Strange mapping at 0xffc00000

[Jim Somerville]

I have started using the 2.2.5 kernel from 2.1.24 previously, and have found
a
new 4 Meg mapping up at 0xffc00000 that was not there before.  This mapping
is
setup in arch/ppc/kernel/pmac_setup.c, in routine pmac_setup_arch with the
following line:

        __ioremap(0xffc00000, 0x400000, pgprot_val(PAGE_READONLY));

It may be related to SMP support on some Macs (9600?) but the comments near
it
are vague.  I have not enabled SMP in the kernel via config, and this
mapping
does not look "config" optional.  I removed this line, rebuilt the kernel,
and
everything is fine, at least on the 7300.  This mapping interferes with a
large
emulator application that I maintain...I don't want junk returned when
dereferencing a bad pointer that happens to point up there.  Can anyone shed
some light, or provide a link to an explanation?  Thanks.

-Jim

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