Nor Flash interface problem

[Fundu]

Hi,

I'm working with a variant of ppc440 taishan board.
here's the setup.
a) 2 spansion 1gb part connected to one of the ~percsX (peripheral select). they are in x16 mode and interlaced.
b) _NO_ byte write/read. _only_ word reads (32 bits). ppc address a0-a4 is  NC.

Problem is percX can only address 128mb (which was realized later).
And the flash part is 256mb. Flash start address 0xf000:0000.

As a mod we have one of the gpio connected to A25 pin of the flash parts.
Such that any of the two 128mb "areas" can be accessed.
gpioX = 0, then f000:0000 -> f7ff:ffff
gpioX = 1, f800:0000 -> ffff:ffff

I'm trying to configure the mtd drivers such that when the the lower 128 is
accessed i.e base + 128mb the gpio is set to 0 and if its above f7ff:ffff 
the i assert A25(using gpio) to access the upper 128.

1) what the best place to have this check ? I'm thinking of having it under include/linux/mtd/cfi.h, cfi_read cfi_write etc..
    a) Or any other ideas to tackle this would also be appreciated.  

Any insights would be appreciated. 
TIA