Subject: Re: [PATCH] ppc32/64: Map prefetchable PCI without guarded bit

From: Benjamin Herrenschmidt <[email-masked]>
Date: Sat, 26 Mar 2005 09:26:52 +1100

On Thu, 2005-03-24 at 19:20 +0100, Segher Boessenkool wrote:
> > While experimenting with framebuffer access performances, we noticed a
> > very significant improvement in write access to it when not setting
> > the "guarded" bit on the MMU mappings. This bit basically says that
> > reads and writes won't have side effects (it allows speculation).
> 
> Unless the data is already in cache.
> 
> > It appears that it also disables write combining.
> 
> When the page is also cache-inhibited, it indeed does.
> 
> 
> Btw, did you ever get to fix the problem with mapping the last page
> of physical address space via /dev/mem ?

I don't think so, but I'll have to double check.

Ben.