Linux IO mapped memory access

APIs

For a device driver, the hardware registers access usually involves following kernel API.

• request_mem_region: Tell kernel that the specific range of physical memory are to be used.
• ioremap: Maps the physical memory to kernel virtual memory that can be accessed by kernel.
• ioreadX, iowriteX: X could be 8, 16, 32, parameter is the kernel virtual memory.
• release_mem_region: tell kernel the range of physical memory is not to be used anymore

ioread & iowirte

There are some drivers for ARM device are using "writel" and  "iowrite32" function to access IO mapped memory.
writeX read X are deprecated functions,  should use ioreadX iowriteX functions.

There is some interesting story about memory barriers of io mapped memory access.
Seems in 2006, writel and iowrite32 is no-barrier.  It is nowadays.

http://lwn.net/Articles/198988/

request_mem_region

About why some code, there is no calling of request_mem_region.
http://stackoverflow.com/questions/7682422/what-does-request-mem-region-actually-do-and-when-it-is-needed

Examples

Some source examples shows how to use io read/write related functions.