comment.
Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Cc: Christian König <ckoenig.leichtzumerken@gmail.com>
Cc: Christian Koenig <christian.koenig@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210519082409.672016-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/ttm/ttm_bo.c

@@ -682,7 +682,9 @@ int ttm_mem_evict_first(struct ttm_device *bdev,
 }
 
 /*
- * Add the last move fence to the BO and reserve a new shared slot.
+ * Add the last move fence to the BO and reserve a new shared slot. We only use
+ * a shared slot to avoid unecessary sync and rely on the subsequent bo move to
+ * either stall or use an exclusive fence respectively set bo->moving.
  */
 static int ttm_bo_add_move_fence(struct ttm_buffer_object *bo,
 				 struct ttm_resource_manager *man,