[RaspberryPi] Raspbian提供的原生开关机按钮

在Raspbian的boot分区有个配置overlays的config.txt文档，参考如下配置就可以实现带状态指示的开关机按钮。

Rasperrypi
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功能：  
Name:   gpio-poweroff
Info:   Drives a GPIO high or low on poweroff (including halt). Enabling this
        overlay will prevent the ability to boot by driving GPIO3 low.
Load:   dtoverlay=gpio-poweroff,<param>=<val>
Params: gpiopin                 GPIO for signalling (default 26)

        active_low              Set if the power control device requires a
                                high->low transition to trigger a power-down.
                                Note that this will require the support of a
                                custom dt-blob.bin to prevent a power-down
                                during the boot process, and that a reboot
                                will also cause the pin to go low.
        input                   Set if the gpio pin should be configured as
                                an input.
        export                  Set to export the configured pin to sysfs
        timeout_ms              Specify (in ms) how long the kernel waits for
                                power-down before issuing a WARN (default 3000).
                                
Name:   gpio-shutdown
Info:   Initiates a shutdown when GPIO pin changes. The given GPIO pin
        is configured as an input key that generates KEY_POWER events.

        This event is handled by systemd-logind by initiating a
        shutdown. Systemd versions older than 225 need an udev rule
        enable listening to the input device:

                ACTION!="REMOVE", SUBSYSTEM=="input", KERNEL=="event*", \
                        SUBSYSTEMS=="platform", DRIVERS=="gpio-keys", \
                        ATTRS{keys}=="116", TAG+="power-switch"

        Alternatively this event can be handled also on systems without
        systemd, just by traditional SysV init daemon. KEY_POWER event
        (keycode 116) needs to be mapped to KeyboardSignal on console
        and then kb::kbrequest inittab action which is triggered by
        KeyboardSignal from console can be configured to issue system
        shutdown. Steps for this configuration are:

            Add following lines to the /etc/console-setup/remap.inc file:

                # Key Power as special keypress
                keycode 116 = KeyboardSignal

            Then add following lines to /etc/inittab file:

                # Action on special keypress (Key Power)
                kb::kbrequest:/sbin/shutdown -t1 -a -h -P now

            And finally reload configuration by calling following commands:

                # dpkg-reconfigure console-setup
                # service console-setup reload
                # init q

        This overlay only handles shutdown. After shutdown, the system
        can be powered up again by driving GPIO3 low. The default
        configuration uses GPIO3 with a pullup, so if you connect a
        button between GPIO3 and GND (pin 5 and 6 on the 40-pin header),
        you get a shutdown and power-up button. Please note that
        Raspberry Pi 1 Model B rev 1 uses GPIO1 instead of GPIO3.
Load:   dtoverlay=gpio-shutdown,<param>=<val>
Params: gpio_pin                GPIO pin to trigger on (default 3)
                                For Raspberry Pi 1 Model B rev 1 set this
                                explicitly to value 1, e.g.:

                                    dtoverlay=gpio-shutdown,gpio_pin=1

        active_low              When this is 1 (active low), a falling
                                edge generates a key down event and a
                                rising edge generates a key up event.
                                When this is 0 (active high), this is
                                reversed. The default is 1 (active low).

        gpio_pull               Desired pull-up/down state (off, down, up)
                                Default is "up".

                                Note that the default pin (GPIO3) has an
                                external pullup. Same applies for GPIO1
                                on Raspberry Pi 1 Model B rev 1.

        debounce                Specify the debounce interval in milliseconds
                                (default 100)
                                
Name:   i2c-rtc-gpio
Info:   Adds support for a number of I2C Real Time Clock devices
        using the software i2c controller
Load:   dtoverlay=i2c-rtc-gpio,<param>=<val>
Params: abx80x                  Select one of the ABx80x family:
                                  AB0801, AB0803, AB0804, AB0805,
                                  AB1801, AB1803, AB1804, AB1805

        ds1307                  Select the DS1307 device

        ds1339                  Select the DS1339 device

        ds3231                  Select the DS3231 device

        m41t62                  Select the M41T62 device

        mcp7940x                Select the MCP7940x device

        mcp7941x                Select the MCP7941x device

        pcf2127                 Select the PCF2127 device

        pcf2129                 Select the PCF2129 device

        pcf8523                 Select the PCF8523 device

        pcf8563                 Select the PCF8563 device

        rv1805                  Select the Micro Crystal RV1805 device

        rv3028                  Select the Micro Crystal RV3028 device

        addr                    Sets the address for the RTC. Note that the
                                device must be configured to use the specified
                                address.

        trickle-diode-type      Diode type for trickle charge - "standard" or
                                "schottky" (ABx80x and RV1805 only)

        trickle-resistor-ohms   Resistor value for trickle charge (DS1339,
                                ABx80x, RV1805, RV3028)

        wakeup-source           Specify that the RTC can be used as a wakeup
                                source

        backup-switchover-mode  Backup power supply switch mode. Must be 0 for
                                off or 1 for Vdd < VBackup (RV3028 only)

        i2c_gpio_sda            GPIO used for I2C data (default "23")

        i2c_gpio_scl            GPIO used for I2C clock (default "24")

        i2c_gpio_delay_us       Clock delay in microseconds
                                (default "2" = ~100kHz)