Change clock frequency of AT91SAM9260

Hello,

On compiling UBoot, I know change the clock frequency.

But I would like to change the clock frequency of processor while linux 
is working .

Do you think that it is possible?

I have AT91SAM9260 processor.

I tried to write in CKGR_PLLAR register and modified MULA value but the
frequency is not changed (however, the register is really changed).


Thanks


Damien