ARM linux DMA alloc question

Hi,

I am using Linux kernel 2.6.12.6 for my ARM project.
I use dma_alloc_writecombine() to allocate DMA uncached
memory. The default consistent memory is define in
CONSISTENT_BASE and CONSISTENT_END as 0xffc00000 and
0xffe00000 which is 2MB. Since I need about 4MB of
DMA memory, so I changed CONSISTENT_BASE to 0xffa00000,
but I got kernel OOPS:

kernel BUG at arch/arm/mm/consistent.c:218!
Unable to handle kernel NULL pointer dereference at virtual address 00000000
pgd = 67f28000
[00000000] *pgd=07ef2031, *pte=00000000, *ppte=00000000
Internel error: Oops: 817 [#1]


Can anyone shed some light on me regarding this problem?
Thanks.
.

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