[PATCH v2 2/2] staging: iio: ad7816: avoid DMA from stack in spi_read

[Abdelnasser Hussein <abdelnasserhussein11@gmail.com>]

The SPI transfer buffer is allocated on the stack, which is unsafe when
the SPI core performs DMA transfers. With VMAP_STACK enabled, this can
lead to DMA mapping failures because the stack is not guaranteed to be
DMA-accessible.

Move the buffer into struct ad7816_chip_info to provide storage with an
appropriate lifetime for DMA, align it with
__aligned(IIO_DMA_MINALIGN), and update the spi_read() sizeof() argument
to reference the relocated buffer.

Fixes: 7924425db04a ("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, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/staging/iio/adc/ad7816.c b/drivers/staging/iio/adc/ad7816.c
index b5a0c2871e00..c58a6bf77020 100644
--- a/drivers/staging/iio/adc/ad7816.c
+++ b/drivers/staging/iio/adc/ad7816.c
@@ -51,6 +51,7 @@ struct ad7816_chip_info {
 	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 {
@@ -66,7 +67,6 @@ static int ad7816_spi_read(struct ad7816_chip_info *chip, u16 *data)
 {
 	struct spi_device *spi_dev = chip->spi_dev;
 	int ret;
-	__be16 buf;
 
 	mutex_lock(&chip->lock);
 
@@ -95,7 +95,7 @@ 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");
 		mutex_unlock(&chip->lock);
@@ -103,7 +103,7 @@ static int ad7816_spi_read(struct ad7816_chip_info *chip, u16 *data)
 		return ret;
 	}
 
-	*data = be16_to_cpu(buf);
+	*data = be16_to_cpu(chip->rx_buf);
 	mutex_unlock(&chip->lock);
 	return ret;
 }
-- 
2.54.0