staging: comedi: das1800: clarify step 4 of the ai (*do_cmdtest)

This step of the (*do_cmdtest) fixes the timing arguments when the
cmd->convert_src is TRIG_TIMER. The code is compact but it's a bit
hard to understand the "burst mode" vs. "non burst mode" (i.e. paced)
timing.

Clarify the fixup by splitting it into separate functions to check
the arguments based on if the command is doing 'paced' or 'burst'
timing. Add some comments to clarify the fixups.

This also fixes a minor issue with the 'burst' timing. When the
cmd->scan_begin_src is also TRIG_TIMER the pacer clock is used to
start each scan. The cmd->scan_begin_arg specifies the scan time.
The minimum (not maximum) value for this time must be large enough
for the 'burst' conversion time to sample all the channels.

The current 'max' check limits the scan time to the minimum time
(i.e. the max conversion speed).
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/das1800.c

@@ -544,33 +544,73 @@ static irqreturn_t das1800_interrupt(int irq, void *d)
 	return IRQ_HANDLED;
 }
 
-/* converts requested conversion timing to timing compatible with
- * hardware, used only when card is in 'burst mode'
- */
-static unsigned int burst_convert_arg(unsigned int convert_arg, int flags)
+static int das1800_ai_fixup_paced_timing(struct comedi_device *dev,
+					 struct comedi_cmd *cmd)
 {
-	unsigned int micro_sec;
+	unsigned int arg = cmd->convert_arg;
+
+	/*
+	 * Paced mode:
+	 *	scan_begin_src is TRIG_FOLLOW
+	 *	convert_src is TRIG_TIMER
+	 *
+	 * The convert_arg sets the pacer sample acquisition time.
+	 * The max acquisition speed is limited to the boards
+	 * 'ai_speed' (this was already verified). The min speed is
+	 * limited by the cascaded 8254 timer.
+	 */
+	comedi_8254_cascade_ns_to_timer(dev->pacer, &arg, cmd->flags);
+	return comedi_check_trigger_arg_is(&cmd->convert_arg, arg);
+}
 
-	/*  in burst mode, the maximum conversion time is 64 microseconds */
-	if (convert_arg > 64000)
-		convert_arg = 64000;
+static int das1800_ai_fixup_burst_timing(struct comedi_device *dev,
+					 struct comedi_cmd *cmd)
+{
+	unsigned int arg = cmd->convert_arg;
+	int err = 0;
+
+	/*
+	 * Burst mode:
+	 *	scan_begin_src is TRIG_TIMER or TRIG_EXT
+	 *	convert_src is TRIG_TIMER
+	 *
+	 * The convert_arg sets burst sample acquisition time.
+	 * The max acquisition speed is limited to the boards
+	 * 'ai_speed' (this was already verified). The min speed is
+	 * limiited to 64 microseconds,
+	 */
+	err |= comedi_check_trigger_arg_max(&arg, 64000);
 
-	/*  the conversion time must be an integral number of microseconds */
-	switch (flags & CMDF_ROUND_MASK) {
+	/* round to microseconds then verify */
+	switch (cmd->flags & CMDF_ROUND_MASK) {
 	case CMDF_ROUND_NEAREST:
 	default:
-		micro_sec = (convert_arg + 500) / 1000;
+		arg = DIV_ROUND_CLOSEST(arg, 1000);
 		break;
 	case CMDF_ROUND_DOWN:
-		micro_sec = convert_arg / 1000;
+		arg = arg / 1000;
 		break;
 	case CMDF_ROUND_UP:
-		micro_sec = (convert_arg - 1) / 1000 + 1;
+		arg = DIV_ROUND_UP(arg, 1000);
 		break;
 	}
+	err |= comedi_check_trigger_arg_is(&cmd->convert_arg, arg * 1000);
+
+	/*
+	 * The pacer can be used to set the scan sample rate. The max scan
+	 * speed is limited by the conversion speed and the number of channels
+	 * to convert. The min speed is limited by the cascaded 8254 timer.
+	 */
+	if (cmd->scan_begin_src == TRIG_TIMER) {
+		arg = cmd->convert_arg * cmd->chanlist_len;
+		err |= comedi_check_trigger_arg_min(&cmd->scan_begin_arg, arg);
 
-	/*  return number of nanoseconds */
-	return micro_sec * 1000;
+		arg = cmd->scan_begin_arg;
+		comedi_8254_cascade_ns_to_timer(dev->pacer, &arg, cmd->flags);
+		err |= comedi_check_trigger_arg_is(&cmd->scan_begin_arg, arg);
+	}
+
+	return err;
 }
 
 static int das1800_ai_check_chanlist(struct comedi_device *dev,
@@ -600,7 +640,6 @@ static int das1800_ai_cmdtest(struct comedi_device *dev,
 {
 	const struct das1800_board *board = dev->board_ptr;
 	int err = 0;
-	unsigned int arg;
 
 	/* Step 1 : check if triggers are trivially valid */
 
@@ -659,31 +698,13 @@ static int das1800_ai_cmdtest(struct comedi_device *dev,
 	if (err)
 		return 3;
 
-	/* step 4: fix up any arguments */
+	/* Step 4: fix up any arguments */
 
-	if (cmd->scan_begin_src == TRIG_FOLLOW &&
-	    cmd->convert_src == TRIG_TIMER) {
-		/* we are not in burst mode */
-		arg = cmd->convert_arg;
-		comedi_8254_cascade_ns_to_timer(dev->pacer, &arg, cmd->flags);
-		err |= comedi_check_trigger_arg_is(&cmd->convert_arg, arg);
-	} else if (cmd->convert_src == TRIG_TIMER) {
-		/* we are in burst mode */
-		arg = burst_convert_arg(cmd->convert_arg, cmd->flags);
-		err |= comedi_check_trigger_arg_is(&cmd->convert_arg, arg);
-
-		if (cmd->scan_begin_src == TRIG_TIMER) {
-			arg = cmd->convert_arg * cmd->chanlist_len;
-			err |= comedi_check_trigger_arg_max(&cmd->
-							    scan_begin_arg,
-							    arg);
-
-			arg = cmd->scan_begin_arg;
-			comedi_8254_cascade_ns_to_timer(dev->pacer, &arg,
-							cmd->flags);
-			err |= comedi_check_trigger_arg_is(&cmd->scan_begin_arg,
-							   arg);
-		}
+	if (cmd->convert_src == TRIG_TIMER) {
+		if (cmd->scan_begin_src == TRIG_FOLLOW)
+			err |= das1800_ai_fixup_paced_timing(dev, cmd);
+		else /* TRIG_TIMER or TRIG_EXT */
+			err |= das1800_ai_fixup_burst_timing(dev, cmd);
 	}
 
 	if (err)