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Re: Does HIGHMEM work on 32-bit MIPS ports?

To: David VomLehn <dvomlehn@cisco.com>
Subject: Re: Does HIGHMEM work on 32-bit MIPS ports?
From: Ralf Baechle <ralf@linux-mips.org>
Date: Wed, 20 Feb 2008 16:18:44 +0000
Cc: linux-mips@linux-mips.org
In-reply-to: <47BBA809.3050505@cisco.com>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <47BBA809.3050505@cisco.com>
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On Tue, Feb 19, 2008 at 08:09:45PM -0800, David VomLehn wrote:

> As we continue to investigate using high memory on MIPS, we keep coming up 
> with odd results. The basic mapping of high memory seems to be working 
> correctly, and if we use an INITRAMFS root filesystem, things seem to work. 
> Things also seem to work with an NFS root filesystem if we disable 
> preemption, though we get someone squirrelly behavior in some minor ways. 
> Has anyone else successfully been able to use high memory on a 32-bit MIPS 
> Linux port?

I've written MIPS highmem support in late 2002 for a customer who back
then wasn't interested in being the first through the 64-bit minefield.
Which back then certainly was justified - but there are now fairly
stable 64-bit Linux kernels available so if you happen to be running on
64-bit hardware don't even spend a nanosecond on thinking about 32-bit
highmem kernels.  Highmem fundamentally sucks rocks through a straw.

Coming back to your question.  Highmem was only ever tested to work on
SB1 and somewhat later PMC-Sierra RM9000 cores, both being 64-bit.  With
the increasing maturity of 64-bit Linux interest in these went away and
as the result the highmem code started a slow bitrot - unnoticed for many
moons.

  Ralf

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