[U-Boot] Flag -mno-cache-volatile to bypass the cache

Hi list, I use readl() or writel() to access the hardware but i found a compiler flag that bypass the cache for all volatile variables, it uses io versions of load/store functions. So, what should i do, make use of write/read functions or insert this flag in the makefile?. It's a nios2 platform. Thank you in advance. Ivan Llopard.