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Kernel space access to 2GB of physical memory

To: linux-mips <linux-mips@linux-mips.org>
Subject: Kernel space access to 2GB of physical memory
From: "Alex Gonzalez" <langabe@gmail.com>
Date: Thu, 2 Aug 2007 14:47:37 +0100
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Hi,

I have a system booting the 2.6.12-rc3 32bits kernel on an RM9000
processor and I am trying to get my head around how to access up to
2GB of physical memory with a mem=256MB command line booting
parameter. I read that on the MIPS kernel this is split 2GB/2GB
between kernel and user space, so the kernel should be able to access
directly 2GB of physical memory.

Kernel drivers will access the memory above 256MB using ioremaps. I
need it this way as the memory above 256MB will be used in special
ways.

What I struggle to understand is whether it would be possible to
access all the memory up to 2GB using the ioremap method.

The only way I can think of achieving this would be to use ksseg and
dynamic TLB entries to access it in 256MB chunks. The fact that I can
access ksseg must mean that the kernel is not clearing the TLB entries
that the bootloader sets up before launching the kernel, so I would
expect to be able to add/remove TLB entries dynamically without
affecting the kernel's own memory management.

1) Is there a simpler mechanism to achieve this?

2) Any ill effect on the kernel from the method described above?

Many thanks,
Alex

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