use of volatile in iounmap()?

[Sam Ravnborg]

While reviewing some CAN driver I stumbled on iounmap
which has following prototype on x86:

extern void iounmap(volatile void __iomem *addr);

I argued that the driver should not use volatile
but then I cannot explain why the argument to
iounmap takes a volatile.

The same goes for many other functions in
the io*.h headers.

Grepping the other archs they mostly follow
same pattern.

Can anyone explain the rational for volatile in this case.

Thanks,
	Sam
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