Set maximum SPI speed for MCP23S17

The linux-raspberrypi kernel update to 4.9.43 included a change to the
default reported max speed of the linux spidev, from 100KHz to 125MHz.

kernel: BCM270X_DT: Set spidev spi-max-frequency to 125MHz.
See raspberrypi/linux#2165.

It appears the reported max speed is used as the default if you don't
specify it for the device. The MCP23S17 chip used in the PiFace digital
board can only handle 10MHz but we were using the default so it was not
working after the recent kernel update. This change explicitly sets the
maximum speed after opening the device.

Author: bulletmark

pifaceio.py

@@ -22,11 +22,13 @@
 
 class _SPIdev(object):
     'Class to package 3 byte write + read transfers to spi device'
-    def __init__(self, devname):
+    def __init__(self, devname, speed):
         'Constructor'
         self.fp = open(devname, 'r+b', buffering=0)
         self.fn = self.fp.fileno()
         self.count = 0
+        # Set the max speed
+        fcntl.ioctl(self.fn, 0x40046B04, struct.pack('L', speed))
 
     def write(self, tx):
         'Write given transfer "tx" to device'
@@ -62,7 +64,8 @@ class PiFace(object):
     _spi = {}
 
     def __init__(self, board=0, pull_ups=0xff, read_polarity=0x00,
-            write_polarity=0xff, init_board=True, bus=0, chip=0):
+            write_polarity=0xff, init_board=True, bus=0, chip=0,
+            speed=int(10e6)):
         '''
         PiFace board constructor.
         board: Piface board number = 0 to 7, default = 0.
@@ -71,6 +74,7 @@ def __init__(self, board=0, pull_ups=0xff, read_polarity=0x00,
         init_board: Set False to inhibit board initialisaion. Default is True.
         bus: SPI bus = 0 or 1, default = 0.
         chip: SPI chip select = 0 or 1, default = 0
+        speed: in Hz. Default is MCP23S17 max speed of 10MHz.
 
         Note that bus 0, chip 0 corresponds to /dev/spidev0.0, etc.
 
@@ -86,7 +90,7 @@ def __init__(self, board=0, pull_ups=0xff, read_polarity=0x00,
         self.busaddr = (bus, chip)
         if self.busaddr not in PiFace._spi:
             PiFace._spi[self.busaddr] = \
-                    _SPIdev('/dev/spidev%d.%d' % self.busaddr)
+                    _SPIdev('/dev/spidev%d.%d' % self.busaddr, speed)
 
         self.spi = PiFace._spi[self.busaddr]
         self.spi.count += 1