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Re: Using more than 256 MB of memory on SB1250 in 32-bit mode

To: Steve Madsen <madsen@tadpole.com>
Subject: Re: Using more than 256 MB of memory on SB1250 in 32-bit mode
From: Ralf Baechle <ralf@linux-mips.org>
Date: Sun, 31 Aug 2003 15:34:34 +0200
Cc: linux-mips@linux-mips.org
In-reply-to: <3F4FCCD5.1000604@tadpole.com>
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References: <3F4FCCD5.1000604@tadpole.com>
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On Fri, Aug 29, 2003 at 02:59:49PM -0700, Steve Madsen wrote:

> Is it possible to use more than 256 MB of system memory with the Broadcom 
> SB1250 in 32-bit mode?  The memory map I'm looking at shows me that the 
> second 256 MB of memory is at physical address 0x80000000.  I suspect that 
> due to the 2G/2G split in the kernel, I can't use memory this high without 
> moving to the 64-bit kernel.

Steve Finney's answer was correct; I'd like to add a few details though.

The explanation you gave isn't exactly right.  A 2GB/2GB split would normally
support 2GB of low memory.  We don't on MIPS due to the very inconvenient and
unchangable mappings of KSEG0/KSEG1 - something that may have been sweet
in '85 when the address map was designed but not today when 32-bit address
spaces are beginning to be fairly tight.

Highmem works ok in 2.4 as long as you have a reasonably low ratio of
highmem to lowmem.  For typical loads that means going beyond 4:1 isn't
sensible but the actual number may vary much based on exact system
configuration or workload.

  Ralf

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