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Subject[patch] x86, voyager: fix ioremap_nocache()

* James Bottomley <> wrote:

> This patch:
> commit 6371b495991debfd1417b17c2bc4f7d7bae05739
> Author: Ingo Molnar <>
> Date: Wed Jan 30 13:33:40 2008 +0100
> x86: change ioremap() to default to uncached
> As far as I can tell went blindly into the x86 tree without being
> shared on any mailing list at all. How can something that completely
> alters the semantics of ioremap on x86 platforms go in without any
> review.

what happened before was that on x86 ioremap() was "lax" about the PTE
cachability and just said "writeback cached". That was utterly false for
most of the real ioremap()s done by drivers, but it happened to work out
fine due to the courtesy of BIOSes setting up UC MTRRs that forced those
areas into uncachable.

with the PAT changes, what used to be this default and careless
ioremap() behavior by x86 turned into a hard cache aliasing conflict. So
we pretty much _have to_ default to uncached - like all other sane
architectures already did forever. This is a direct consequence of the
PAT changes which were discussed on lkml.

But with PAT we take over from the MTRRs on x86 and using a cacheable
ioremap() would give us aliasing conflicts and trouble all around the

In the Voyager case we should use ioremap_cache(), and thanks for
pointing out that dependency of the QIC. Does the patch below fix it for


Subject: x86, voyager: fix ioremap_nocache()
From: Ingo Molnar <>
Date: Sun Apr 27 23:21:03 CEST 2008

James Bottomley reported that the following PAT related commit:

| commit 6371b495991debfd1417b17c2bc4f7d7bae05739
| Author: Ingo Molnar <>
| Date: Wed Jan 30 13:33:40 2008 +0100
| x86: change ioremap() to default to uncached

broke Voyager.

James says:

" it broke a class of voyager machines: those which
rely on the quad interrupt controller (QIC). The precis of why they
broke is because the QIC does IPIs (or CPIs in its terminology) via
cache line interference: you interrupt a processor by moving a
designated memory area to write exclusive in the cache (by simply
writing to the line) and the CPU acks the interrupt by moving it back to
read shared (by reading from it). That area, is, of course, mapped by
ioremap, so reversing the ioremap semantics and adding the uncached bit
completely breaks the QIC. "

Sorry about that!

Signed-off-by: Ingo Molnar <>
arch/x86/mach-voyager/voyager_cat.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

Index: linux-x86.q/arch/x86/mach-voyager/voyager_cat.c
--- linux-x86.q.orig/arch/x86/mach-voyager/voyager_cat.c
+++ linux-x86.q/arch/x86/mach-voyager/voyager_cat.c
@@ -602,7 +602,7 @@ void __init voyager_cat_init(void)

request_resource(&iomem_resource, &res);
voyager_SUS = (struct voyager_SUS *)
- ioremap(addr, 0x400);
+ ioremap_cache(addr, 0x400);
printk(KERN_NOTICE "Voyager SUS mailbox version 0x%x\n",
voyager_SUS->kernel_version = VOYAGER_MAILBOX_VERSION;
@@ -877,7 +877,7 @@ void __init voyager_cat_init(void)
request_resource(&iomem_resource, res);

- qic_addr = (unsigned long)ioremap(qic_addr, 0x400);
+ qic_addr = (unsigned long)ioremap_cache(qic_addr, 0x400);

for (j = 0; j < 4; j++) {
__u8 cpu;

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