简单看一下设备树Overlay
之前接触的设备树是固定的，每个引脚都是在设备树中固定配置好，编译完成以后不能在进行修改，当我们引出的引脚是固定的，想要对引脚复用的时候，这个缺陷就变得很明显了，譬如我们引出的引脚带有I2C或者GPIO两个可选功能，需要在运用到的时候在进行修改，这就要用到设备树的Overlays功能了。
- LWN 616859: 设备树动态叠加技术
- Device Trees, Overlays and Parameters
- 设备树叠加层
- 树莓派 -- i2c学习 续（1） DeviceTree Overlay实例化rtc
- Device Tree Overlay Notes
- OSD335x Lesson 2: Linux Device Tree Overlay
Supported overlays and parameters
pi@raspberrypi:/boot/overlays $ ls
adau1977-adc.dtbo dpi24.dtbo hy28b.dtbo mcp3202.dtbo qca7000.dtbo spi1-1cs.dtbo
adau7002-simple.dtbo dwc2.dtbo i2c0-bcm2708.dtbo media-center.dtbo README spi1-2cs.dtbo
ads1015.dtbo dwc-otg.dtbo i2c1-bcm2708.dtbo midi-uart0.dtbo rotary-encoder.dtbo spi1-3cs.dtbo
ads1115.dtbo enc28j60.dtbo i2c-bcm2708.dtbo midi-uart1.dtbo rpi-backlight.dtbo spi2-1cs.dtbo
ads7846.dtbo enc28j60-spi2.dtbo i2c-gpio.dtbo mmc.dtbo rpi-cirrus-wm5102.dtbo spi2-2cs.dtbo
adv7282m.dtbo exc3000.dtbo i2c-mux.dtbo mpu6050.dtbo rpi-dac.dtbo spi2-3cs.dtbo
adv728x-m.dtbo fe-pi-audio.dtbo i2c-pwm-pca9685a.dtbo mz61581.dtbo rpi-display.dtbo spi-gpio35-39.dtbo
akkordion-iqdacplus.dtbo goodix.dtbo i2c-rtc.dtbo ov5647.dtbo rpi-ft5406.dtbo spi-rtc.dtbo
allo-boss-dac-pcm512x-audio.dtbo googlevoicehat-soundcard.dtbo i2c-rtc-gpio.dtbo papirus.dtbo rpi-poe.dtbo superaudioboard.dtbo
allo-digione.dtbo gpio-fan.dtbo i2c-sensor.dtbo pi3-act-led.dtbo rpi-proto.dtbo sx150x.dtbo
allo-katana-dac-audio.dtbo gpio-ir.dtbo i2s-gpio28-31.dtbo pi3-disable-bt.dtbo rpi-sense.dtbo tc358743-audio.dtbo
allo-piano-dac-pcm512x-audio.dtbo gpio-ir-tx.dtbo iqaudio-dac.dtbo pi3-disable-wifi.dtbo rpi-tv.dtbo tc358743.dtbo
allo-piano-dac-plus-pcm512x-audio.dtbo gpio-key.dtbo iqaudio-dacplus.dtbo pi3-miniuart-bt.dtbo rra-digidac1-wm8741-audio.dtbo tinylcd35.dtbo
applepi-dac.dtbo gpio-no-bank0-irq.dtbo iqaudio-digi-wm8804-audio.dtbo pibell.dtbo sc16is750-i2c.dtbo uart0.dtbo
at86rf233.dtbo gpio-no-irq.dtbo jedec-spi-nor.dtbo piscreen2r.dtbo sc16is752-i2c.dtbo uart1.dtbo
audioinjector-addons.dtbo gpio-poweroff.dtbo justboom-dac.dtbo piscreen.dtbo sc16is752-spi1.dtbo upstream-aux-interrupt.dtbo
audioinjector-ultra.dtbo gpio-shutdown.dtbo justboom-digi.dtbo pisound.dtbo sdhost.dtbo upstream.dtbo
audioinjector-wm8731-audio.dtbo hd44780-lcd.dtbo lirc-rpi.dtbo pitft22.dtbo sdio-1bit.dtbo vc4-fkms-v3d.dtbo
audremap.dtbo hifiberry-amp.dtbo ltc294x.dtbo pitft28-capacitive.dtbo sdio.dtbo vc4-kms-kippah-7inch.dtbo
balena-fin.dtbo hifiberry-dac.dtbo mbed-dac.dtbo pitft28-resistive.dtbo sdtweak.dtbo vc4-kms-v3d.dtbo
bmp085_i2c-sensor.dtbo hifiberry-dacplus.dtbo mcp23017.dtbo pitft35-resistive.dtbo smi-dev.dtbo vga666.dtbo
dht11.dtbo hifiberry-digi.dtbo mcp23s17.dtbo pps-gpio.dtbo smi.dtbo w1-gpio.dtbo
dionaudio-loco.dtbo hifiberry-digi-pro.dtbo mcp2515-can0.dtbo pwm-2chan.dtbo smi-nand.dtbo w1-gpio-pullup.dtbo
dionaudio-loco-v2.dtbo hy28a.dtbo mcp2515-can1.dtbo pwm.dtbo spi0-cs.dtbo wittypi.dtbo
dpi18.dtbo hy28b-2017.dtbo mcp3008.dtbo pwm-ir-tx.dtbo spi0-hw-cs.dtbo- 编写overlay dts:
rasp-rtc.dts/dts-v1/; /plugin/; / { compatible = "brcm,bcm2835", "brcm,bcm2708", "brcm,bcm2709"; fragment@0 { target = <&i2c1>; __overlay__ { #address-cells = <1>; #size-cells = <0>; rtc@68 { compatible = "maxim,ds3231"; reg = <0x68>; #address-cells = <2>; #size-cells = <1>; }; }; }; }; - 编译:
dtc -I dts -O dtb -o /boot/overlays/rasp-rtc.dtbo rasp-rtc.dts； - 将编译出来的
rasp-rtc.dtbo放到/boot/overlays； - 更新
/boot/config.txt:dtparam=i2c_arm=on # i2c interface dtoverlay=rasp-rtc # Overlay