i2c_gpio: Get actual direction register from device instead internal …

…state

Until now we have tried to infer the state of the output register from an
internal state of the GPIO driver.
But: This means that the assumed state and the actual state can diverge.

With this change we always to a read-modify-write on this register.

Signed-off-by: Chris Fiege <[email protected]>

usbsdmux/i2c_gpio.py

@@ -41,10 +41,6 @@ def __init__(self, sg):
         """
         self._usb = Usb2642I2C(sg)
 
-        # After POR all Pins are Inputs.
-        # This value mirrors the value of the GPIO configuration
-        self._directionMask = 0xFF
-
     def _write_register(self, register, value):
         """
         Writes a register on the Pca9536 with a given value.
@@ -64,8 +60,9 @@ def set_pin_to_output(self, pins):
         Arguments:
         pins -- Combination of Pca9536.gpio_*
         """
-        self._directionMask = self._directionMask & (~pins)
-        self._write_register(self._register_configuration, self._directionMask)
+        direction = self.read_register(self._register_configuration)[0]
+        direction = (direction & ~pins) & 0xFF
+        self._write_register(self._register_configuration, direction)
 
     def set_pin_to_input(self, pins):
         """
@@ -74,8 +71,9 @@ def set_pin_to_input(self, pins):
         Arguments:
         pins -- Combination of Pca9536.gpio_*
         """
-        self._directionMask = self._directionMask | pins
-        self._write_register(self._register_configuration, self._directionMask)
+        direction = self.read_register(self._register_configuration)[0]
+        direction = direction | pins
+        self._write_register(self._register_configuration, direction)
 
     def output_values(self, values: int, bitmask: int = 0xFF):
         """