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

[Thomas Schlichter <thomas.schlichter@web.de>]

Suresh Siddha wrote:
> 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.

Yes, maybe it's better to drop it for ioremap() and set_memory_wc(). But I'd like
to keep it for mmapping the PCI region. It should help all the people with
PAT-incapable CPUs and graphics chips without DRM support (for them there
simply is no driver that should set up the MTRR entries...).

> 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.

I don't think this is a good idea, Robert Hancock wrote there may be millions of
such Laptops (Core Solo/Duo erratum AE7, Pentium M erratum Y31) :
  http://marc.info/?l=linux-kernel&m=125537136105246&w=2

> or Perhaps just try to add mtrr only for the pci mmap case like the 4th
> patch in this series..

I'd prefer this! ;-)

Kind regards,
  Thomas