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

On 2020-09-17 2:20 p.m., Christian König wrote:

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?

One thing is that AFAIK some (ARM?) CPUs can get very upset when there's both a cached and uncacheable mapping for the same physical page.