staging: comedi: mf6x4: use comedi_subdevice 'readback'

Use the new comedi_subdevice 'readback' member and the core provided
(*insn_read) for the readback of the analog output subdevice channels.
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/mf6x4.c

@@ -100,9 +100,6 @@ struct mf6x4_private {
 	 * offsets -- this variable makes the access easier
 	 */
 	void __iomem *gpioc_R;
-
-	/* DAC value cache -- used for insn_read function */
-	int ao_readback[8];
 };
 
 static int mf6x4_di_insn_bits(struct comedi_device *dev,
@@ -181,7 +178,7 @@ static int mf6x4_ao_insn_write(struct comedi_device *dev,
 {
 	struct mf6x4_private *devpriv = dev->private;
 	unsigned int chan = CR_CHAN(insn->chanspec);
-	unsigned int val = devpriv->ao_readback[chan];
+	unsigned int val = s->readback[chan];
 	uint32_t gpioc;
 	int i;
 
@@ -194,21 +191,7 @@ static int mf6x4_ao_insn_write(struct comedi_device *dev,
 		val = data[i];
 		iowrite16(val, dev->mmio + MF6X4_DAC_R(chan));
 	}
-	devpriv->ao_readback[chan] = val;
-
-	return insn->n;
-}
-
-static int mf6x4_ao_insn_read(struct comedi_device *dev,
-			      struct comedi_subdevice *s,
-			      struct comedi_insn *insn, unsigned int *data)
-{
-	struct mf6x4_private *devpriv = dev->private;
-	unsigned int chan = CR_CHAN(insn->chanspec);
-	int i;
-
-	for (i = 0; i < insn->n; i++)
-		data[i] = devpriv->ao_readback[chan];
+	s->readback[chan] = val;
 
 	return insn->n;
 }
@@ -276,7 +259,11 @@ static int mf6x4_auto_attach(struct comedi_device *dev, unsigned long context)
 	s->maxdata = 0x3fff; /* 14 bits DAC */
 	s->range_table = &range_bipolar10;
 	s->insn_write = mf6x4_ao_insn_write;
-	s->insn_read = mf6x4_ao_insn_read;
+	s->insn_read = comedi_readback_insn_read;
+
+	ret = comedi_alloc_subdev_readback(s);
+	if (ret)
+		return ret;
 
 	/* DIN */
 	s = &dev->subdevices[2];