staging: comedi: ni_labpc: set up command4 register *after* command3

Tuomas <tvainikk _at_ gmail _dot_ com> reported problems getting
meaningful output from a Lab-PC+ in differential mode for AI cmds, but
AI insn reads gave correct readings.  He tracked it down to two
problems, one of which is addressed by this patch.

It seems that writing to the command3 register after writing to the
command4 register in `labpc_ai_cmd()` messes up the differential
reference bit setting in the command4 register.  Set up the command4
register after the command3 register (as in `labpc_ai_rinsn()`) to avoid
the problem.

Thanks to Tuomas for suggesting the fix.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: <stable@vger.kernel.org> # 3.7.x, 3.8.x
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/ni_labpc.c

@@ -1219,21 +1219,6 @@ static int labpc_ai_cmd(struct comedi_device *dev, struct comedi_subdevice *s)
 		devpriv->write_byte(devpriv->command1_bits,
 				    dev->iobase + COMMAND1_REG);
 	}
-	/*  setup any external triggering/pacing (command4 register) */
-	devpriv->command4_bits = 0;
-	if (cmd->convert_src != TRIG_EXT)
-		devpriv->command4_bits |= EXT_CONVERT_DISABLE_BIT;
-	/* XXX should discard first scan when using interval scanning
-	 * since manual says it is not synced with scan clock */
-	if (labpc_use_continuous_mode(cmd, mode) == 0) {
-		devpriv->command4_bits |= INTERVAL_SCAN_EN_BIT;
-		if (cmd->scan_begin_src == TRIG_EXT)
-			devpriv->command4_bits |= EXT_SCAN_EN_BIT;
-	}
-	/*  single-ended/differential */
-	if (aref == AREF_DIFF)
-		devpriv->command4_bits |= ADC_DIFF_BIT;
-	devpriv->write_byte(devpriv->command4_bits, dev->iobase + COMMAND4_REG);
 
 	devpriv->write_byte(cmd->chanlist_len,
 			    dev->iobase + INTERVAL_COUNT_REG);
@@ -1313,6 +1298,22 @@ static int labpc_ai_cmd(struct comedi_device *dev, struct comedi_subdevice *s)
 		devpriv->command3_bits &= ~ADC_FNE_INTR_EN_BIT;
 	devpriv->write_byte(devpriv->command3_bits, dev->iobase + COMMAND3_REG);
 
+	/*  setup any external triggering/pacing (command4 register) */
+	devpriv->command4_bits = 0;
+	if (cmd->convert_src != TRIG_EXT)
+		devpriv->command4_bits |= EXT_CONVERT_DISABLE_BIT;
+	/* XXX should discard first scan when using interval scanning
+	 * since manual says it is not synced with scan clock */
+	if (labpc_use_continuous_mode(cmd, mode) == 0) {
+		devpriv->command4_bits |= INTERVAL_SCAN_EN_BIT;
+		if (cmd->scan_begin_src == TRIG_EXT)
+			devpriv->command4_bits |= EXT_SCAN_EN_BIT;
+	}
+	/*  single-ended/differential */
+	if (aref == AREF_DIFF)
+		devpriv->command4_bits |= ADC_DIFF_BIT;
+	devpriv->write_byte(devpriv->command4_bits, dev->iobase + COMMAND4_REG);
+
 	/*  startup acquisition */
 
 	/*  command2 reg */