[PATCH v7 2/3] staging: iio: adc: ad7816: Serialize SPI operations

[Abdelnasser Hussein <abdelnasserhussein11@gmail.com>]

The ad7816_spi_read() and ad7816_spi_write() functions perform a
sequence of GPIO state changes followed by an SPI transfer. If multiple
operations occur simultaneously, the GPIO state could be changed by one
thread while another is in the middle of a transfer, leading to a race
condition.

Introduce a mutex to serialize the operations, ensuring that the GPIO
toggling and the SPI transfer are treated as a single atomic operation.
The mutex is placed right after the pointers in the device state
structure to avoid padding holes.

Signed-off-by: Abdelnasser Hussein <abdelnasserhussein11@gmail.com>
---
 drivers/staging/iio/adc/ad7816.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/staging/iio/adc/ad7816.c b/drivers/staging/iio/adc/ad7816.c
index 9e43ce83e071..c18093ca8a82 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>
@@ -47,6 +49,7 @@ struct ad7816_chip_info {
 	struct gpio_desc *rdwr_pin;
 	struct gpio_desc *convert_pin;
 	struct gpio_desc *busy_pin;
+	struct mutex lock; /* protect device state during SPI transfers */
 	u8  oti_data[AD7816_CS_MAX + 1];
 	u8  channel_id;	/* 0 always be temperature */
 	u8  mode;
@@ -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));
@@ -107,6 +112,8 @@ static int ad7816_spi_write(struct ad7816_chip_info *chip, u8 data)
 	struct spi_device *spi_dev = chip->spi_dev;
 	int ret;
 
+	guard(mutex)(&chip->lock);
+
 	gpiod_set_value(chip->rdwr_pin, 1);
 	gpiod_set_value(chip->rdwr_pin, 0);
 	ret = spi_write(spi_dev, &data, sizeof(data));
@@ -360,6 +367,10 @@ 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