Linux Device Drivers Development

MMIO device access

Memory-mapped I/O resides in the same address space as memory. The kernel uses part of the address space normally used by RAM (HIGH_MEM actually) to map the device registers, so that instead of having real memory (that is, RAM) at that address, the I/O device takes its place. Thus, communicating to an I/O device becomes like reading and writing to memory addresses devoted to that I/O device.

In other words if I need to access for say the 4 MB of memory-mapped space assigned to IPU-2 (from 0x02A00000 to 0x02DFFFFF) of the i.MX6, the CPU (by mean of the MMU) may assign me address range 0x10000000 to 0x10400000, which is virtual of course. This is not consuming physical RAM (except for building and storing page table entries), ...