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

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Date:	Fri, 17 Jun 2011 10:41:25 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
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>,
	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 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.
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