Re: pre2.0.31-2

Philip Gladstone (philip@raptor.com)
Wed, 04 Jun 1997 16:28:04 -0400

Alan Cox wrote:
>
> > 1) The memory mapped by 'vremap' is marked cacheable. This
> > is (almost certainly) incorrect as it is memory that is
> > actually device memory. Note that the mmap call used by
> > X servers marks the memory as uncached.
>
> The PC architecture is supposed to handle this for you. It could well
> be an issue on MIPS and friends. The X one is hack and shouldnt be needed

I think that the problem arises that some of the motherboard
chipsets get it right, and some get it wrong. Having just spent
a considerable amount of time trying to track down a problem
with a memory mapped flash rom -- I could drive it from user-space
using mmap, but it was flaky when driven from the kernel with
vremap. You could argue that it is a bug on the motherboard --
and indeed I would argue that. However, the pragmatic solution
is to disable the caching for everything that isn't memory.

With a pentium II, things will become more complex as there is
more control over the cache type -- e.g. write combining cache.

Philip

-- 
Philip Gladstone                           +1 617 487 7700
Raptor Systems, Waltham, MA         http://www.raptor.com/