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Re: Should /dev/kmem support above 0x80000000 area?

To: Guido Guenther <>
Subject: Re: Should /dev/kmem support above 0x80000000 area?
From: Jun Sun <>
Date: Mon, 11 Dec 2000 18:26:51 -0800
Cc: "Maciej W. Rozycki" <>,
References: <> <> <>
Guido Guenther wrote:
> On Mon, Dec 11, 2000 at 12:28:19PM +0100, Maciej W. Rozycki wrote:
> > friends) by glibc.  At least XFree86 and SVGATextMode make use of these
> > features.  I suppose it's the same for MIPS (I haven't checked, though).
> Yes. xf86MapVidMem & friends use /dev/mem to mmap videomemory & iospace
> independent of architecure.
>  -- Guido

I am surprised.  I thought /dev/mem is for accessing SYSTEM RAM.  (do a 'man'
on /dev/mem)  It is also confirmed by the code in drivers/char/mem.c.  If you
want to access anything beyond 'high_memory", nothing is read.  

Note that drivers/char/mem.c is cross-platform code.  I am not sure how X
would access video memory through /dev/mem on either MIPS or other platforms.

That reason I want to fix /dev/kmem is that in some cases before a driver is
written people want to play with the hardware directly from the userland
(especially for demo purpose. :-0)  Very useful for embedded systems.

Potentially fixing /dev/mem can do the same job.  However /dev/mem cannot
differentiate cached or uncached accesses.  With /dev/kmem, we just specify
0x8.. or 0xa....


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