mmap'ing /dev/mem and directly poking at registers is fine as long as you know what you are doing and one of the best ways to explore and play with something no

mmap'ing /dev/mem and directly poking at registers is fine as long as you know what you are doing and one of the best ways to explore and play with something not yet "properly" supported... ;-) But of course for this you'll have to map whole memory pages (there's no smaller granularity in providing direct memory access to registers), and this typically includes all registers of (here:) the GPIO peripheral. Only toggling outputs via the set/clear registers has little potential for damage, but anything requiring a read-modify-write will race against anything the kernel might want to do with GPIO pins in this bank. And only a particular boards' configuration can tell you if there's anything important on any other pin. A serial bus might for example want to change the direction of a pin, a keyboard might enable/disable interrupts on pin-changes, there's SD-card insertion, control of a WiFi cards' transmit functionality (rfkill), power to USB busses, detection of USB OTG cables. A good compromise, if you really have to toggle pins fast, might be to first reserve and configure a GPIOs functionality via sysfs, and then only use the mmap() for the actual input/output. Or write a proper kernel module. There are inline functions/macros (for in-kernel-use) that basically do the single-register-write.