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

From: Segher Boessenkool <[email-masked]>
Date: Thu, 24 Mar 2005 19:20:43 +0100

> 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 ?

Segher