[media] v4l2-fh: add v4l2_fh_is_singular

Several drivers need to do something when the first filehandle is opened
or the last filehandle is closed. Most implement some use count mechanism,
but if they use v4l2_fh, then you can also just check if this is the only
filehandle for the device node. A simple helper function can do this.

Signed-off-by: Hans Verkuil <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/video/v4l2-fh.c

···
 	return 0;
 }
 EXPORT_SYMBOL_GPL(v4l2_fh_release);
+
+int v4l2_fh_is_singular(struct v4l2_fh *fh)
+{
+	unsigned long flags;
+	int is_singular;
+
+	if (fh == NULL || fh->vdev == NULL)
+		return 0;
+	spin_lock_irqsave(&fh->vdev->fh_lock, flags);
+	is_singular = list_is_singular(&fh->list);
+	spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);
+	return is_singular;
+}
+EXPORT_SYMBOL_GPL(v4l2_fh_is_singular);

include/media/v4l2-fh.h

···
  * v4l2_fh struct) is NULL. This function always returns 0.
  */
 int v4l2_fh_release(struct file *filp);
+/*
+ * Returns 1 if this filehandle is the only filehandle opened for the
+ * associated video_device. If fh is NULL, then it returns 0.
+ */
+int v4l2_fh_is_singular(struct v4l2_fh *fh);
+/*
+ * Helper function with struct file as argument. If filp->private_data is
+ * NULL, then it will return 0.
+ */
+static inline int v4l2_fh_is_singular_file(struct file *filp)
+{
+	return v4l2_fh_is_singular(filp->private_data);
+}
 
 #endif /* V4L2_EVENT_H */