How to get Physical Address in uClinux with MMU

For the same physical memory, the address trhe user space program sees is different from the one the device driver sees. To access user space memory (e.g. for pointers provided by the user space program when calling the driver, I understand that (usually) a standard function is called, that _copies_ the appropriate memory block. I don't know how to make the driver determine the physical address or the Kernel virtual address of a pointer it gets from a user memory block from/to the Kernel space. Maybe in fact this is not provided as it asks for nasty side effects.