[PATCH v2] iio: temperature: tsys02d: Fix state desynchronization on resolution write error

[Salah Triki <salah.triki@gmail.com>]

In tsys02d_write_raw(), dev_data->res_index is updated before checking
the return value of ms_sensors_write_resolution(). If the hardware I2C
write operation fails, dev_data->res_index remains updated with the
new index despite the physical sensor remaining in its previous state.

This leads to a state desynchronization where subsequent reads via
tsys02d_read_raw() return an incorrect sampling frequency.

Fix this by handling the error path first, updating dev_data->res_index
only on success, and using guard(mutex) to clean up locking.

This was found through manual code review.

Fixes: 53bf4d067d51 ("Add tsys02d meas-spec driver support")
Signed-off-by: Salah Triki <salah.triki@gmail.com>
---
v1 -> v2:
 - Use error-first pattern (if (ret) return ret) instead of if (!ret).
 - Use guard(mutex) for clean locking and simplified return path.
 - Add note stating the issue was found through manual code review.
 - Add proper blank lines.

 drivers/iio/temperature/tsys02d.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/iio/temperature/tsys02d.c b/drivers/iio/temperature/tsys02d.c
index 3ef72347456e..20f67134affd 100644
--- a/drivers/iio/temperature/tsys02d.c
+++ b/drivers/iio/temperature/tsys02d.c
@@ -72,12 +72,14 @@ static int tsys02d_write_raw(struct iio_dev *indio_dev,
 				break;
 		if (i < 0)
 			return -EINVAL;
-		mutex_lock(&dev_data->lock);
-		dev_data->res_index = i;
+
+		guard(mutex)(&dev_data->lock);
+
 		ret = ms_sensors_write_resolution(dev_data, i);
-		mutex_unlock(&dev_data->lock);
+		if (ret)
+			return ret;
 
-		return ret;
+		dev_data->res_index = i;
 	default:
 		return -EINVAL;
 	}
-- 
2.43.0