[PATCH v2 1/2] staging: iio: ad7816: serialize ad7816_spi_read() with a mutex

[Abdelnasser Hussein <abdelnasserhussein11@gmail.com>]

The ad7816_spi_read() path performs a sequence of SPI transfers and GPIO
state changes that must not be interleaved with another read operation.

Without serialization, concurrent callers can interfere with each other,
leading to inconsistent device state and incorrect data being returned.

Add a mutex to struct ad7816_chip_info and hold it across the entire read
sequence to ensure exclusive access to the device.

Signed-off-by: Abdelnasser Hussein <abdelnasserhussein11@gmail.com>
---
 drivers/staging/iio/adc/ad7816.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/staging/iio/adc/ad7816.c b/drivers/staging/iio/adc/ad7816.c
index 0e32a2295990..b5a0c2871e00 100644
--- a/drivers/staging/iio/adc/ad7816.c
+++ b/drivers/staging/iio/adc/ad7816.c
@@ -50,6 +50,7 @@ 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 */
 };
 
 enum ad7816_type {
@@ -67,11 +68,14 @@ static int ad7816_spi_read(struct ad7816_chip_info *chip, u16 *data)
 	int ret;
 	__be16 buf;
 
+	mutex_lock(&chip->lock);
+
 	gpiod_set_value(chip->rdwr_pin, 1);
 	gpiod_set_value(chip->rdwr_pin, 0);
 	ret = spi_write(spi_dev, &chip->channel_id, sizeof(chip->channel_id));
 	if (ret < 0) {
 		dev_err(&spi_dev->dev, "SPI channel setting error\n");
+		mutex_unlock(&chip->lock);
 		return ret;
 	}
 	gpiod_set_value(chip->rdwr_pin, 1);
@@ -94,11 +98,13 @@ static int ad7816_spi_read(struct ad7816_chip_info *chip, u16 *data)
 	ret = spi_read(spi_dev, &buf, sizeof(*data));
 	if (ret < 0) {
 		dev_err(&spi_dev->dev, "SPI data read error\n");
+		mutex_unlock(&chip->lock);
+
 		return ret;
 	}
 
 	*data = be16_to_cpu(buf);
-
+	mutex_unlock(&chip->lock);
 	return ret;
 }
 
@@ -359,7 +365,7 @@ static int ad7816_probe(struct spi_device *spi_dev)
 	if (!indio_dev)
 		return -ENOMEM;
 	chip = iio_priv(indio_dev);
-
+	mutex_init(&chip->lock);
 	chip->spi_dev = spi_dev;
 	for (i = 0; i <= AD7816_CS_MAX; i++)
 		chip->oti_data[i] = 203;
-- 
2.54.0