linux-kernel - Re: [PATCH 00/10] Enhance /dev/mem to allow read/write of arbitrary physical addresses

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Date:	Mon, 20 Jun 2011 09:02:17 +1000
From:	Ryan Mallon <rmallon@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Petr Tesarik <ptesarik@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Fenghua Yu <fenghua.yu@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tony Luck <tony.luck@...el.com>, x86@...nel.org,
	linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
	linux-sh@...r.kernel.org, linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>,
	Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 00/10] Enhance /dev/mem to allow read/write of arbitrary
 physical addresses

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

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