iio: trigger: warn about non-registered iio trigger getting attempt

As a part of patch series about wrong trigger register() and get()
calls order in the some IIO drivers trigger initialization path:

https://lore.kernel.org/all/20220524181150.9240-1-ddrokosov@sberdevices.ru/

runtime WARN_ONCE() is added to alarm IIO driver authors who make such
a mistake.

When an IIO driver allocates a new IIO trigger, it should register it
before calling the get() operation. In other words, each IIO driver
must abide by IIO trigger alloc()/register()/get() calls order.

Signed-off-by: Dmitry Rokosov <ddrokosov@sberdevices.ru>
Link: https://lore.kernel.org/r/20220607183907.20017-1-ddrokosov@sberdevices.ru
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-trigger.c

@@ -576,6 +576,8 @@ struct iio_trigger *viio_trigger_alloc(struct device *parent,
 	if (trig->name == NULL)
 		goto free_descs;
 
+	INIT_LIST_HEAD(&trig->list);
+
 	trig->subirq_chip.name = trig->name;
 	trig->subirq_chip.irq_mask = &iio_trig_subirqmask;
 	trig->subirq_chip.irq_unmask = &iio_trig_subirqunmask;

include/linux/iio/trigger.h

@@ -93,6 +93,11 @@ static inline void iio_trigger_put(struct iio_trigger *trig)
 static inline struct iio_trigger *iio_trigger_get(struct iio_trigger *trig)
 {
 	get_device(&trig->dev);
+
+	WARN_ONCE(list_empty(&trig->list),
+		  "Getting non-registered iio trigger %s is prohibited\n",
+		  trig->name);
+
 	__module_get(trig->owner);
 
 	return trig;