staging: comedi: usbduxsigma: don't clobber ao_timer in command test

commit c04a1f17 upstream.

`devpriv->ao_timer` is used while an asynchronous command is running on
the AO subdevice.  It also gets modified by the subdevice's `cmdtest`
handler for checking new asynchronous commands,
`usbduxsigma_ao_cmdtest()`, which is not correct as it's allowed to
check new commands while an old command is still running.  Fix it by
moving the code which sets up `devpriv->ao_timer` into the subdevice's
`cmd` handler, `usbduxsigma_ao_cmd()`.

** This backported patch also moves the code that sets up
`devpriv->ao_sample_count` from `usbduxsigma_ao_cmdtest()` to
`usbduxsigma_ao_cmd()` for the same reason as above.  (This was not
needed in the upstream commit.) **

Note that the removed code in `usbduxsigma_ao_cmdtest()` checked that
`devpriv->ao_timer` did not end up less that 1, but that could not
happen due because `cmd->scan_begin_arg` or `cmd->convert_arg` had
already been range-checked.

Also note that we tested the `high_speed` variable in the old code, but
that is currently always 0 and means that we always use "scan" timing
(`cmd->scan_begin_src == TRIG_TIMER` and `cmd->convert_src == TRIG_NOW`)
and never "convert" (individual sample) timing (`cmd->scan_begin_src ==
TRIG_FOLLOW` and `cmd->convert_src == TRIG_TIMER`).  The moved code
tests `cmd->convert_src` instead to decide whether "scan" or "convert"
timing is being used, although currently only "scan" timing is
supported.

Fixes: fb1ef622 ("staging: comedi: usbduxsigma: tidy up analog output command support")
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/staging/comedi/drivers/usbduxsigma.c

@@ -946,11 +946,26 @@ static int usbduxsigma_ao_cmdtest(struct comedi_device *dev,
 	if (err)
 		return 3;
 
-	/* Step 4: fix up any arguments */
+	return 0;
+}
 
-	/* we count in timer steps */
-	if (high_speed) {
-		/* timing of the conversion itself: every 125 us */
+static int usbduxsigma_ao_cmd(struct comedi_device *dev,
+			      struct comedi_subdevice *s)
+{
+	struct usbduxsigma_private *devpriv = dev->private;
+	struct comedi_cmd *cmd = &s->async->cmd;
+	int ret;
+
+	down(&devpriv->sem);
+
+	/* set current channel of the running acquisition to zero */
+	s->async->cur_chan = 0;
+
+	if (cmd->convert_src == TRIG_TIMER) {
+		/*
+		 * timing of the conversion itself: every 125 us
+		 * at high speed (not used yet)
+		 */
 		devpriv->ao_timer = cmd->convert_arg / 125000;
 	} else {
 		/*
@@ -959,12 +974,9 @@ static int usbduxsigma_ao_cmdtest(struct comedi_device *dev,
 		 */
 		devpriv->ao_timer = cmd->scan_begin_arg / 1000000;
 	}
-	if (devpriv->ao_timer < 1)
-		err |= -EINVAL;
-
 	if (cmd->stop_src == TRIG_COUNT) {
 		/* not continuous, use counter */
-		if (high_speed) {
+		if (cmd->convert_src == TRIG_TIMER) {
 			/* high speed also scans everything at once */
 			devpriv->ao_sample_count = cmd->stop_arg *
 						   cmd->scan_end_arg;
@@ -981,24 +993,6 @@ static int usbduxsigma_ao_cmdtest(struct comedi_device *dev,
 		devpriv->ao_sample_count = 0;
 	}
 
-	if (err)
-		return 4;
-
-	return 0;
-}
-
-static int usbduxsigma_ao_cmd(struct comedi_device *dev,
-			      struct comedi_subdevice *s)
-{
-	struct usbduxsigma_private *devpriv = dev->private;
-	struct comedi_cmd *cmd = &s->async->cmd;
-	int ret;
-
-	down(&devpriv->sem);
-
-	/* set current channel of the running acquisition to zero */
-	s->async->cur_chan = 0;
-
 	devpriv->ao_counter = devpriv->ao_timer;
 
 	if (cmd->start_src == TRIG_NOW) {