[PATCH v4 2/2] staging: iio: adc: ad7816: Use DMA-safe buffer for SPI read

[Abdelnasser Hussein <abdelnasserhussein11@gmail.com>]

Stack buffers are not guaranteed to be cache-coherent and must not
be used with SPI reads, which can trigger issues with VMAP_STACK.
Fix this by using a dedicated DMA-safe rx_buf aligned with
IIO_DMA_MINALIGN. Also, correct the sizeof() argument from *data
to chip->rx_buf to match the buffer size.

Fixes: 7024425db64a ("staging: iio: adc: new driver for AD7816 devices")
Signed-off-by: Abdelnasser Hussein <abdelnasserhussein11@gmail.com>
---
 drivers/staging/iio/adc/ad7816.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/staging/iio/adc/ad7816.c b/drivers/staging/iio/adc/ad7816.c
index ab80b3a889bb..e14ae805f076 100644
--- a/drivers/staging/iio/adc/ad7816.c
+++ b/drivers/staging/iio/adc/ad7816.c
@@ -52,6 +52,8 @@ struct ad7816_chip_info {
 	u8  oti_data[AD7816_CS_MAX + 1];
 	u8  channel_id;	/* 0 always be temperature */
 	u8  mode;
+	struct mutex lock; /* protect device state during SPI transfers */
+	__be16 rx_buf __aligned(IIO_DMA_MINALIGN);
 };
 
 enum ad7816_type {
@@ -94,13 +96,13 @@ static int ad7816_spi_read(struct ad7816_chip_info *chip, u16 *data)
 
 	gpiod_set_value(chip->rdwr_pin, 0);
 	gpiod_set_value(chip->rdwr_pin, 1);
-	ret = spi_read(spi_dev, &buf, sizeof(*data));
+	ret = spi_read(spi_dev, &chip->rx_buf, sizeof(chip->rx_buf));
 	if (ret < 0) {
 		dev_err(&spi_dev->dev, "SPI data read error\n");
 		return ret;
 	}
 
-	*data = be16_to_cpu(buf);
+	*data = be16_to_cpu(chip->rx_buf);
 
 	return ret;
 }
-- 
2.54.0