How can I write or read from a specific GPIO pin in from a kernel-module?

Question

I'm using a Raspberry Pi 4 with standard 32 bit Raspbian. I'm trying to write a driver in c that writes AT commands to some GPIO pins and reads the response from other pins. For that I want to use the open() and write() commands from Linux onto a character device f.e. /dev/gpio or similar.

However these devices do not exist, all I could find so far was /dev/gpiochip0 and /dev/gpiochip1 but no gpiohip2 or beyond. I've looked for some source codes which seems to suggest that a character device for each pin should exist (or did exist in the past...), but no matter where (/dev/, /sys/class/gpio/, /sys/bus/gpio/devices/) I look there are a maximum of two pins represented as char devices.

One of the codes that I found was: https://elinux.org/RPi_GPIO_Code_Samples

snprintf(path, DIRECTION_MAX, "/sys/class/gpio/gpio%d/direction", pin);
fd = open(path, O_WRONLY);

where the path on my Raspbian only contains a link to "gpiochip0" and "gpiochip504" folders, neither of which contain a file called direction.

I'm guessing that either I have to activate something to make the char devices appear or use a different access method for each pin. How can I write or read from a specific GPIO pin in from a kernel-module?

Answer 1

I think you have misinterpreted what you are seeing.

On the Pi /dev/gpiochip0 is the main GPIO block of 54 GPIO and corresponds to Broadcom GPIO 0 to 53. This includes all the GPIO you can access from the extension header.

Pi400 /dev/gpiochip0

lines=54 name=gpiochip0 label=pinctrl-bcm2835
 
offset=0 flags=0 name=ID_SDA user=
offset=1 flags=0 name=ID_SCL user=
offset=2 flags=0 name=SDA1 user=
offset=3 flags=0 name=SCL1 user=
offset=4 flags=0 name=GPIO_GCLK user=
offset=5 flags=0 name=GPIO5 user=
offset=6 flags=0 name=GPIO6 user=
offset=7 flags=7 name=SPI_CE1_N user=spi0 CS1
offset=8 flags=7 name=SPI_CE0_N user=spi0 CS0
offset=9 flags=0 name=SPI_MISO user=
offset=10 flags=0 name=SPI_MOSI user=
offset=11 flags=0 name=SPI_SCLK user=
offset=12 flags=0 name=GPIO12 user=
offset=13 flags=0 name=GPIO13 user=
offset=14 flags=0 name=TXD1 user=
offset=15 flags=0 name=RXD1 user=
offset=16 flags=0 name=GPIO16 user=
offset=17 flags=3 name=GPIO17 user=gpio-fan@0
offset=18 flags=0 name=GPIO18 user=
offset=19 flags=0 name=GPIO19 user=
offset=20 flags=0 name=GPIO20 user=
offset=21 flags=0 name=GPIO21 user=
offset=22 flags=0 name=GPIO22 user=
offset=23 flags=0 name=GPIO23 user=
offset=24 flags=0 name=GPIO24 user=
offset=25 flags=0 name=GPIO25 user=
offset=26 flags=0 name=GPIO26 user=
offset=27 flags=0 name=GPIO27 user=
offset=28 flags=0 name=RGMII_MDIO user=
offset=29 flags=0 name=RGMIO_MDC user=
offset=30 flags=0 name=CTS0 user=
offset=31 flags=0 name=RTS0 user=
offset=32 flags=0 name=TXD0 user=
offset=33 flags=0 name=RXD0 user=
offset=34 flags=0 name=SD1_CLK user=
offset=35 flags=0 name=SD1_CMD user=
offset=36 flags=0 name=SD1_DATA0 user=
offset=37 flags=0 name=SD1_DATA1 user=
offset=38 flags=0 name=SD1_DATA2 user=
offset=39 flags=0 name=SD1_DATA3 user=
offset=40 flags=0 name=PWM0_MISO user=
offset=41 flags=0 name=PWM1_MOSI user=
offset=42 flags=3 name=STATUS_LED_G_CLK user=led0
offset=43 flags=0 name=SPIFLASH_CE_N user=
offset=44 flags=0 name=SDA0 user=
offset=45 flags=0 name=SCL0 user=
offset=46 flags=0 name=RGMII_RXCLK user=
offset=47 flags=0 name=RGMII_RXCTL user=
offset=48 flags=0 name=RGMII_RXD0 user=
offset=49 flags=0 name=RGMII_RXD1 user=
offset=50 flags=0 name=RGMII_RXD2 user=
offset=51 flags=0 name=RGMII_RXD3 user=
offset=52 flags=0 name=RGMII_TXCLK user=
offset=53 flags=0 name=RGMII_TXCTL user=

/dev/gpiochip1 is a secondary block of GPIO.

Pi400 /dev/gpiochip1

lines=8 name=gpiochip1 label=raspberrypi-exp-gpio

offset=0 flags=2 name=BT_ON user=
offset=1 flags=2 name=WL_ON user=
offset=2 flags=7 name=PWR_LED_OFF user=led1
offset=3 flags=2 name=GLOBAL_RESET user=
offset=4 flags=3 name=VDD_SD_IO_SEL user=vdd-sd-io
offset=5 flags=3 name=CAM_GPIO user=power_ctrl
offset=6 flags=3 name=SD_PWR_ON user=sd_vcc_reg
offset=7 flags=0 name=SD_OC_N user=

Answer 2

After searching endlessly to control a GPIO pin on Raspberry Pi 4 (cm4 module) from a kernel driver module (with kernel version: 5.19.17), I came up with the following simple solution directly accessing gpiolib:-

static int is_right_chip(struct gpio_chip *chip, void *data)
{
    if (strcmp(data, chip->label) == 0)
            return 1;
    return 0;
}


void waggleGPIO(void) 
{
    // point to gpiochip0.. use chip name given by userspace gpio utility: gpiodetect (in this example: pinctrl-bcm2711)
    struct gpio_chip *chip = gpiochip_find("pinctrl-bcm2711", is_right_chip);
    if (!chip) {
        printk("FAIL to file pinctrl-bcm2711 gpiochip0!\n");    
    }
    else {
        printk("GOOD..found gpiochip0\n");
        // using userspace utility: gpioinfo, decide which line you want to read or write.. in this example gpio24 (gpiochip0 index 24)
        chip->direction_output(chip, 24, 0);  //set direction to output and drive pin low... this calls bcm2835_gpio_direction_output()
        chip->set(chip, 24, 1);  // this drives gpio24 to a high level
        chip->set(chip, 24, 0);  // this drives gpio24 to a low level
    }
}

Similar gpiolib functions can be used to read the state of the GPIO line using:-

int gpio24Value = chip->direction_input(chip, 24);
gpio24Value = chip->get(chip, 24);

Note that this example does poke around with the internals of gpiolib.

I hope this helps someone!

tags: drive GPIO in Linux kernel

Answer 3

The interface described here no longer works on Raspberry Pi OS bookworm.

echo 20 > /sys/class/gpio/export 
-bash: echo: write error: Invalid argument