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

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

Am Samstag 10 Oktober 2009 06:24:41 schrieb Arjan van de Ven:
> Thomas Schlichter wrote:
> > Hi,
> >
> > I've found a problem with X.org not setting up MTRR for the framebuffer
> > memory. After I investigated I think this is not a X.org problem, but a
> > kernel issue.
> 
> is there any CPU left that does not support PAT ?
> 
> eg in other words, should we try instead to get PAT working for you?

Hmm, my CPU (VIA Nano) seems to support PAT, but Ubuntu disables this
feature in its kernel config. I'm using 9.04, but even 9.10 will have PAT
disabled:

http://kernel.ubuntu.com/git?p=ubuntu/ubuntu-
karmic.git;a=blob;f=debian.master/config/config.common.ubuntu;h=a43cbabd02bfdf5412de6fea232db86aa5d3e742;hb=HEAD

The patch I sent does nothing if PAT is enabled, so it should be quite
safe for this case. And for the case when PAT is not enabled, it simply
tries to set up MTRR entries, if this fails it will still behave like
it does today...

So, convincing Ubuntu to enable PAT would be a workaround, but I think
the problem would still exist and should be fixed.

Kind regards,
  Thomas