On Mon, 2004-05-10 at 23:49, Ralf Baechle wrote: > 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. You mean that after basic probing you always access PCI devices thru the TLB? > > > 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 ... Well my 4KEc core has a CP0 register called EBase (15 select 1) that allows to put the vectors anywhere (excepted reset/softreset/nmi/ejtag). > > > 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@xxxxxxxxxxxxxxxxxx:/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. Thanks Sincerely yours, E.M. > > Ralf > >
