staging: comedi: comedi_test: saturate fake waveform values

While an asynchronous command is running on the analog input subdevice,
fake waveform generators are connected to each channel to provide the
input voltages that are converted to sample value.  Channel 0 is
connected to a sawtooth generator, channel 1 is connected to a
squarewave generator, and the remaining channels are connected to a
flatline generator.  The non-flatline generators share the same
amplitude (in microvolts) and period (in microseconds) which are
configured when the COMEDI device is attached.  All waveforms are
centered around 0 microvolts and the non-flatline waveforms go between
-amplitude and +amplitude.

It is possible for the waveforms to swing outside the input range of the
channels to which they are connected.  When that happens, the sample
values resulting from simulated A-to-D conversion will wrap around due
to integer overflow.  Prevent that by clamping the sample values that
would go out of range.  This is closer to how a real hardware device
would behave (assuming the input voltage is not high enough to damage
the hardware!).
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/comedi_test.c

@@ -104,9 +104,17 @@ static unsigned short fake_sawtooth(struct comedi_device *dev,
 	value = current_time;
 	value *= binary_amplitude * 2;
 	do_div(value, devpriv->usec_period);
-	value -= binary_amplitude;	/* get rid of sawtooth's dc offset */
+	value += offset;
+	/* get rid of sawtooth's dc offset and clamp value */
+	if (value < binary_amplitude) {
+		value = 0;			/* negative saturation */
+	} else {
+		value -= binary_amplitude;
+		if (value > s->maxdata)
+			value = s->maxdata;	/* positive saturation */
+	}
 
-	return offset + value;
+	return value;
 }
 
 static unsigned short fake_squarewave(struct comedi_device *dev,
@@ -119,16 +127,25 @@ static unsigned short fake_squarewave(struct comedi_device *dev,
 	u64 value;
 	const struct comedi_krange *krange =
 	    &s->range_table->range[range_index];
-	current_time %= devpriv->usec_period;
 
+	current_time %= devpriv->usec_period;
 	value = s->maxdata;
 	value *= devpriv->uvolt_amplitude;
 	do_div(value, krange->max - krange->min);
 
-	if (current_time < devpriv->usec_period / 2)
-		value *= -1;
+	/* get one of two values for square-wave and clamp */
+	if (current_time < devpriv->usec_period / 2) {
+		if (offset < value)
+			value = 0;		/* negative saturation */
+		else
+			value = offset - value;
+	} else {
+		value += offset;
+		if (value > s->maxdata)
+			value = s->maxdata;	/* positive saturation */
+	}
 
-	return offset + value;
+	return value;
 }
 
 static unsigned short fake_flatline(struct comedi_device *dev,