http://blog.csdn.net/DroidPhone/article/details/23367051
https://www.kernel.org/doc/html/v4.14/driver-api/spi.html
https://armbedded.taskit.de/node/318
https://www.kernel.org/doc/html/v4.14/driver-api/i2c.html
http://www.cnblogs.com/lknlfy/p/3265019.html
https://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus
https://www.arduino.cc/en/Reference/SPI
http://blog.csdn.net/u013983194/article/details/75257592
http://blog.csdn.net/u013983194/article/category/6940608
https://github.com/christianeisendle/linux/blob/rpi-4.4.y_bal/arch/arm/boot/dts/overlays/bal-overlay.dts
https://github.com/christianeisendle/nxprdlib-kernel-bal