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@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. Ralf
