staging: comedi: adq12b: fix ctreg (ai channel/range) programming

This driver only updates the ctreg (ai channel/range) register if the
desired channel or range has changed since the last analog input read
operation. It does this becuase the hardware requires an udelay to
allow the multiplexor to settle. Unfortunatly the current code never
updates the 'last_channel' and 'last_range' in the private data so
the ctreg gets updated every time.

Fix this and simplify it a bit by just storing the last ctreg value
in the private data.
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/adq12b.c

@@ -116,8 +116,7 @@ static const struct comedi_lrange range_adq12b_ai_unipolar = {
 struct adq12b_private {
 	int unipolar;		/* option 2 of comedi_config (1 is iobase) */
 	int differential;	/* option 3 of comedi_config */
-	int last_channel;
-	int last_range;
+	unsigned int last_ctreg;
 };
 
 static int adq12b_ai_eoc(struct comedi_device *dev,
@@ -134,20 +133,23 @@ static int adq12b_ai_eoc(struct comedi_device *dev,
 }
 
 static int adq12b_ai_rinsn(struct comedi_device *dev,
-			   struct comedi_subdevice *s, struct comedi_insn *insn,
+			   struct comedi_subdevice *s,
+			   struct comedi_insn *insn,
 			   unsigned int *data)
 {
 	struct adq12b_private *devpriv = dev->private;
+	unsigned int chan = CR_CHAN(insn->chanspec);
+	unsigned int range = CR_RANGE(insn->chanspec);
+	unsigned int val;
 	int n;
-	int range, channel;
 	unsigned char hi, lo, status;
 	int ret;
 
 	/* change channel and range only if it is different from the previous */
-	range = CR_RANGE(insn->chanspec);
-	channel = CR_CHAN(insn->chanspec);
-	if (channel != devpriv->last_channel || range != devpriv->last_range) {
-		outb((range << 4) | channel, dev->iobase + ADQ12B_CTREG);
+	val = (range << 4) | chan;
+	if (val != devpriv->last_ctreg) {
+		outb(val, dev->iobase + ADQ12B_CTREG);
+		devpriv->last_ctreg = val;
 		udelay(50);	/* wait for the mux to settle */
 	}
 
@@ -226,12 +228,7 @@ static int adq12b_attach(struct comedi_device *dev, struct comedi_devconfig *it)
 
 	devpriv->unipolar = it->options[1];
 	devpriv->differential = it->options[2];
-	/*
-	 * initialize channel and range to -1 so we make sure we
-	 * always write at least once to the CTREG in the instruction
-	 */
-	devpriv->last_channel = -1;
-	devpriv->last_range = -1;
+	devpriv->last_ctreg = -1;	/* force ctreg update */
 
 	ret = comedi_alloc_subdevices(dev, 3);
 	if (ret)