serialhdl: Clear "hupcl" bit from serial port

The arduino style serial port interfaces can reset the MCU when the
serial port is opened.  Clearing the HUPCL flag makes this less
likely.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/serialhdl.py

@@ -6,7 +6,7 @@
 import time, logging, threading
 import serial
 
-import msgproto, chelper
+import msgproto, chelper, util
 
 class SerialReader:
     BITS_PER_BYTE = 10.
@@ -298,6 +298,7 @@ class SerialBootStrap:
 # Attempt to place an AVR stk500v2 style programmer into normal mode
 def stk500v2_leave(ser, reactor):
     logging.debug("Starting stk500v2 leave programmer sequence")
+    util.clear_hupcl(ser.fileno())
     origbaud = ser.baudrate
     # Request a dummy speed first as this seems to help reset the port
     ser.baudrate = 2400

klippy/util.py

@@ -16,6 +16,12 @@ def set_nonblock(fd):
     fcntl.fcntl(fd, fcntl.F_SETFL
                 , fcntl.fcntl(fd, fcntl.F_GETFL) | os.O_NONBLOCK)
 
+# Clear HUPCL flag
+def clear_hupcl(fd):
+    attrs = termios.tcgetattr(fd)
+    attrs[2] = attrs[2] & ~termios.HUPCL
+    termios.tcsetattr(fd, termios.TCSADRAIN, attrs)
+
 # Support for creating a pseudo-tty for emulating a serial port
 def create_pty(ptyname):
     mfd, sfd = pty.openpty()