Re: [RFC Patch] use MTRR for write combining if PAT is not available

[Ingo Molnar <mingo@elte.hu>]

* Suresh Siddha <suresh.b.siddha@intel.com> wrote:

> On Mon, 2009-10-19 at 02:16 -0700, Jan Beulich wrote:
> > Functionality-wise this looks fine to me
> 
> If we are going to make ioremap() and set_memory_wc() add mtrr's in 
> non-pat case, then we need to delete the added mtrr(s) in the 
> corresponding iounmap() and set_memory_wb() aswell.
> 
> hmm, this is becoming too complex. The way i915 and other graphics 
> drivers are using set_memory_wc(), it is def a bad idea to start 
> adding mtrr's behind the back for non-pat case.

Touching MTRRs beyond working around basic bugs like non-cached RAM 
sounds like madness. The interactions with PAT are ... countless.

> Can't we just force PAT option always and we probably don't care about 
> ioremap_wc() on processors were PAT doesn't get enabled because of 
> known errata.

We can make PAT configurability dependent on EMBEDDED-y - mind sending a 
patch for that?

	Ingo