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