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Re: HIGHMEM

To: ralf@linux-mips.org
Subject: Re: HIGHMEM
From: Jon Anders Haugum <jonah@omegav.ntnu.no>
Date: Tue, 7 Dec 2004 11:29:04 +0100 (CET)
Cc: linux-mips@linux-mips.org
In-reply-to: <003801c4dc45$f9212690$2203a8c0@qfree.com>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <003801c4dc45$f9212690$2203a8c0@qfree.com>
Sender: linux-mips-bounce@linux-mips.org
In 2.4 the support for CONFIG_DISCONTIG and CONFIG_NUMA are a bit tangled
with each other because IP27 is the only platform to uses these features and
it needs both.  Other than that you can also just setup your system as 0x0 -
0x10000000 being RAM, 0x10000000 - 0x20000000 being reserved memory and
0x20000000 - 0x30000000 being highmem.  Which works but is a bit wasteful.

Issue #2 is that we don't support the combination of CONFIG_DISCONTIG and
CONFIG_HIGHMEM.  And highmem is a lobotomized solution for lobotomized
silicon anyway.  You have a 64-bit processor - use it's capabilities :-)

Issue #3 - As I recall the TX4937's H3 core is suffering from cache aliases.
Handling those efficiently for highmem is not easily possible and so we
don't even try.  More recent kernels will refuse to enable highmem on such
cache configurations but something like 2.4.18 which by now is an almost 3
year old antique doesn't know about that and will happily crash.

I recommend you should go for a 64-bit kernel instead.  And 64-bit support
is certainly better in 2.6 than in 2.4.  Especially the area of 32-bit
binary compatibility has been improved significantly.

What about on a processor like the AMD au1550?

I've tried the latest from 2.4 branch in cvs, and I haven't been successful in geting past INIT either...

Can I place all the memory from address 0x20000000 to get more than 256Meg?


--
Jon Anders Haugum


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