scd30_core.c currently uses manual mutex_lock() and mutex_unlock()
calls. Replace them with the newer guard(mutex)() for cleaner RAII
patterns and to improve maintainability.

Add new helper function scd30_trigger_handler_helper_locked() containing
the critical section for scd30_trigger_handler(). After moving
scd30_trigger_handler()'s critical section into the new helper, tune up
control logic to return ret early and not memcpy() if it's an error
condition.

In addition, small refactor to replace "?:" operator with regular
if/else returns.

Signed-off-by: Maxwell Doose <m32285159@gmail.com>
---
 v2:
 - Fix callback issue as noted by Jonathan v1.
 - Refactor critical section of scd30_trigger_handler() into helper
   called scd30_trigger_handler_helper_locked().
 - Revert removal of goto in scd30_trigger_handler().

 v3:
 - Tune up helper to return early on failure condition per Jonathan's
   suggestion.

 v4:
 - Forgot to commit changes listed in v3, now fixed.

 drivers/iio/chemical/scd30_core.c | 68 ++++++++++++++++++++-----------
 1 file changed, 44 insertions(+), 24 deletions(-)

diff --git a/drivers/iio/chemical/scd30_core.c b/drivers/iio/chemical/scd30_core.c
index a665fcb78806..8dbba9a7c426 100644
--- a/drivers/iio/chemical/scd30_core.c
+++ b/drivers/iio/chemical/scd30_core.c
@@ -368,11 +368,13 @@ static ssize_t calibration_auto_enable_show(struct device *dev, struct device_at
 	int ret;
 	u16 val;
 
-	mutex_lock(&state->lock);
-	ret = scd30_command_read(state, CMD_ASC, &val);
-	mutex_unlock(&state->lock);
+	guard(mutex)(&state->lock);
 
-	return ret ?: sysfs_emit(buf, "%d\n", val);
+	ret = scd30_command_read(state, CMD_ASC, &val);
+	if (ret)
+		return ret;
+
+	return sysfs_emit(buf, "%d\n", val);
 }
 
 static ssize_t calibration_auto_enable_store(struct device *dev, struct device_attribute *attr,
@@ -387,11 +389,13 @@ static ssize_t calibration_auto_enable_store(struct device *dev, struct device_a
 	if (ret)
 		return ret;
 
-	mutex_lock(&state->lock);
-	ret = scd30_command_write(state, CMD_ASC, val);
-	mutex_unlock(&state->lock);
+	guard(mutex)(&state->lock);
 
-	return ret ?: len;
+	ret = scd30_command_write(state, CMD_ASC, val);
+	if (ret)
+		return ret;
+
+	return len;
 }
 
 static ssize_t calibration_forced_value_show(struct device *dev, struct device_attribute *attr,
@@ -402,11 +406,13 @@ static ssize_t calibration_forced_value_show(struct device *dev, struct device_a
 	int ret;
 	u16 val;
 
-	mutex_lock(&state->lock);
-	ret = scd30_command_read(state, CMD_FRC, &val);
-	mutex_unlock(&state->lock);
+	guard(mutex)(&state->lock);
 
-	return ret ?: sysfs_emit(buf, "%d\n", val);
+	ret = scd30_command_read(state, CMD_FRC, &val);
+	if (ret)
+		return ret;
+
+	return sysfs_emit(buf, "%d\n", val);
 }
 
 static ssize_t calibration_forced_value_store(struct device *dev, struct device_attribute *attr,
@@ -424,11 +430,13 @@ static ssize_t calibration_forced_value_store(struct device *dev, struct device_
 	if (val < SCD30_FRC_MIN_PPM || val > SCD30_FRC_MAX_PPM)
 		return -EINVAL;
 
-	mutex_lock(&state->lock);
-	ret = scd30_command_write(state, CMD_FRC, val);
-	mutex_unlock(&state->lock);
+	guard(mutex)(&state->lock);
 
-	return ret ?: len;
+	ret = scd30_command_write(state, CMD_FRC, val);
+	if (ret)
+		return ret;
+
+	return len;
 }
 
 static IIO_DEVICE_ATTR_RO(sampling_frequency_available, 0);
@@ -579,24 +587,36 @@ static irqreturn_t scd30_irq_thread_handler(int irq, void *priv)
 	return IRQ_HANDLED;
 }
 
+/* Meant ONLY for scd30_trigger_handler() */
+static int scd30_trigger_handler_helper_locked(struct iio_dev *indio_dev,
+					       int *scan_data, int arr_size)
+{
+	struct scd30_state *state = iio_priv(indio_dev);
+	int ret;
+
+	guard(mutex)(&state->lock);
+
+	if (!iio_trigger_using_own(indio_dev))
+		ret = scd30_read_poll(state);
+	else
+		ret = scd30_read_meas(state);
+	if (ret)
+		return ret;
+	memcpy(scan_data, state->meas, arr_size);
+	return 0;
+}
+
 static irqreturn_t scd30_trigger_handler(int irq, void *p)
 {
 	struct iio_poll_func *pf = p;
 	struct iio_dev *indio_dev = pf->indio_dev;
-	struct scd30_state *state = iio_priv(indio_dev);
 	struct {
 		int data[SCD30_MEAS_COUNT];
 		aligned_s64 ts;
 	} scan = { };
 	int ret;
 
-	mutex_lock(&state->lock);
-	if (!iio_trigger_using_own(indio_dev))
-		ret = scd30_read_poll(state);
-	else
-		ret = scd30_read_meas(state);
-	memcpy(scan.data, state->meas, sizeof(state->meas));
-	mutex_unlock(&state->lock);
+	ret = scd30_trigger_handler_helper_locked(indio_dev, scan.data, sizeof(scan.data));
 	if (ret)
 		goto out;
 
-- 
2.54.0