AM625: Ensuring Consistent GPIO Numbering with Removable I2C Expansion Module

Hi Bin，

I think you may not fully understand my question. In our APP, we do not rely on the gpiochip name but rather on their GPIO numbers.

Here is a simple example of how we use it: we control an LED using main_gpio0_57.

In this case, if the I2C expansion IO module is plugged in, MAIN_GPIO0_BASE needs to be defined as 386. However, if the module is removed, MAIN_GPIO0_BASE needs to be defined as 394 to control the same GPIO pin.

#define MAIN_GPIO0_BASE 386
#define GPIO_TO_PIN(base, gpio) ((base) + (gpio))
#define GPIO_LED_ALARM	  GPIO_TO_PIN(MAIN_GPIO0_BASE, 57)
....


if (temp > 0) {
	gpio_direction_output(GPIO_LED_ALARM, ON);
} else {
	gpio_direction_output(GPIO_LED_ALARM, OFF);
}

Hi Stephen,

My point is that the user space applications should not rely on the gpiochip IDs or gpio pin numbers which vary from kernel to kernel or even platform to platform. To completely decouple the applications from kernel, gpio pin names should be used instead of gpiochip IDs and gpio pin numbers.

Here is example on AM64x EVM (sorry to use AM64x an example, I just have the EVM running on my desk at the moment).

In kernel device tree k3-am642-evm.dts, the I/O expander pin names are defined as:

  &main_i2c1 {                                                                                                                                                                                                                                                                                                                                                                                                                            
          status = "okay";                                                                                                                                                                                                                                                                                                                                                                                                                
          pinctrl-names = "default";                                                                                                                                                                                                                                                                                                                                                                                                      
          pinctrl-0 = <&main_i2c1_pins_default>;                                                                                                                                                                                                                                                                                                                                                                                          
          clock-frequency = <400000>;                                                                                                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                                                                                                          
          exp1: gpio@22 {                                                                                                                                                                                                                                                                                                                                                                                                                 
                  compatible = "ti,tca6424";                                                                                                                                                                                                                                                                                                                                                                                              
                  reg = <0x22>;                                                                                                                                                                                                                                                                                                                                                                                                           
                  gpio-controller;                                                                                                                                                                                                                                                                                                                                                                                                        
                  #gpio-cells = <2>;                                                                                                                                                                                                                                                                                                                                                                                                      
                  gpio-line-names = "GPIO_eMMC_RSTn", "CAN_MUX_SEL",                                                                                                                                                                                                                                                                                                                                                                      
                                    "GPIO_CPSW1_RST", "GPIO_RGMII1_RST",                                                                                                                                                                                                                                                                                                                                                                  
                                    "GPIO_RGMII2_RST", "GPIO_PCIe_RST_OUT",                                                                                                                                                                                                                                                                                                                                                               
                                    "MMC1_SD_EN", "FSI_FET_SEL",
                ...

Using pin "GPIO_OLED_RESETn" as an example in Linux:

root@am64xx-evm:~# gpiofind GPIO_OLED_RESETn
gpiochip0 14
root@am64xx-evm:~# gpioget 0 14
0

Hi Bin，

Thanks for providing the new method. I will test this approach for controlling the GPIO later.
Additionally, if I want to continue using the previous method, is there a way to avoid the issues mentioned above?

Hi Stephen,

Unfortunately I am not aware of any way to prevent kernel dynamically assigning the gpiochip IDs and pin numbers.

Hi Bin,

If I want to call these unique names in a .c file, which header file needs to be included? Which function should be used to operate it? Are there examples of usage in user applications and kernel drivers? Thanks.

Hi Stephen,

gpiofind command is part of the libgpiod package. Please check the libgpiod project source code for examples.