iio: trigger: allow immutable triggers to be assigned

There are times when an assigned trigger to a device shouldn't ever
change after intialization.

Examples of this being used is when an provider device has a trigger
that is assigned to an ADC, which uses it populate data into a callback
buffer.

Signed-off-by: Matt Ranostay <matt@ranostay.consulting>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/industrialio-trigger.c

@@ -119,6 +119,22 @@ void iio_trigger_unregister(struct iio_trigger *trig_info)
 }
 EXPORT_SYMBOL(iio_trigger_unregister);
 
+int iio_trigger_set_immutable(struct iio_dev *indio_dev, struct iio_trigger *trig)
+{
+	if (!indio_dev || !trig)
+		return -EINVAL;
+
+	mutex_lock(&indio_dev->mlock);
+	WARN_ON(indio_dev->trig_readonly);
+
+	indio_dev->trig = iio_trigger_get(trig);
+	indio_dev->trig_readonly = true;
+	mutex_unlock(&indio_dev->mlock);
+
+	return 0;
+}
+EXPORT_SYMBOL(iio_trigger_set_immutable);
+
 /* Search for trigger by name, assuming iio_trigger_list_lock held */
 static struct iio_trigger *__iio_trigger_find_by_name(const char *name)
 {
@@ -384,6 +400,10 @@ static ssize_t iio_trigger_write_current(struct device *dev,
 		mutex_unlock(&indio_dev->mlock);
 		return -EBUSY;
 	}
+	if (indio_dev->trig_readonly) {
+		mutex_unlock(&indio_dev->mlock);
+		return -EPERM;
+	}
 	mutex_unlock(&indio_dev->mlock);
 
 	trig = iio_trigger_find_by_name(buf, len);

include/linux/iio/iio.h

@@ -483,6 +483,7 @@ struct iio_buffer_setup_ops {
  * @scan_timestamp:	[INTERN] set if any buffers have requested timestamp
  * @scan_index_timestamp:[INTERN] cache of the index to the timestamp
  * @trig:		[INTERN] current device trigger (buffer modes)
+ * @trig_readonly	[INTERN] mark the current trigger immutable
  * @pollfunc:		[DRIVER] function run on trigger being received
  * @pollfunc_event:	[DRIVER] function run on events trigger being received
  * @channels:		[DRIVER] channel specification structure table
@@ -523,6 +524,7 @@ struct iio_dev {
 	bool				scan_timestamp;
 	unsigned			scan_index_timestamp;
 	struct iio_trigger		*trig;
+	bool				trig_readonly;
 	struct iio_poll_func		*pollfunc;
 	struct iio_poll_func		*pollfunc_event;
 

include/linux/iio/trigger.h

@@ -131,6 +131,15 @@ int iio_trigger_register(struct iio_trigger *trig_info);
  **/
 void iio_trigger_unregister(struct iio_trigger *trig_info);
 
+/**
+ * iio_trigger_set_immutable() - set an immutable trigger on destination
+ *
+ * @indio_dev - IIO device structure containing the device
+ * @trig - trigger to assign to device
+ *
+ **/
+int iio_trigger_set_immutable(struct iio_dev *indio_dev, struct iio_trigger *trig);
+
 /**
  * iio_trigger_poll() - called on a trigger occurring
  * @trig:	trigger which occurred