staging: comedi: comedi_test: move modulo operations for waveform

The fake waveform generator functions, `fake_sawtooth()` and
`fake_squarewave()`, called from `fake_waveform()`, have a
`current_time` parameter which is the time since the start of a waveform
period.  The parameter value may be greater than the waveform period so
they do a modulo operation to bring it into range.  Do the modulo
operations outside the functions in `waveform_ai_interrupt()` so that
the waveform generator functions always get a `current_time` parameter
less than the waveform period and do not have to do the modulo operation
themselves.  Also, only do the modulo operations when the time since the
start of a waveform exceeds the waveform period.  Usually, several
samples are produced in each waveform period and modulo operations are
typically more expensive than a simple comparison.
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

@@ -97,7 +97,6 @@ static unsigned short fake_sawtooth(struct comedi_device *dev,
 	binary_amplitude *= devpriv->uvolt_amplitude;
 	do_div(binary_amplitude, krange->max - krange->min);
 
-	current_time %= devpriv->usec_period;
 	value = current_time;
 	value *= binary_amplitude * 2;
 	do_div(value, devpriv->usec_period);
@@ -125,7 +124,6 @@ static unsigned short fake_squarewave(struct comedi_device *dev,
 	const struct comedi_krange *krange =
 	    &s->range_table->range[range_index];
 
-	current_time %= devpriv->usec_period;
 	value = s->maxdata;
 	value *= devpriv->uvolt_amplitude;
 	do_div(value, krange->max - krange->min);
@@ -206,19 +204,23 @@ static void waveform_ai_interrupt(unsigned long arg)
 
 	num_scans = comedi_nscans_left(s, num_scans);
 	for (i = 0; i < num_scans; i++) {
+		unsigned long scan_remain_period = devpriv->scan_period;
+
 		for (j = 0; j < cmd->chanlist_len; j++) {
 			unsigned short sample;
 
+			if (devpriv->usec_current >= devpriv->usec_period)
+				devpriv->usec_current %= devpriv->usec_period;
 			sample = fake_waveform(dev, CR_CHAN(cmd->chanlist[j]),
 					       CR_RANGE(cmd->chanlist[j]),
-					       devpriv->usec_current +
-						   i * devpriv->scan_period +
-						   j * devpriv->convert_period);
+					       devpriv->usec_current);
 			comedi_buf_write_samples(s, &sample, 1);
+			devpriv->usec_current += devpriv->convert_period;
+			scan_remain_period -= devpriv->convert_period;
 		}
+		devpriv->usec_current += scan_remain_period;
 	}
-
-	devpriv->usec_current += elapsed_time;
+	if (devpriv->usec_current >= devpriv->usec_period)
 		devpriv->usec_current %= devpriv->usec_period;
 
 	if (cmd->stop_src == TRIG_COUNT && async->scans_done >= cmd->stop_arg)