Add require_repeated_start function to I2C as workaround for I2C issue on Raspberry Pi.

Author: tdicola

Adafruit_GPIO/I2C.py

@@ -19,8 +19,8 @@
 # LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 # THE SOFTWARE.
-
 import logging
+import subprocess
 
 import smbus
 
@@ -65,6 +65,24 @@ def get_i2c_device(address, busnum=None, **kwargs):
         busnum = get_default_bus()
     return Device(address, busnum, **kwargs)
 
+def require_repeated_start():
+    """Enable repeated start conditions for I2C register reads.  This is the
+    normal behavior for I2C, however on some platforms like the Raspberry Pi
+    there are bugs which disable repeated starts unless explicitly enabled with
+    this function.  See this thread for more details:
+      http://www.raspberrypi.org/forums/viewtopic.php?f=44&t=15840
+    """
+    plat = Platform.platform_detect()
+    if plat == Platform.RASPBERRY_PI:
+        # On the Raspberry Pi there is a bug where register reads don't send a
+        # repeated start condition like the kernel smbus I2C driver functions
+        # define.  As a workaround this bit in the BCM2708 driver sysfs tree can
+        # be changed to enable I2C repeated starts.
+        subprocess.check_call('chmod 666 /sys/module/i2c_bcm2708/parameters/combined', shell=True)
+        subprocess.check_call('echo -n 1 > /sys/module/i2c_bcm2708/parameters/combined', shell=True)
+    # Other platforms are a no-op because they (presumably) have the correct
+    # behavior and send repeated starts.
+
 
 class Device(object):
     """Class for communicating with an I2C device using the smbus library.