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

[Eugen Hristev]

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.hris...@microchip.com>
---

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+5f...@mail.gmail.com/T/#t

Thanks !

 drivers/iio/adc/at91-sama5d2_adc.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/iio/adc/at91-sama5d2_adc.c 
b/drivers/iio/adc/at91-sama5d2_adc.c
index de9583d6cddd..b5196797dcb8 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,
 };
 
@@ -1586,7 +1585,11 @@ 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
+        */
+       return at91_adc_buffer_prepare(indio_dev);
 }
 
 static int at91_adc_update_scan_mode(struct iio_dev *indio_dev,
-- 
2.25.1