i've found out that in a kernel module gpio_set_value and gpio_get_value are very slow (60 useconds). According to this site, http://codeandlife.com/2012/07/03/benchmarking-raspberry-pi-gpio-speed/ it's possible to access gpio in less than 75 nanoseconds...consider that in the kernel module i've disabled interrupts, so it should not care about the other processes... I've found a guide about accessing gpio using memory registers.
Do you have any idea about how to use this code in a kernel module? Thnaks in advance.