We're working on a custom board using imx8mp processor. In our new design, our hardware engineers as a solution for pin expansion are using some kind of multiplexing/demultiplexing using shift registers; setting one gpio one set of input output becomes available, unsetting it become the latter. For what I could see, this is called runtime pinmuxing. I am using yocto kirkstone, and as far as I could see there is something called runtime pinmuxing via debugfs.
Can something like this could work?
Runtime Pinmuxing
Runtime pinmuxing is a technique for configuring the IOMUXC register set dynamically at runtime. This is useful for applications that need to change the pinmux configuration based on runtime conditions, such as the boot stage or the application's needs.
Complete example of a .dts file that demonstrates how to use runtime pinmuxing with the i.MX 8MP SoC. This example configures the GPIO1_IO03 pin to be used as a GPIO input pin.
DTS
/dts-v1/;
#include "imx8mp-pinfunc.h"
/*
* Set up the IOMUXC register for GPIO1_IO03 to be used as a GPIO input pin.
*/
&iomuxc {
pinctrl_gpio1_io03: gpio1_io03_gpio {
fsl,pins = <
MX8MP_IOMUXC_GPIO1_IO03_GPIO1_IO03 /* GPIO1_IO03 */
>;
};
};
Using Runtime Pinmuxing
To use runtime pinmuxing, you will need to write a driver that sets the IOMUXC registers at runtime. The driver will need to be able to identify the pin that needs to be configured and the desired pinmux setting.
Here is an example of a driver that sets the pinmux configuration for GPIO1_IO03 to be used as a GPIO input pin:
#include <linux/gpio.h>
#include <linux/iomuxc.h>
static int gpio1_io03_pinmux_set(struct gpio_chip *chip, unsigned int offset,
int value)
{
if (value) {
/* Set the IOMUXC register for GPIO1_IO03 to be used as a GPIO input pin. */
iomuxc_set_gpr_register( IOMUXC_GPR1, MX8MP_GPIO1_IOMUXC_GPR1_GPIO1_IO03_GPIO, 1);
} else {
/* Set the IOMUXC register for GPIO1_IO03 to be used as GPIO1_IO03 */
iomuxc_set_gpr_register( IOMUXC_GPR1, MX8MP_GPIO1_IOMUXC_GPR1_GPIO1_IO03_GPIO, 0);
}
return 0;
}
static struct gpio_chip gpio1_io03_chip = {
.label = "gpio1_io03",
.base = GPIO1_IO03,
.ngpio = 1,
.set = gpio1_io03_pinmux_set,
};
static int __init gpio1_io03_init(void)
{
int ret;
ret = gpiochip_add(&gpio1_io03_chip);
if (ret) {
return ret;
}
return 0;
}
arch_initcall(gpio1_io03_init);
Here are the steps on how to create a debugfs file for runtime pinmuxing:
1. Create a debugfs directory:
sudo mkdir /sys/kernel/debug/pinctrl/<controller>
where <controller> is the name of the pinctrl controller you want to use. For example, to create a debugfs directory for the IMX 8MP iomuxc controller, you would use the following command:
sudo mkdir /sys/kernel/debug/pinctrl/imx8mp-iomuxc
2. Create a debugfs file:
sudo touch /sys/kernel/debug/pinctrl/<controller>/pinmux-select
where <controller> is the name of the pinctrl controller you want to use. For example, to create a debugfs file for the IMX 8MP iomuxc controller, you would use the following command:
sudo touch /sys/kernel/debug/pinctrl/imx8mp-iomuxc/pinmux-select
3. Modify the permissions of the debugfs file:
sudo chmod 666 /sys/kernel/debug/pinctrl/<controller>/pinmux-select
where <controller> is the name of the pinctrl controller you want to use. For example, to modify the permissions of the IMX 8MP iomuxc controller's debugfs file, you would use the following command:
sudo chmod 666 /sys/kernel/debug/pinctrl/imx8mp-iomuxc/pinmux-select
4. Create a script to set the pinmux configuration:
#!/bin/bash
pinmux="<pinmux-setting>"
echo "$pinmux" > /sys/kernel/debug/pinctrl/<controller>/pinmux-select
where <pinmux-setting> is the desired pinmux setting. For example, the following script would set the GPIO1_IO03 pin to be used as a GPIO input pin:
#!/bin/bash
pinmux="imx8mp-iomuxc-44:gpio1_io03:1"
echo "$pinmux" > /sys/kernel/debug/pinctrl/imx8mp-iomuxc/pinmux-select
5. Save the script and make it executable:
sudo chmod +x <script-name>
./set-pinmux.sh
