staging: comedi: usbduxfast: TRIG_EXT is only valid for cmd->start_src

According to the code, and verified with the author Bernd Porr, the
TRIG_EXT trigger is only valid for the cmd->start_src. Remove this
trigger from the cmd->scan_begin_src and cmd->convert_src.

The only valid scan_begin_src is then TRIG_FOLLOW  and the convert_src
is only TRIG_TIMER. Remove the redundant unique trigger checks in Step
2b of the (*do_cmdtest) as well as any other unnecessary checks of
those trigger sources.

Also remove the incorrect comment for a cmd->stop_src of TRIG_EXT.
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/usbduxfast.c

@@ -366,10 +366,8 @@ static int usbduxfast_ai_cmdtest(struct comedi_device *dev,
 
 	err |= comedi_check_trigger_src(&cmd->start_src,
 					TRIG_NOW | TRIG_EXT | TRIG_INT);
-	err |= comedi_check_trigger_src(&cmd->scan_begin_src,
-					TRIG_FOLLOW | TRIG_EXT);
-	err |= comedi_check_trigger_src(&cmd->convert_src,
-					TRIG_TIMER | TRIG_EXT);
+	err |= comedi_check_trigger_src(&cmd->scan_begin_src, TRIG_FOLLOW);
+	err |= comedi_check_trigger_src(&cmd->convert_src, TRIG_TIMER);
 	err |= comedi_check_trigger_src(&cmd->scan_end_src, TRIG_COUNT);
 	err |= comedi_check_trigger_src(&cmd->stop_src, TRIG_COUNT | TRIG_NONE);
 
@@ -379,16 +377,10 @@ static int usbduxfast_ai_cmdtest(struct comedi_device *dev,
 	/* Step 2a : make sure trigger sources are unique */
 
 	err |= comedi_check_trigger_is_unique(cmd->start_src);
-	err |= comedi_check_trigger_is_unique(cmd->scan_begin_src);
-	err |= comedi_check_trigger_is_unique(cmd->convert_src);
 	err |= comedi_check_trigger_is_unique(cmd->stop_src);
 
 	/* Step 2b : and mutually compatible */
 
-	/* can't have external stop and start triggers at once */
-	if (cmd->start_src == TRIG_EXT && cmd->stop_src == TRIG_EXT)
-		err |= -EINVAL;
-
 	if (err)
 		return 2;
 
@@ -412,34 +404,21 @@ static int usbduxfast_ai_cmdtest(struct comedi_device *dev,
 	else
 		min_sample_period = MIN_SAMPLING_PERIOD;
 
-	if (cmd->convert_src == TRIG_TIMER) {
-		steps = cmd->convert_arg * 30;
-		if (steps < (min_sample_period * 1000))
-			steps = min_sample_period * 1000;
+	steps = cmd->convert_arg * 30;
+	if (steps < (min_sample_period * 1000))
+		steps = min_sample_period * 1000;
 
-		if (steps > (MAX_SAMPLING_PERIOD * 1000))
-			steps = MAX_SAMPLING_PERIOD * 1000;
+	if (steps > (MAX_SAMPLING_PERIOD * 1000))
+		steps = MAX_SAMPLING_PERIOD * 1000;
 
-		/* calc arg again */
-		tmp = steps / 30;
-		err |= comedi_check_trigger_arg_is(&cmd->convert_arg, tmp);
-	}
+	/* calc arg again */
+	tmp = steps / 30;
+	err |= comedi_check_trigger_arg_is(&cmd->convert_arg, tmp);
 
-	/* stop source */
-	switch (cmd->stop_src) {
-	case TRIG_COUNT:
+	if (cmd->stop_src == TRIG_COUNT)
 		err |= comedi_check_trigger_arg_min(&cmd->stop_arg, 1);
-		break;
-	case TRIG_NONE:
+	else	/* TRIG_NONE */
 		err |= comedi_check_trigger_arg_is(&cmd->stop_arg, 0);
-		break;
-		/*
-		 * TRIG_EXT doesn't care since it doesn't trigger
-		 * off a numbered channel
-		 */
-	default:
-		break;
-	}
 
 	if (err)
 		return 3;
@@ -508,9 +487,7 @@ static int usbduxfast_ai_cmd(struct comedi_device *dev,
 	 */
 	devpriv->ignore = PACKETS_TO_IGNORE;
 
-	steps = 0;
-	if (cmd->convert_src == TRIG_TIMER)
-		steps = (cmd->convert_arg * 30) / 1000;
+	steps = (cmd->convert_arg * 30) / 1000;
 
 	if ((steps < MIN_SAMPLING_PERIOD) && (cmd->chanlist_len != 1)) {
 		dev_err(dev->class_dev,