On Fri, Jun 17, 2011 at 11:30:32AM +0200, Ingo Molnar wrote:

I'm aware of these current /dev/mem uses:

 - Xorg maps below 4G non-RAM addresses and the video BIOS

 - It used to have some debugging role but these days kexec and kgdb 
   has largely taken over that role - partly due to the 4G limit.

 - there's some really horrible out-of-tree drivers that do mmap()s
   via /dev/mem, those should be fixed if they want to move beyond 
   4G: their char device should be mmap()able.

 - all distro kernel's i'm aware of use CONFIG_STRICT_DEVMEM=y, which
   restricts /dev/mem to non-RAM pages of physical memory.
   [ With the sad inclusion of the first 1MB, which Xorg needs. ]

There's another use case for /dev/mem:

 - debugging via devmem2 on embedded platforms, where you want to be able
   to boot a kernel, and then peek and poke at MMIO registers either to
   verify register values or test things out.