Writing to a device at a known physical address


I know this is a pretty basic question but bear with me.

I'm running Linux (2.6.10) on a Freescale Coldfire (M5484) chip. I
ported the Colilo boot loader and made some mods to the kernel to run
on a custom board and things are working really well. Now I need to
access particular hardware that are configured (on a Flexbus) at
particular addresses. A simple example are the LED's that are used for
indicating life in the system. They are mapped to 0x4200 0000 and I
can access them easily from the boot loader. However once Linux is up
and running I get seg faults when I try to write to that address. This
makes sense since the MMU is remapping where things are but how do I
work with that? How do I know where these devices are in memory and
how do I get to them? Must it be done as a driver in kernel space
rather than user space?

Any help much appreciated!!
.

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