[PATCH v6 2/3] staging: iio: adc: ad7816: Serialize SPI read operations

[Abdelnasser Hussein <abdelnasserhussein11@gmail.com>]

The ad7816_spi_read() function performs a sequence of GPIO state
changes followed by an SPI transfer. If multiple read operations
occur simultaneously, the GPIO state could be changed by one thread
while another is in the middle of a read. This leads to a race
condition where the sensor state is disrupted.

Introduce a mutex to serialize the read sequence, ensuring that the
GPIO toggling and the SPI transfer are treated as a single atomic
operation.

Signed-off-by: Abdelnasser Hussein <abdelnasserhussein11@gmail.com>
---
 drivers/staging/iio/adc/ad7816.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/staging/iio/adc/ad7816.c b/drivers/staging/iio/adc/ad7816.c
index 9e43ce83e071..acf26c5a267a 100644
--- a/drivers/staging/iio/adc/ad7816.c
+++ b/drivers/staging/iio/adc/ad7816.c
@@ -5,12 +5,14 @@
  * Copyright 2010 Analog Devices Inc.
  */
 
+#include <linux/cleanup.h>
 #include <linux/device.h>
 #include <linux/gpio/consumer.h>
 #include <linux/interrupt.h>
 #include <linux/kernel.h>
 #include <linux/list.h>
 #include <linux/module.h>
+#include <linux/mutex.h>
 #include <linux/slab.h>
 #include <linux/spi/spi.h>
 #include <linux/sysfs.h>
@@ -50,6 +52,7 @@ 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 {
@@ -67,6 +70,8 @@ static int ad7816_spi_read(struct ad7816_chip_info *chip, u16 *data)
 	int ret;
 	__be16 buf;
 
+	guard(mutex)(&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));
@@ -360,6 +365,10 @@ static int ad7816_probe(struct spi_device *spi_dev)
 		return -ENOMEM;
 	chip = iio_priv(indio_dev);
 
+	ret = devm_mutex_init(&spi_dev->dev, &chip->lock);
+	if (ret)
+		return ret;
+
 	chip->spi_dev = spi_dev;
 	for (i = 0; i <= AD7816_CS_MAX; i++)
 		chip->oti_data[i] = 203;
-- 
2.54.0