TTM placement & caching issue/questions

[Michel Dänzer]

On 04.09.2014 10:55, Jerome Glisse wrote:
>
> While i agree about the issue of incoherent double map of same page, i
> think we have more issue. For instance lattely AMD have been pushing a
> lot of patches to move things to use uncached memory for radeon and as
> usual thoses patches comes with no comment to the motivations of those
> changes.

That would have been a fair review comment...


> What i understand is that uncached mapping for some frequently use buffer
> give a significant performance boost (i am assuming this has to do with
> all the snoop pci transaction overhead).

Exactly, although it's a win even if the data is written by the CPU only 
once and read by the GPU only once.


> This also means that we need to fix ttm_tt_set_placement_caching so that
> when it returns an error it switches to cached mapping. Which will always
> work.

GTT with AGP being one exception.


-- 
Earthling Michel Dänzer            |                  http://www.amd.com
Libre software enthusiast          |                Mesa and X developer