mx6ul gpio 在device tree 中的配置

内核版本：linux-3.14.38

以GPIO1_IO13为例:
1. 在arch/arm/boot/dts/imx6ul-14x14-evk.dts中：
 pinctrl_hog_1: hoggrp-1 {
                        fsl,pins = <
                                MX6UL_PAD_LCD_RESET__WDOG1_WDOG_ANY   0x30b0
                                MX6UL_PAD_UART1_RTS_B__GPIO1_IO19       0x17059
                                MX6UL_PAD_GPIO1_IO05__USDHC1_VSELECT    0x17059
                                MX6UL_PAD_GPIO1_IO09__GPIO1_IO09        0x17059

                                MX6UL_PAD_JTAG_TDI__GPIO1_IO13          0xC0000000     
                        >;
                };
MX6UL_PAD_JTAG_TDI__GPIO1_IO13:宏定义引脚JTAG_TDI功能复用为GPIO13；

0xC0000000:

• 0x80000000 is special and means "I don't know and don't change from the default"
• Bit 30 set to 1 means: software input on (SION) whcih means that the logical value of an output can be read back

注意：要把SION设为1，才可以用在gpio为output时读回，否则读回将为0。但根据Refernce Manual中，SION应该在software mux control registers的第四位，而0xC0000000设置的却是SW_PAD_CTL Register，而且Manual中也没看见这个寄存器能设置SION，这里很奇怪。

2. 在用户空间可控制gpio13:

echo 13 > /sys/class/gpio/export

echo out > /sys/class/gpio/gpio2/direction

echo 1 > /sys/class/gpio/gpio2/value

echo 0 > /sys/class/gpio/gpio2/value

参考链接:

imx6ul gpio_spi用法

  (2016-04-01 14:14:33)

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在imx6ul-14x14-evk.dts中有这个一个设备树节点：
     spi4 {
                compatible = "spi-gpio";
                pinctrl-names = "default";
                pinctrl-0 = <&pinctrl_spi4>;
                pinctrl-assert-gpios = <&gpio5 8 GPIO_ACTIVE_LOW>;
                status = "okay";
                gpio-sck = <&gpio5 11 0>;
                gpio-mosi = <&gpio5 10 0>;
                cs-gpios = <&gpio5 7 0>;
                num-chipselects = <1>;
                #address-cells = <1>;
                #size-cells = <0>;

                gpio_spi: gpio_spi@0 {
                        compatible = "fairchild,74hc595";
                        gpio-controller;
                        #gpio-cells = <2>;
                        reg = <0>;
                        registers-number = <1>;
                        registers-default = /bits/ 8 <0xb7>;
                        spi-max-frequency = <100000>;
                };
        };

这个设备对应的是移位寄存器（74hc595），作用是拓展gpio口，linux中驱动代码位于drivers/gpio/gpio-74x164.c。
SN74HC595N是德州仪器公司生产的集成电路芯片，是一个8位串行输入变串行输出或并行输出移位寄存器，具有高阻关断，高电平和低电平三态输出。在IO扩充上，可以最多串联15片，也就是高达120个IO扩充。



那么这种扩展出来的gpio应如何使用呢？原理图中Q0~Q7对应&gpio_spi0~7
举两个栗子说明：
1. 在用户空间控制Q7 -- LCD_nPWREN：
   ~# cat /sys/kernel/debug/gpio
   GPIOs 248-255, spi/spi32766.0, 74hc595, can sleep:
   所以，Q0～Q7对应gpio248~255

   ~# echo 255 > /sys/class/gpio/export
   ~# echo 0/1 > /sys/class/gpio/gpio255/value
 
 2. 在device tree中添加控制节点：
  backlight {
                compatible = "pwm-backlight";
                pwms = <&pwm1 0 5000000>;
                brightness-levels = <0 4 8 16 32 64 128 255>;
                default-brightness-level = <6>;
                enable-gpios = <&gpio_spi 7 GPIO_ACTIVE_LOW>;
                status = "okay";
        };
 
  在backlight的驱动中drivers/video/backlight/pwm_bl.c中已经有enable-gpio的获取和使用。
  系统启动后可以看到：
~# cat /sys/kernel/debug/gpio

GPIOs 248-255, spi/spi32766.0, 74hc595, can sleep:
 gpio-255 (enable              ) out lo