Re: TTM placement & caching issue/questions

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