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Re: new platform

To: Emmanuel Michon <em@realmagic.fr>
Subject: Re: new platform
From: Ralf Baechle <ralf@linux-mips.org>
Date: Mon, 10 May 2004 23:49:27 +0200
Cc: linux-mips@linux-mips.org
In-reply-to: <1084199090.12536.1314.camel@avalon.france.sdesigns.com>
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On Mon, May 10, 2004 at 04:24:51PM +0200, Emmanuel Michon wrote:

> I plan to port linux-mips to a a 32bit 4KEc based (little endian)
> hardware design.
> 
> I have three questions:
> 
> Q1- The nice book `see mips run' states that it's better that the
> physical address map fits entirely in kseg1 (in 0x0-0x2000_0000).
> 
> I would not be the first to plan for a lot of RAM and I understand
> HIGHMEM patch is ok if an extra RAM area is out of reach of kseg1.

Using highmem in general is a baaad idea.  The option only exists at all
for MIPS because of a user who didn't want to try something as unorthodox
as 64-bit kernels ...

Highmem implies significant extra overhead and complexity for software
that runs in kernel space.  Avoid like the plague if you can.

> But what if my PCI devices I/O do not lie in kseg1? I may program the
> TLB to see them thru kseg2 (but kseg2 seems to be the place where page
> tables are stored...)

Doesn't really matter.  It's nice to have devices in the lower 512MB of
physical address space because that means the TLB will not be used - a
nice performance bonus.

Whatever - the driver API to use is ioremap.

> Q2- Most hardware platforms have their SDRAM chips mapped at
> physical address 0x0. Mine does not. Am I going ahead of problems?

It won't work ;-)

You at least need some memory at physical address zero because exception
vectors are located in the first few k of physical address space.  Of
course you could avoid that by having the BEV bit set in the status
register so exceptions would go via 0xbfc00000 - but that's an uncached
address, likely even in a flash so performance would go down the drain ...

> It seems to be assumed at a lot of places (I have already ported YAMON).
> 
> Q3- I'd rather stick to a 2.4.x linux port. But... should I use:

Depending on what exactly you want to do you should take a look at 2.6.

> a- the latest official 2.4.x kernel
> b- the latest 2.4.x-preY kernel

kernel.org kernels won't work out of the box or at least your chances
are worse due to the lag in merging MIPS code from to kernel.org.

> c- the latest linux-mips.org 2.4.x kernel
> d- cvs -z3 -d :pserver:cvs@ftp.linux-mips.org:/home/cvs co -r linux_2_4

D - where C and D are the basically the same anyway - I've stopped making
snapshot tarballs years ago, so you'll have to fetch from cvs.

  Ralf

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