On 17/06/11 19:30, Ingo Molnar wrote:
* Petr Tesarik<ptesarik@xxxxxxx> wrote:This patch series enhances /dev/mem, so that read and write is possible at any address. The patchset includes actual implementation for x86.This series lacks a description of why this is desired. My strong opinion is that it's not desired at all: /dev/mem never worked beyond 4G addresses so by today it has become largely obsolete and is on the way out really. 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.
There are drivers where this makes sense. For example an FPGA device with a proprietary register layout on the memory bus can be done this way. The FPGA can simply be mapped in user-space via /dev/mem and handled there. If the device requires no access other than memory bus reads and writes then writing a custom char device driver just to get an mmap function seems a bit overkill.
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