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

[Michel Dänzer]

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.


-- 
Earthling Michel Dänzer               |               https://redhat.com
Libre software enthusiast             |             Mesa and X developer