PXA270 and frequency change

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Hi,

I'm currently trying to change the frequency of a PXA270 processor,
the system I'm working on is a Linux 2.6.24, and I would like to write
a code that allows an user to change the frequency, to do so I need to
write in a register of coprocessor 14. According to ARM documentation
i need to use MRC/MCR instructions, I tried but I got an error :
"Illegal instruction".

It seems that I should use Software Interrupt in order to switch from
user mode to supervisor mode and then use MRC/MCR instructions,  but I
don't know how the OS will handle that interrupt.
I'm not even sure that what I want to do is possible, but if it is,
does someone have any idea of the code I should write to switch to
supervisor mode and use MRC or MCR instruction?

Thanks,

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