[PATCH v2] iio: adc: at91-sama5d2_adc: fix DMA conversion crash

[Eugen Hristev <eugen.hristev@microchip.com>]

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: f3c034f61775 ("iio: at91-sama5d2_adc: adjust iio_triggered_buffer_{predisable,postenable} positions")
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
---
Changes in v2:
- check return value of buffer prepare, and in case of error, unwind DMA
- validated by trying to force an error :

# echo 100 > /sys/bus/iio/devices/iio\:device0/buffer/length
# echo 100 > /sys/bus/iio/devices/iio\:device0/buffer/watermark
# iio_generic_buffer -n fc030000.adc -t fc030000.adc-dev0-external_rising -c 5
iio device number being used is 0
iio trigger number being used is 0
/sys/bus/iio/deviciio io:device0 fc030000.adc-dev0-external_risingiio:device0: using dma0chan10 for rx DMA transfers

iio iio:device0: continuing without DMA support
505.151367
983.715820
1352.709961
1485.644531


Original patch message:

Hi,

This crash is in the kernel since 5.8-rc1

Please have a look at discussion here:
https://lore.kernel.org/linux-iio/CA+U=DsqRUtjjoe5nevP_wNxTgr27+O2V1h9w7d3QijBQ+5f3XA@mail.gmail.com/T/#t

Thanks !

 drivers/iio/adc/at91-sama5d2_adc.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/iio/adc/at91-sama5d2_adc.c b/drivers/iio/adc/at91-sama5d2_adc.c
index de9583d6cddd..f94641193b98 100644
--- a/drivers/iio/adc/at91-sama5d2_adc.c
+++ b/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,
-- 
2.25.1