Use ioctl for half-duplex writes

Might as well use the ioctl for writes as well. Allows for more
specifications (speed, bits per word, delay) and avoids any buffering
and flushing issues with normal writes to the spidev character device.

Author: tomstokes

spi.py

@@ -362,14 +362,25 @@ def mode(self):
     def mode(self, mode):
         self._set_mode(mode)
 
-    def write(self, data):
+    def write(self, data, speed=0, bits_per_word=0, delay=0):
         """Perform half-duplex SPI write.
 
         Args:
             data: Binary string of data to write
+            speed: Optional temporary bitrate override in Hz. 0 (default)
+                uses existing spidev speed setting.
+            bits_per_word: Optional temporary bits_per_word override. 0 (
+                default) is equivalent to 8 bits per word.
+            delay: Optional delay in usecs between sending the last bit and
+                deselecting the chip select line. 0 (default) for no delay.
         """
-        self.handle.write(data)
-        self.handle.flush()
+        length = len(data)
+        transmit_buffer = ctypes.create_string_buffer(str(data))
+        spi_ioc_transfer = struct.pack(SPI._IOC_TRANSFER_FORMAT,
+                                       ctypes.addressof(transmit_buffer), 0,
+                                       length, speed, delay, bits_per_word, 0,
+                                       0, 0, 0)
+        fcntl.ioctl(self.handle, SPI._IOC_MESSAGE, spi_ioc_transfer)
 
     def read(self, length, speed=0, bits_per_word=0, delay=0):
         """Perform half-duplex SPI read