Test devmem in rpi2 by shell script

#!/bin/bash

#Test devmem (Toggle RED LED) in rpi2 by shell script

GPIO_BASE=$((0x3F200000))
LED_GPFSEL=4
LED_GPFBIT=21
LED_GPSET=8
LED_GPCLR=11
LED_GPIO_BIT=3

#addr=$(($GPIO_BASE + $LED_GPFSEL * 4))
#rvalue=$(devmem $(($addr)) 32)

addr=$(($GPIO_BASE + $LED_GPCLR * 4))
devmem $((addr)) 32 $((1 << $LED_GPIO_BIT))

sleep 5

addr=$(($GPIO_BASE + $LED_GPSET * 4))
devmem $((addr)) 32 $((1 << $LED_GPIO_BIT))

Add devmem in OpenWRT

make menuconfig

1) Select Global build settings

2) Select Kernel build options

3) Select [*] /dev/mem virtual device support

4) Return to main menu

5) select Base System

6) select <*> busybox

7) select <*>Customize busybox options

8) select Miscellaneous Utilities

9) select <*> devmem (2.5 kb)

Usage: devmem ADDRESS [WIDTH [VALUE]]

Read/write from physical address

    ADDRESS Address to act upon, for example 0x????????
    WIDTH   Width (8/16/...)
    VALUE   Data to be written, for example 0x????????

read 32bit register: devmem address

read 16bit register: devmem address 16

read 8bit register: devmem address 8

write 32bit register: devmem address 32 value

write 16bit register: devmem address 16 value

write 8bit register: devmem address 8 value