[RFC v2 1/4] v4l: return POLLERR on V4L2 sub-devices if no events are subscribed

[Sakari Ailus <sakari.ailus@linux.intel.com>]

From: Teemu Tuominen <teemux.tuominen@intel.com>

Add check and return POLLERR from subdev_poll() in case of no events
subscribed and wakeup once the last event subscription is removed.

This change is essentially done to add possibility to wakeup polling
with concurrent unsubscribe.

Signed-off-by: Teemu Tuominen <teemux.tuominen@intel.com>

Move the check after calling poll_wait(). Otherwise it's possible that we go
to sleep without getting notified if the subscription went away between the
two.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Tested-by: Teemu Tuominen <teemux.tuominen@intel.com>
---
 drivers/media/v4l2-core/v4l2-event.c  | 15 +++++++++++++++
 drivers/media/v4l2-core/v4l2-subdev.c |  3 +++
 include/media/v4l2-event.h            |  1 +
 3 files changed, 19 insertions(+)

diff --git a/drivers/media/v4l2-core/v4l2-event.c b/drivers/media/v4l2-core/v4l2-event.c
index 86dcb54..b53897e 100644
--- a/drivers/media/v4l2-core/v4l2-event.c
+++ b/drivers/media/v4l2-core/v4l2-event.c
@@ -107,6 +107,19 @@ static struct v4l2_subscribed_event *v4l2_event_subscribed(
 	return NULL;
 }
 
+bool v4l2_event_has_subscribed(struct v4l2_fh *fh)
+{
+	unsigned long flags;
+	bool rval;
+
+	spin_lock_irqsave(&fh->vdev->fh_lock, flags);
+	rval = !list_empty(&fh->subscribed);
+	spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);
+
+	return rval;
+}
+EXPORT_SYMBOL(v4l2_event_has_subscribed);
+
 static void __v4l2_event_queue_fh(struct v4l2_fh *fh, const struct v4l2_event *ev,
 		const struct timespec *ts)
 {
@@ -299,6 +312,8 @@ int v4l2_event_unsubscribe(struct v4l2_fh *fh,
 			fh->navailable--;
 		}
 		list_del(&sev->list);
+		if (list_empty(&fh->subscribed))
+			wake_up_all(&fh->wait);
 	}
 
 	spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);
diff --git a/drivers/media/v4l2-core/v4l2-subdev.c b/drivers/media/v4l2-core/v4l2-subdev.c
index 996c248..7d72389 100644
--- a/drivers/media/v4l2-core/v4l2-subdev.c
+++ b/drivers/media/v4l2-core/v4l2-subdev.c
@@ -382,6 +382,9 @@ static unsigned int subdev_poll(struct file *file, poll_table *wait)
 	if (v4l2_event_pending(fh))
 		return POLLPRI;
 
+	if (!v4l2_event_has_subscribed(fh))
+		return POLLERR | POLLPRI;
+
 	return 0;
 }
 
diff --git a/include/media/v4l2-event.h b/include/media/v4l2-event.h
index be05d01..a9ca2b5 100644
--- a/include/media/v4l2-event.h
+++ b/include/media/v4l2-event.h
@@ -121,6 +121,7 @@ struct v4l2_subscribed_event {
 
 int v4l2_event_dequeue(struct v4l2_fh *fh, struct v4l2_event *event,
 		       int nonblocking);
+bool v4l2_event_has_subscribed(struct v4l2_fh *fh);
 void v4l2_event_queue(struct video_device *vdev, const struct v4l2_event *ev);
 void v4l2_event_queue_fh(struct v4l2_fh *fh, const struct v4l2_event *ev);
 int v4l2_event_pending(struct v4l2_fh *fh);
-- 
1.8.3.2