The following error:

eric@xyzzy:~/freesa/openwrt-trunk/build_mipsel/linux$ ARCH=mips make
  CHK     include/linux/version.h
  CHK     include/linux/utsrelease.h
  CHK     include/linux/compile.h
  MODPOST vmlinux
  Building modules, stage 2.
  MODPOST 283 modules
WARNING: "local_flush_data_cache_page" [drivers/ide/ide-core.ko] undefined!
make[1]: *** [__modpost] Error 1
make: *** [modules] Error 2
eric@xyzzy:~/freesa/openwrt-trunk/build_mipsel/linux 

Is resolved by editing:

 openwrt-trunk/build_mipsel/linux/arch/mips/mm/cache.c

and adding the line:

EXPORT_SYMBOL(local_flush_data_cache_page);