[PATCHv2 1/2] v4l2-dev: add /sys media_dev attr for video devices

[Hans Verkuil <hverkuil-cisco@xs4all.nl>]

Create a media_dev attribute in /sys for each video device
which contains the media device major and minor number (or
is empty if there is no associated media device).

It is not created if the CONFIG_MEDIA_CONTROLLER is not
defined.

This makes it possible for applications like v4l2-compliance
to find the associated media controller of a video device.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
---
 drivers/media/v4l2-core/v4l2-dev.c | 48 +++++++++++++++++++++++++++++-
 1 file changed, 47 insertions(+), 1 deletion(-)

diff --git a/drivers/media/v4l2-core/v4l2-dev.c b/drivers/media/v4l2-core/v4l2-dev.c
index b6a72d297775..85b94b25aba2 100644
--- a/drivers/media/v4l2-core/v4l2-dev.c
+++ b/drivers/media/v4l2-core/v4l2-dev.c
@@ -87,13 +87,59 @@ static ssize_t name_show(struct device *cd,
 }
 static DEVICE_ATTR_RO(name);
 
+#if defined(CONFIG_MEDIA_CONTROLLER)
+static ssize_t media_dev_show(struct device *cd,
+			 struct device_attribute *attr, char *buf)
+{
+	struct video_device *vdev = to_video_device(cd);
+	struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
+
+	buf[0] = '\0';
+	if (!v4l2_dev->mdev)
+		return 0;
+	return sprintf(buf, "%u:%u\n",
+		       MAJOR(v4l2_dev->mdev->devnode->dev.devt),
+		       MINOR(v4l2_dev->mdev->devnode->dev.devt));
+}
+
+static DEVICE_ATTR_RO(media_dev);
+#endif
+
+static umode_t video_device_attr_is_visible(struct kobject *kobj,
+					    struct attribute *attr, int n)
+{
+	struct video_device *vdev = to_video_device(kobj_to_dev(kobj));
+
+#if defined(CONFIG_MEDIA_CONTROLLER)
+	if (attr == &dev_attr_media_dev.attr) {
+		struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
+
+		if (!v4l2_dev->mdev)
+			return 0;
+	}
+#endif
+	return attr->mode;
+}
+
 static struct attribute *video_device_attrs[] = {
 	&dev_attr_name.attr,
 	&dev_attr_dev_debug.attr,
 	&dev_attr_index.attr,
+#if defined(CONFIG_MEDIA_CONTROLLER)
+	&dev_attr_media_dev.attr,
+#endif
 	NULL,
 };
-ATTRIBUTE_GROUPS(video_device);
+
+static const struct attribute_group video_device_group = {
+	.is_visible = video_device_attr_is_visible,
+	.attrs = video_device_attrs,
+};
+
+static const struct attribute_group *video_device_groups[] = {
+	&video_device_group,
+	NULL
+};
 
 /*
  *	Active devices
-- 
2.29.2