[PATCH v2] iio: adc: mcp3422: fix locking scope

[Angelo Compagnucci <angelo.compagnucci@gmail.com>]

From: Angelo Compagnucci <angelo.compagnucci@gmail.com>

Locking should be held for the entire reading sequence involving setting
the channel, waiting for the channel switch and reading from the
channel.
If not, reading from a channel can result mixing with the reading from
another channel.

Fixes: 07914c84ba30 ("iio: adc: Add driver for Microchip MCP3422/3/4 high resolution ADC")
Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
---
 drivers/iio/adc/mcp3422.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/drivers/iio/adc/mcp3422.c b/drivers/iio/adc/mcp3422.c
index d86c0b5d80a3..02a60fb164cd 100644
--- a/drivers/iio/adc/mcp3422.c
+++ b/drivers/iio/adc/mcp3422.c
@@ -96,16 +96,12 @@ static int mcp3422_update_config(struct mcp3422 *adc, u8 newconfig)
 {
 	int ret;
 
-	mutex_lock(&adc->lock);
-
 	ret = i2c_master_send(adc->i2c, &newconfig, 1);
 	if (ret > 0) {
 		adc->config = newconfig;
 		ret = 0;
 	}
 
-	mutex_unlock(&adc->lock);
-
 	return ret;
 }
 
@@ -138,6 +134,8 @@ static int mcp3422_read_channel(struct mcp3422 *adc,
 	u8 config;
 	u8 req_channel = channel->channel;
 
+	mutex_lock(&adc->lock);
+
 	if (req_channel != MCP3422_CHANNEL(adc->config)) {
 		config = adc->config;
 		config &= ~MCP3422_CHANNEL_MASK;
@@ -150,7 +148,11 @@ static int mcp3422_read_channel(struct mcp3422 *adc,
 		msleep(mcp3422_read_times[MCP3422_SAMPLE_RATE(adc->config)]);
 	}
 
-	return mcp3422_read(adc, value, &config);
+	ret = mcp3422_read(adc, value, &config);
+
+	mutex_unlock(&adc->lock);
+
+	return ret;
 }
 
 static int mcp3422_read_raw(struct iio_dev *iio,
-- 
2.25.1