Invalid path for USR LED when setup as input

[pdp7]

This error occurs for built-in LEDs like USR1. It currently preventing the built-in LEDs from being used in the CircuitPython Blinka library: adafruit/Adafruit_Blinka#96

The error is can be triggered by calling GPIO.setup("USR1", GPIO.IN) instead of GPIO.setup("USR1", GPIO.OUT):

import Adafruit_BBIO.GPIO as GPIO
GPIO.setup("USR1", GPIO.IN)

results in error:

Traceback (most recent call last):
  File "/home/debian/test-usr.py", line 2, in <module>
    GPIO.setup("USR1", GPIO.IN)
ValueError: Set gpio mode failed, missing file or invalid permissions.

and strace shows the bad path:

open("/sys/devices/platform/ocp/ocp:USR01_pinmux/state", O_WRONLY|O_CREAT|O_TRUNC, 0666) = -1 ENOENT (No such file or directory)

While it is the case that INPUT is not valid state for built-in LEDs, this error is caused by accessing the incorrect path for the LED.

The correct path is:

/sys/class/leds/beaglebone:green:usr0/brightness

[pdp7]

For reference, the Linux kernel has a LED subsystem which is different than the GPIO subsystem. Therefore, the built-in LED devices show up under /sys/class/leds/ instead of /sys/class/gpio.

This requires a special case in Adafruit_BBIO to handle the built-in LEDs:
https://github.com/adafruit/adafruit-beaglebone-io-python/blob/master/source/event_gpio.c#L220

    // create file descriptor of value file
    if ((gpio >= USR_LED_GPIO_MIN) && (gpio <=  USR_LED_GPIO_MAX)) {
        snprintf(filename, sizeof(filename), "/sys/class/leds/beaglebone:green:usr%d/brightness", gpio -  USR_LED_GPIO_MIN);
    } else if (beaglebone_blue()) {
        //syslog(LOG_DEBUG, "Adafruit_BBIO: gpio open_value_file: beaglebone_blue() is true\n");
        switch(gpio) {
            case USR_LED_RED:
                snprintf(filename, sizeof(filename), "/sys/class/leds/red/brightness");
                break;
            case USR_LED_GREEN:
                snprintf(filename, sizeof(filename), "/sys/class/leds/green/brightness");
                break;
            case BAT25:
                snprintf(filename, sizeof(filename), "/sys/class/leds/bat25/brightness");
                break;
            case BAT50:
                snprintf(filename, sizeof(filename), "/sys/class/leds/bat50/brightness");
                break;
            case BAT75:
                snprintf(filename, sizeof(filename), "/sys/class/leds/bat75/brightness");
                break;
            case BAT100:
                snprintf(filename, sizeof(filename), "/sys/class/leds/bat100/brightness");
                break;
            case WIFI:
                snprintf(filename, sizeof(filename), "/sys/class/leds/wifi/brightness");
                break;
            default:
                snprintf(filename, sizeof(filename), "/sys/class/gpio/gpio%d/value", gpio);
                break;
        }
    } else {
        //syslog(LOG_DEBUG, "Adafruit_BBIO: gpio open_value_file: default gpio path\n");
        snprintf(filename, sizeof(filename), "/sys/class/gpio/gpio%d/value", gpio);
    }

[pdp7]

The above commit fixes adafruit/Adafruit_Blinka#96

This Blinka example now runs OK:

import board
import digitalio
import busio
import time

led = digitalio.DigitalInOut(board.LED_USR0)
led.direction = digitalio.Direction.OUTPUT


while True:
    led.value = True
    time.sleep(0.5)
    led.value = False
    time.sleep(0.5)