• Platform/operating system: Beaglebone Black rev C, BeagleBoard.org Debian Image 2018-01-28, no capes.

• Python version (run python -version or python3 -version): Python 2.7.13

• Error message you are receiving, including any Python exception traces:

Our MWE:

 import Adafruit_BBIO.PWM as PWM
 PWM.start("P9_14",0,10000)

on the first run after reboot, would fail with:

 RuntimeError: Problem with a sysfs file

The second (and all subsequent) runs would work.

Here is the relevant sequence in strace:

stat64("/sys/devices/platform/ocp/48302000.epwmss/48302200.pwm/pwm/pwmchip3/pwm0", 0xbe8dcd28) = -1 ENOENT (No such file or directory)
stat64("/sys/devices/platform/ocp/48302000.epwmss/48302200.pwm/pwm/pwmchip3/pwm-3:0", {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
open("/sys/devices/platform/ocp/48302000.epwmss/48302200.pwm/pwm/pwmchip3/pwm-3:0/period", O_RDWR) = -1 EACCES (Permission denied)

The code is failing the stat on line 435, going successfully through the stat in line 441 (middle line), then failing at line 515.

I wondered if there was a delay in udev's operation, I added a 100ms delay (pull request pending) which resolved this issue. I don't think this is the best solution, but it hints at a solution.