iio: adc: at91-sama5d2_adc: fix DMA conversion crash

After the move of the postenable code to preenable, the DMA start was
done before the DMA init, which is not correct.
The DMA is initialized in set_watermark. Because of this, we need to call
the DMA start functions in set_watermark, after the DMA init, instead of
preenable hook, when the DMA is not properly setup yet.

Fixes: f3c034f6 ("iio: at91-sama5d2_adc: adjust iio_triggered_buffer_{predisable,postenable} positions")
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Link: https://lore.kernel.org/r/20200923121748.49384-1-eugen.hristev@microchip.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/at91-sama5d2_adc.c

@@ -884,7 +884,7 @@ static bool at91_adc_current_chan_is_touch(struct iio_dev *indio_dev)
 			       AT91_SAMA5D2_MAX_CHAN_IDX + 1);
 }
 
-static int at91_adc_buffer_preenable(struct iio_dev *indio_dev)
+static int at91_adc_buffer_prepare(struct iio_dev *indio_dev)
 {
 	int ret;
 	u8 bit;
@@ -901,7 +901,7 @@ static int at91_adc_buffer_preenable(struct iio_dev *indio_dev)
 	/* we continue with the triggered buffer */
 	ret = at91_adc_dma_start(indio_dev);
 	if (ret) {
-		dev_err(&indio_dev->dev, "buffer postenable failed\n");
+		dev_err(&indio_dev->dev, "buffer prepare failed\n");
 		return ret;
 	}
 
@@ -989,7 +989,6 @@ static int at91_adc_buffer_postdisable(struct iio_dev *indio_dev)
 }
 
 static const struct iio_buffer_setup_ops at91_buffer_setup_ops = {
-	.preenable = &at91_adc_buffer_preenable,
 	.postdisable = &at91_adc_buffer_postdisable,
 };
 
@@ -1563,6 +1562,7 @@ static void at91_adc_dma_disable(struct platform_device *pdev)
 static int at91_adc_set_watermark(struct iio_dev *indio_dev, unsigned int val)
 {
 	struct at91_adc_state *st = iio_priv(indio_dev);
+	int ret;
 
 	if (val > AT91_HWFIFO_MAX_SIZE)
 		return -EINVAL;
@@ -1586,7 +1586,15 @@ static int at91_adc_set_watermark(struct iio_dev *indio_dev, unsigned int val)
 	else if (val > 1)
 		at91_adc_dma_init(to_platform_device(&indio_dev->dev));
 
-	return 0;
+	/*
+	 * We can start the DMA only after setting the watermark and
+	 * having the DMA initialization completed
+	 */
+	ret = at91_adc_buffer_prepare(indio_dev);
+	if (ret)
+		at91_adc_dma_disable(to_platform_device(&indio_dev->dev));
+
+	return ret;
 }
 
 static int at91_adc_update_scan_mode(struct iio_dev *indio_dev,