Re: [libcamera-devel] [PATCH 2/4] media: v4l2-dev: Add v4l2_device_register_ro_subdev_node()

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Hi Jacopo,

Thank you for your patch set!

On 24.03.2020 23:28, Jacopo Mondi wrote:
Add to the V4L2 code a function to register device nodes for video
subdevices in read-only mode.

Registering a device node in read-only mode is useful to expose to
userspace the current sub-device configuration, without allowing
application to change it by using the V4L2 subdevice ioctls.

Signed-off-by: Jacopo Mondi <jacopo@xxxxxxxxxx>
---
  drivers/media/v4l2-core/v4l2-device.c | 16 +++++++++++++++-
  drivers/media/v4l2-core/v4l2-subdev.c | 19 +++++++++++++++++++
  include/media/v4l2-dev.h              |  7 +++++++
  include/media/v4l2-device.h           | 10 ++++++++++
  4 files changed, 51 insertions(+), 1 deletion(-)

diff --git a/drivers/media/v4l2-core/v4l2-device.c b/drivers/media/v4l2-core/v4l2-device.c
index 63d6b147b21e..6f9dba36eda1 100644
--- a/drivers/media/v4l2-core/v4l2-device.c
+++ b/drivers/media/v4l2-core/v4l2-device.c
@@ -188,7 +188,8 @@ static void v4l2_device_release_subdev_node(struct video_device *vdev)
  	kfree(vdev);
  }
-int v4l2_device_register_subdev_nodes(struct v4l2_device *v4l2_dev)
+int __v4l2_device_register_subdev_nodes(struct v4l2_device *v4l2_dev,
+					bool read_only)
  {
  	struct video_device *vdev;
  	struct v4l2_subdev *sd;
@@ -217,6 +218,8 @@ int v4l2_device_register_subdev_nodes(struct v4l2_device *v4l2_dev)
  		vdev->fops = &v4l2_subdev_fops;
  		vdev->release = v4l2_device_release_subdev_node;
  		vdev->ctrl_handler = sd->ctrl_handler;
+		if (read_only)
+			vdev->flags |= V4L2_FL_RO_DEVNODE;

<snip>

@@ -331,6 +331,7 @@ static long subdev_do_ioctl(struct file *file, unsigned int cmd, void *arg)
  	struct v4l2_fh *vfh = file->private_data;
  #if defined(CONFIG_VIDEO_V4L2_SUBDEV_API)
  	struct v4l2_subdev_fh *subdev_fh = to_v4l2_subdev_fh(vfh);
+	bool ro_devnode = !!(vdev->flags & V4L2_FL_RO_DEVNODE);

So V4L2_FL_RO_DEVNODE is a bit mask, ...

<snip>

diff --git a/include/media/v4l2-dev.h b/include/media/v4l2-dev.h
index 48531e57cc5a..029873a338f2 100644
--- a/include/media/v4l2-dev.h
+++ b/include/media/v4l2-dev.h
@@ -82,11 +82,18 @@ struct v4l2_ctrl_handler;
   *	but the old crop API will still work as expected in order to preserve
   *	backwards compatibility.
   *	Never set this flag for new drivers.
+ * @V4L2_FL_RO_DEVNODE:
+ *	indicates that the video device node is registered in read-only mode.
+ *	The flag only applies to device nodes registered for sub-devices, it is
+ *	set by the core when the sub-devices device nodes are registered with
+ *	v4l2_device_register_ro_subdev_nodes() and used by the sub-device ioctl
+ *	handler to restrict access to some ioctl calls.
   */
  enum v4l2_video_device_flags {
  	V4L2_FL_REGISTERED		= 0,
  	V4L2_FL_USES_V4L2_FH		= 1,
  	V4L2_FL_QUIRK_INVERTED_CROP	= 2,
+	V4L2_FL_RO_DEVNODE		= 3,

... then V4L2_FL_RO_DEVNODE should rather be equal to 4, than to (V4L2_FL_USES_V4L2_FH | V4L2_FL_QUIRK_INVERTED_CROP)

Thanks,
Andrey

  };
/* Priority helper functions */
diff --git a/include/media/v4l2-device.h b/include/media/v4l2-device.h
index e0b8f2602670..0df667ba9938 100644
--- a/include/media/v4l2-device.h
+++ b/include/media/v4l2-device.h
@@ -183,6 +183,16 @@ void v4l2_device_unregister_subdev(struct v4l2_subdev *sd);
  int __must_check
  v4l2_device_register_subdev_nodes(struct v4l2_device *v4l2_dev);
+/**
+ * v4l2_device_register_ro_subdev_nodes - Registers read-only device nodes for
+ *      all subdevs of the v4l2 device that are marked with the
+ *      %V4L2_SUBDEV_FL_HAS_DEVNODE flag.
+ *
+ * @v4l2_dev: pointer to struct v4l2_device
+ */
+int __must_check
+v4l2_device_register_ro_subdev_nodes(struct v4l2_device *v4l2_dev);
+
  /**
   * v4l2_subdev_notify - Sends a notification to v4l2_device.
   *




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