iio: adc: stm32: fix scan of multiple channels with DMA

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.gasnier@st.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/iio/adc/stm32-adc.c

@@ -1289,6 +1289,7 @@ static int stm32_adc_set_watermark(struct iio_dev *indio_dev, unsigned int val)
 {
 	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.
@@ -1297,7 +1298,7 @@ static int stm32_adc_set_watermark(struct iio_dev *indio_dev, unsigned int val)
 	 * - 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;
 }