1 GB lowmem

Hi

I am trying to enable 1 GB of lowmem on a Freescale 8280.
In arch/ppc this was easilly done by:
CONFIG_ADVANCED_OPTIONS=3Dy
CONFIG_HIGHMEM_START=3D0xfe000000
CONFIG_LOWMEM_SIZE_BOOL=3Dy
CONFIG_LOWMEM_SIZE=3D0x40000000
CONFIG_KERNEL_START_BOOL=3Dy
CONFIG_KERNEL_START=3D0xa0000000

This does not work in arch/powerpc. CPU hangs as soon as init starts.
Any ideas what to look at, what to change?

The reason I want to do this is because I cannot get highmem support to
work when using CONFIG_PREEMPT_RT.
(hitting a bug_on in kmap_atomic)