commaai/openpilot · error · SerialException

could not configure port: {e}

Error message

could not configure port: {e}

What it means

Raised by Serial._configure when termios.tcsetattr fails while applying the raw 8N1 mode, baud constant, and VMIN/VTIME=0 settings. The kernel rejects the requested attribute set (typically EINVAL) because the combination is not supported by this tty driver.

Source

Thrown at openpilot/common/serial.py:264

    cflag |= termios.CS8
    cflag &= ~(termios.PARENB | termios.PARODD | termios.CSTOPB)

    if hasattr(termios, "CRTSCTS"):
      if self._rtscts:
        cflag |= termios.CRTSCTS
      else:
        cflag &= ~termios.CRTSCTS

    speed = self._baud_constant(self._baudrate)
    cc = list(cc)
    # Non-blocking reads are handled via select + O_NONBLOCK; keep VMIN/VTIME at 0.
    cc[termios.VMIN] = 0
    cc[termios.VTIME] = 0

    try:
      termios.tcsetattr(self._fd, termios.TCSANOW, [iflag, oflag, cflag, lflag, speed, speed, cc])
    except termios.error as e:
      raise SerialException(f"could not configure port: {e}") from e

    # Keep the fd non-blocking so timeout=0 and select work consistently.
    flags = fcntl.fcntl(self._fd, fcntl.F_GETFL)
    fcntl.fcntl(self._fd, fcntl.F_SETFL, flags | os.O_NONBLOCK)

  def _set_line(self, _bit: int, packed: bytes, enabled: bool) -> None:
    request = TIOCMBIS if enabled else TIOCMBIC
    fcntl.ioctl(self._fd, request, packed)

View on GitHub (pinned to 516ec1e682)

Solutions

1. Try a standard baud rate (9600/115200) to confirm the port works at all
2. Disable flow control in the Serial constructor if the adapter lacks RTS/CTS lines
3. Confirm the device is still attached (dmesg) and reopen

Example fix

// before
ser = Serial(devpath, 921600, xonxoff=False, rtscts=True)

// after
ser = Serial(devpath, 115200, rtscts=False)

import termios

def baud_supported(baudrate: int) -> bool:
    return hasattr(termios, f'B{baudrate}')

from common.serial import SerialException
try:
    ser = Serial(devpath, baudrate, rtscts=True)
except SerialException as e:
    ser = Serial(devpath, 115200, rtscts=False)  # fall back to safe settings

When it happens

Trigger: Passing a baud rate whose B<rate> constant exists but the driver cannot set (some USB CDC adapters only support standard rates), enabling CRTSCTS on a 2-wire adapter without RTS/CTS, or the device disconnecting between tcgetattr and tcsetattr.

Common situations: Nonstandard baud rates (e.g., 500000 or 921600) on cheap USB-serial chips, flow-control mismatch, or a disappearing device mid-open.

Related errors

• could not get port attributes: {e}
• AT command timed out
• AT command failed: {line}
• Failed to open ISD-R after retries
• Missing +CGLA response

AI-assisted analysis of commaai/openpilot@516ec1e682 (2026-08-15). Data as JSON: /api/errors/9e080e5338ddd905. Report an issue: GitHub.