add write_then_readinto support for linux (and possibly others!)

Author: ladyada

adafruit_bus_device/i2c_device.py

@@ -101,6 +101,41 @@ def write(self, buf, **kwargs):
  """
  self.i2c.writeto(self.device_address, buf, **kwargs)
 
+ def write_then_readinto(self, out_buffer, in_buffer,
+ out_start=0, out_end=None, in_start=0, in_end=None, stop=True):
+ """
+ Write the bytes from ``out_buffer`` to the device, then immediately
+ reads into ``in_buffer`` from the device. The number of bytes read
+ will be the length of ``in_buffer``.
+ Transmits a stop bit after the write, if ``stop`` is set.
+
+ If ``out_start`` or ``out_end`` is provided, then the output buffer
+ will be sliced as if ``out_buffer[out_start:out_end]``. This will
+ not cause an allocation like ``buffer[out_start:out_end]`` will so
+ it saves memory.
+
+ If ``in_start`` or ``in_end`` is provided, then the input buffer
+ will be sliced as if ``in_buffer[in_start:in_end]``. This will not
+ cause an allocation like ``in_buffer[in_start:in_end]`` will so
+ it saves memory.
+
+ :param bytearray out_buffer: buffer containing the bytes to write
+ :param bytearray in_buffer: buffer containing the bytes to read into
+ :param int out_start: Index to start writing from
+ :param int out_end: Index to read up to but not include
+ :param int in_start: Index to start writing at
+ :param int in_end: Index to write up to but not include
+ :param bool stop: If true, output an I2C stop condition after the buffer is written
+ """
+ if hasattr(self.i2c, 'writeto_then_readfrom'):
+ # In linux, at least, this is a special kernel function call
+ self.i2c.writeto_then_readfrom(self.device_address, out_buffer, in_buffer,
+ out_start, out_end, in_start, in_end, stop)
+ else:
+ # If we don't have a special implementation, we can fake it with two calls
+ self.write(out_buffer, start=out_start, end=out_end, stop=stop)
+ self.readinto(in_buffer, start=in_start, end=in_end)
+
  def __enter__(self):
  while not self.i2c.try_lock():
  pass