i2c_gpio: Extend output_values to only update part of the register

With this change we are able to update only part of the output pins
of a I2C port expander.

This will be needed when we want to preserve some state in the USB-SD-Mux.
For example: When updating the multiplexer we want to keep the state of the
outputs.

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

usbsdmux/i2c_gpio.py

@@ -77,15 +77,25 @@ def set_pin_to_input(self, pins):
         self._directionMask = self._directionMask | pins
         self._write_register(self._register_configuration, self._directionMask)
 
-    def output_values(self, values):
+    def output_values(self, values: int, bitmask: int = 0xFF):
         """
         Writes the given values to the GPIO-expander.
         Pins configured as Inputs are not affected by this.
 
         Arguments:
         values -- Combination of Pca9536.gpio_*
+        bitmask -- Only update bits in the register that are '1' in the bitmask
         """
-        self._write_register(self._register_outputPort, values)
+
+        if bitmask == 0xFF:
+            # trivial case: Let's just write the value
+            self._write_register(self._register_outputPort, values)
+        else:
+            # complex case: Let's do a read-modify-write
+            val = self.read_register(self.register_outputPort, 1)[0]
+            val = (val & ~bitmask) & 0xFF  # reset masked bits
+            val = val | (values & bitmask)  # set bits set in values and bitmask
+            self._write_register(self.register_outputPort, val)
 
 
 class Pca9536(I2cGpio):