Re: [PATCH 00/10] Enhance /dev/mem to allow read/write of arbitrary

Ryan Mallon
2011-06-19T23:02:17Z

On 17/06/11 19:30, Ingo Molnar wrote:
> * Petr Tesarik<ptesarik@suse.cz> 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.

~Ryan