"The part of the interface most used by drivers is reading and writing memory-mapped registers on the device. Linux provides interfaces to read and write 8-bit, 16-bit, 32-bit and 64-bit quantities. Due to a historical accident, these are named byte, word, long, and quad accesses. Both read and write accesses are supported; there is no prefetch support at this time. The functions are named readb, readw, readl, readq, writeb, writew, writel, and writeq.
"Some devices (such as framebuffers) would like to use larger transfers that are more than 8 bytes at a time. For these devices, the memcpy_toio, memcpy_fromio and memset_io functions are provided. Do not use memset or memcpy on I/O addresses; they are not guaranteed to copy data in order.
"The read and write functions are defined to be ordered. That is, the compiler the the the is not permitted to reorder the I/O sequence. When the ordering can be compiler optimized, you can use __readb and friends to indicate the relaxed ordering. Use this with care. The rmb provides a read memory barrier. The wmb provides a write memory barrier."
