Why can't ttm_tt_swapout() handle uncached or WC BOs?

Hi guys,

Michel once submitted a patch to fix triggering this BUG_ON in ttm_tt_swapout():

BUG_ON(ttm->caching_state != tt_cached);

Now my question is does anybody know why we have that in the first place?

The only problematic thing I can see is calling copy_highpage() and that one is just doing a kmap_atomic()/kunmap_atomic() on the source and destination.

I can't see why it should be problematic for this temporary mapping to be cached instead of uncached or WC?

Does anybody has any idea?

Thanks, Christian.