实现在树莓派的GPIO接口上用跳线(短路帽)轻松选择启用或者禁用overlayfs影子系统
前面我发过一篇blog说明如何使用overlayfs打造一个只读的不怕意外关机的树莓派Raspberry Pi。
https://blog.csdn.net/zhufu86/article/details/78906046
下面我来实现在树莓派的GPIO接口上用跳线(短路帽)选择启用或者禁用overlayfs影子系统。
【接线】
我是用了用了如下材料
--- 跳线帽(短路帽) * 1
--- 连接线 * 2 (可选)
--- LED * 1 (可选)
--- 电阻 1k欧姆 * 1 (可选)
如上图所示，在BCM GPIO 4 以及和右边的一位针脚（GND）上使用跳线帽，在BCM GPIO 18和其右边的一位针脚（GND）上连接LED，并串联1K电阻，LED的阴极接GND，阳极接电阻，电阻的另一端接GPIO 18。
接LED是为了在启动时指示脚本的运行状态，并不是必须要接的，如果不接也可以。
【原理】
需要在原overlayRoot.sh脚本的基础上，代码开头增加检测GPIO端口的代码。
首先，要mount sysfs，以便于后面访问GPIO
mount -t sysfs sysfs /sys
if [ $? -ne 0 ]; then
fail "ERROR: could not mount sysfs"
fi
设置GPIO引脚变量，可以按照你自己的实际接线更改设置。
# The jumper is on BCM GPIO 4
GPIO_PIN_JP=4
# The LED is connected on BCM GPIO 18
GPIO_PIN_LED=18
定义闪LED的函数，功能是点亮LED 0.4秒钟并熄灭。
# Function - Flash/light-up the LED one time
FlashLED()
{
# Export LED pin
echo $GPIO_PIN_LED > /sys/class/gpio/export
# Set LED pin as output port
echo out > /sys/class/gpio/gpio$GPIO_PIN_LED/direction
# Set LED pin output HIGH
echo 1 > /sys/class/gpio/gpio$GPIO_PIN_LED/value
sleep 0.4
# Set LED pin output LOW
echo 0 > /sys/class/gpio/gpio$GPIO_PIN_LED/value
# Set LED pin as input port
echo in > /sys/class/gpio/gpio$GPIO_PIN_LED/direction
# Unexport LED pin
echo $GPIO_PIN_LED > /sys/class/gpio/unexport
sleep 0.4
}
如果不打算连接LED，而且GPIO 18也可能别有他用，可以把这段代码改为无用代码：
# Function - Flash/light-up the LED one time
FlashLED()
{
echo I am a LED.
}
后面的代码开始干正事儿。
首先点亮LED 0.4秒后熄灭。
# Flash the LED one time
FlashLED
然后每隔50ms检测一次有没有跳线短接GPIO 4到GND，连续检测10次
# Export Jumper pin
echo "$GPIO_PIN_JP" > /sys/class/gpio/export
# Set Jumper pin as input port
echo in > /sys/class/gpio/gpio$GPIO_PIN_JP/direction
i=1
count=0
while [ "$i" -le 10 ]
do
# Detect the jumper pin every 0.05sec
sleep 0.05
# If the Jumper is shorted to GND
if [ $(cat "/sys/class/gpio/gpio${GPIO_PIN_JP}/value") = 0 ]; then
count=$((count+1))
fi
i=$((i+1))
done
# Uncomment to print the count value for test purpose
#echo $count
# Unexport the Jumper pin
echo $GPIO_PIN_JP > /sys/class/gpio/unexport如果发现10次检测有9次以上为低电平（即GPIO短路到GND），则点亮LED 0.4秒后熄灭，然后继续常规的init过程（/sbin/init）而不开启overlayfs模式，否则继续后面的开启overlayfs模式的代码。 继续之前不忘记unmount sysfs。
if [ $count -ge 9 ]; then
# if the Jumper is shorted to GND, exit to the normal init.
# Flash the LED one time
FlashLED
umount /sys
echo [Info.] Overlayfs jumper setting DISABLE.
# Continue with regular init
exec /sbin/init
#exit 0
else
umount /sys
echo [Info.] Overlayfs jumper setting ENABLE.
# Continue with overlayfs activation process.
fi【完整的overlayRoot.sh代码】
如下：
#!/bin/sh
# Read-only Root-FS for Raspian using overlayfs
# Version 1.0
#
# Created 2017 by Pascal Suter @ DALCO AG, Switzerland
# to work on Raspian as custom init script
# (raspbian does not use an initramfs on boot)
#
# Modified 2017-Apr-21 by Tony McBeardsley
#
# * Modified 2018-Jul-18 by Yang Chen
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program. If not, see
#
【备注】
启动之前使用跳线帽短路GPIO 4和GND，则启动过程会看到LED闪烁两次，启动后可以看到overlayfs并未启用。
启动之前如果空开GPIO 4和GND，即不短接跳线帽，则启动过程只会看到LED闪烁一次，启动后可以看到overlayfs已启用。
不连接LED，脚本也是可以正常工作的。GPIO 18如果别有他用，则需要按文中所述方法修改闪烁LED的那段函数。
理论上来说应该在内核启动时设置跳线帽连接的那个GPIO（我的例子里为GPIO 4）为上拉状态，不过我没有做任何处理，似乎也没问题。
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