Re: [PATCH] ioremap_cached()

Andi Kleen
2006-03-30T19:50:20Z

On Thursday 30 March 2006 21:34, Matthew Wilcox wrote:
> On Thu, Mar 30, 2006 at 08:27:53PM +0200, Andi Kleen wrote:
> > Matthew Wilcox <matthew@wil.cx> writes:
> >
> > > We currently have three ways for getting access to device memory --
> > > ioremap(), ioremap_nocache() and pci_iomap(). 99% of the callers of
> > > ioremap() are doing it to access device registers, and really, really
> > > want to use ioremap_nocache() instead. I presume nobody notices on PCs
> > > because they have write-through caches, but it ought to trip up people
> > > trying to flush writes.
> >
> > Actually MTRRs take care of that on x86.
> > So essentially on x86 ioremap() for devices is already ioremap_uncached()
> > And ioremap on memory is cached.
> >
> > That's nice and simple semantics that other platforms can emulate too.
> > Doing things differently will just cause pain for the other platforms
> > when they have to fix up drivers all the time.
>
> That doesn't make any sense. What's the point of ioremap_nocache() if
> ioremap() does magic things that make things uncached?

In theory ioremap_nocache would force uncached even if there is no MTRR
and is better/clearer.

But on x86 there normally is, so lots of code gets it wrong.

My point is just that forcing a semantics that's not enforced
on x86 would be a uphill battle for everybody else. Probably not
a good idea. Better fake x86.

> And who says
> you're allowed to ioremap() memory anyway?

Why not? There are cases when it's needed.

> > It all works fine until someone wants WC too. I would rather add a
> > ioremap_wc(), that would be more useful.
>
> ioremap_wc() sounds like a good idea.

It's unfortunately tricky to get it fully right on x86. The issue
is to have a good way avoid illegal cache aliases. There were some
attempts, but so far they never were polished up from the initial
prototypes.

-Andi