2GB address space limit on 32-bit PowerPC Macintosh

Benjamin Herrenschmidt benh at kernel.crashing.org
Mon May 16 09:34:29 EST 2005

On Mon, 2005-05-16 at 09:29 +1000, Paul Mackerras wrote:
> Benjamin Herrenschmidt writes:
> > We "inherited" from some historic junk in the prep and chrp support,
> > that a lot of embedded platforms blindly copied, where archs use
> > io_block_mapping() early during boot to hard-wire various IO stuffs in
> > various places in the address space, including just after 2Gb. It's
> > totally bogus, but nobody really cared to fix it so far. The 2Gb
> > TASK_SIZE limit doesn't seem to have ever been an issue for ppc32 users
> > so far I must say, at least you are the first one to complain ;)
> I believe that prep and chrp are also now OK with a 3GB TASK_SIZE
> limit, it's just various embedded board ports that will blow up with
> 3GB.  We should change the default to 3GB to encourage the embedded
> guys to fix their ports properly (or else to put in the appropriate
> Kconfig stuff to force it back to 2GB for their port).

static void __init
	io_block_mapping(0x80000000, PREP_ISA_IO_BASE, 0x10000000, _PAGE_IO);
	io_block_mapping(0xf0000000, PREP_ISA_MEM_BASE, 0x08000000, _PAGE_IO);

We need to fix that too :) Though I suppose we can just switch that to
page tables, I don't really see the point of using a BAT here...


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