Sorry. I didn't commit TX39, R5900 configuration.
I prefer to specify CPU architecture rather than ISA. Because
nowadays instruction set architecture don't reflect its cache
implementation.
Do you think about this patch? 
ftp://ftp.netbsd.org/pub/NetBSD/misc/uch/mips.cache.conf.patch
And this patch reduce kernel size.(don't link unneeded code)
it is important for small memory machines.
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UCHIYAMA Yasushi
uch@vnop.net