4.15-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Fabrice Gasnier <fabrice.gasn...@st.com>

commit 04e491ca9df60ffe8637d00d68e5ab8bc73b30d5 upstream.

By default, watermark is set to '1'. Watermark is used to fine tune
cyclic dma buffer period. In case watermark is left untouched (e.g. 1)
and several channels are being scanned, buffer period is wrongly set
(e.g. to 1 sample). As a consequence, data is never pushed to upper layer.
Fix buffer period size, by taking scan channels number into account.

Fixes: 2763ea0585c9 ("iio: adc: stm32: add optional dma support")

Signed-off-by: Fabrice Gasnier <fabrice.gasn...@st.com>
Signed-off-by: Jonathan Cameron <jonathan.came...@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 drivers/iio/adc/stm32-adc.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/drivers/iio/adc/stm32-adc.c
+++ b/drivers/iio/adc/stm32-adc.c
@@ -1315,6 +1315,7 @@ static int stm32_adc_set_watermark(struc
 {
        struct stm32_adc *adc = iio_priv(indio_dev);
        unsigned int watermark = STM32_DMA_BUFFER_SIZE / 2;
+       unsigned int rx_buf_sz = STM32_DMA_BUFFER_SIZE;
 
        /*
         * dma cyclic transfers are used, buffer is split into two periods.
@@ -1323,7 +1324,7 @@ static int stm32_adc_set_watermark(struc
         * - one buffer (period) driver can push with iio_trigger_poll().
         */
        watermark = min(watermark, val * (unsigned)(sizeof(u16)));
-       adc->rx_buf_sz = watermark * 2;
+       adc->rx_buf_sz = min(rx_buf_sz, watermark * 2 * adc->num_conv);
 
        return 0;
 }


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