[PATCH 1/2] hwmon: (ads7871) Add mutex to serialize SPI transactions

[Tabrez Ahmed <tabreztalks@gmail.com>]

If userspace reads multiple channels concurrently, the driver can
overwrite ongoing REG_GAIN_MUX writes before the conversion completes.

Add a mutex to the driver's private data structure to serialize
hardware access and prevent concurrent userspace reads from corrupting
the ADC multiplexer state. Update error paths to ensure the mutex
is properly unlocked.

Suggested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Tabrez Ahmed <tabreztalks@gmail.com>
---
 drivers/hwmon/ads7871.c | 26 ++++++++++++++++++--------
 1 file changed, 18 insertions(+), 8 deletions(-)

diff --git a/drivers/hwmon/ads7871.c b/drivers/hwmon/ads7871.c
index 9bfdf9e6bcd77..02e69bef2ca12 100644
--- a/drivers/hwmon/ads7871.c
+++ b/drivers/hwmon/ads7871.c
@@ -59,11 +59,13 @@
 #include <linux/hwmon-sysfs.h>
 #include <linux/err.h>
 #include <linux/delay.h>
+#include <linux/mutex.h>
 
 #define DEVICE_NAME	"ads7871"
 
 struct ads7871_data {
 	struct spi_device *spi;
+	struct mutex lock;
 };
 
 static int ads7871_read_reg8(struct spi_device *spi, int reg)
@@ -98,6 +100,8 @@ static ssize_t voltage_show(struct device *dev, struct device_attribute *da,
 	uint8_t channel, mux_cnv;
 
 	channel = attr->index;
+
+	mutex_lock(&pdata->lock);
 	/*
 	 * TODO: add support for conversions
 	 * other than single ended with a gain of 1
@@ -107,11 +111,11 @@ static ssize_t voltage_show(struct device *dev, struct device_attribute *da,
 	ret = ads7871_write_reg8(spi, REG_GAIN_MUX,
 				 (MUX_CNV_BM | MUX_M3_BM | channel));
 	if (ret < 0)
-		return ret;
+		goto out_unlock;
 
 	ret = ads7871_read_reg8(spi, REG_GAIN_MUX);
 	if (ret < 0)
-		return ret;
+		goto out_unlock;
 	mux_cnv = ((ret & MUX_CNV_BM) >> MUX_CNV_BV);
 	/*
 	 * on 400MHz arm9 platform the conversion
@@ -121,21 +125,27 @@ static ssize_t voltage_show(struct device *dev, struct device_attribute *da,
 		i++;
 		ret = ads7871_read_reg8(spi, REG_GAIN_MUX);
 		if (ret < 0)
-			return ret;
+			goto out_unlock;
 		mux_cnv = ((ret & MUX_CNV_BM) >> MUX_CNV_BV);
 		msleep_interruptible(1);
 	}
 
 	if (mux_cnv == 0) {
 		val = ads7871_read_reg16(spi, REG_LS_BYTE);
-		if (val < 0)
-			return val;
+		if (val < 0) {
+			ret = val;
+			goto out_unlock;
+		}
 		/*result in volts*10000 = (val/8192)*2.5*10000*/
 		val = ((val >> 2) * 25000) / 8192;
-		return sysfs_emit(buf, "%d\n", val);
+		ret = sysfs_emit(buf, "%d\n", val);
+		goto out_unlock;
 	}
 
-	return -ETIMEDOUT;
+	ret = -ETIMEDOUT;
+out_unlock:
+	mutex_unlock(&pdata->lock);
+	return ret;
 }
 
 static SENSOR_DEVICE_ATTR_RO(in0_input, voltage, 0);
@@ -194,7 +204,7 @@ static int ads7871_probe(struct spi_device *spi)
 		return -ENOMEM;
 
 	pdata->spi = spi;
-
+	mutex_init(&pdata->lock);
 	hwmon_dev = devm_hwmon_device_register_with_groups(dev, spi->modalias,
 							   pdata,
 							   ads7871_groups);
-- 
2.43.0