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:email@example.com:/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.