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If suddenly it happens that you need to free GPIOs on UART(Serial Tx and Rx pins), then this topic is for you.

The fact that on processors AR71xx/AR913x/AR724X/AR933X - UART is locked by default and use internal resources (buffer, interrupt) for interaction with the kernel driver.
Therefore, when we trying to use these GPIOs(tx and rx) via sysfs - it fails.

So, it is possible to disable the internal UART via simple script (without the need to patch / recompile firmware):

Install io via opkg:

opkg update && opkg install io

If we use console, create empty file(or use sftp, scp, WinSCP):

touch /usr/sbin/uart_gpio

Script itself (copy this code into empty file):

#!/bin/sh
# Bitwise operations: & = And, | = Or, ^ = xOr, << = Left Shift

detect_addr="0x18060090"
rev_id_maj_msk="0xfff0"
func_addr="0x18040028"
bit1="1<<1"            # using bit1 for AR724x/AR933x
bit8="1<<8"            # using bit8 for AR71xx/AR913x

detect_value=0x`io -4 $detect_addr | cut -f3 -d' '`
detected_result=$(printf "0x%4.4x" $(($detect_value & $rev_id_maj_msk)))
func_value=0x`io -4 $func_addr | cut -f3 -d' '`

# depending on the detected rev_id of CPU -
# it will be use specific bit# as case_bit variable, or exit
case "$detected_result" in
# AR7240/AR7241/AR7242/AR9330/AR9331
0x00c0 | \
0x0100 | \
0x1100 | \
0x0110 | \
0x1110 )
    case_bit=$bit1
    ;;
# AR71xx/AR913x
0x00a0 | \
0x00b0 )
    case_bit=$bit8
    ;;
# AR9341/AR9342/AR9344
0x0120 | \
0x1120 | \
0x2120 )
    echo -e "No need to disable UART on AR934x processors,\n \
    just use sysfs to reprogram GPIOs."
    break
    exit 0
    ;;
* )
    echo "Can't detect your CPU, must be Atheros!"
    break
    exit 1
    ;;
esac

# we using Bitwise xOr operation to switching bit# state (0 or 1)
io -4 $func_addr $(printf "0x%8.8x" $(($func_value ^ $case_bit)))

# read bit# state and depending on the state - print some info
if [ $(($func_value & $case_bit)) = $(($case_bit)) ]; then
    echo "Hardware UART is turned OFF"
    # You can use this line for automatic configuring GPIOs via sysfs
    # or you can load other modules that use these GPIOs
else
    echo "Hardware UART is turned ON"
fi

Rights to execute the script:

chmod +x /usr/sbin/uart_gpio

Execute uart_gpio, or use /etc/rc.local for autoexecute.
(if we re-execute uart_gpio this will turn ON the HW-UART)

Source.

(UPDATE: cutted code in script - No need to unbind serial driver!)

(Last edited by Dioptimizer on 6 Jan 2014, 20:44)