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-rw-r--r--Documentation/userspace-api/media/v4l/dev-subdev.rst5
1 files changed, 5 insertions, 0 deletions
diff --git a/Documentation/userspace-api/media/v4l/dev-subdev.rst b/Documentation/userspace-api/media/v4l/dev-subdev.rst
index 0c1a5f50ee21..134d2fb909fa 100644
--- a/Documentation/userspace-api/media/v4l/dev-subdev.rst
+++ b/Documentation/userspace-api/media/v4l/dev-subdev.rst
@@ -39,6 +39,11 @@ will feature a character device node on which ioctls can be called to
Sub-device character device nodes, conventionally named
``/dev/v4l-subdev*``, use major number 81.
+Drivers may opt to limit the sub-device character devices to only expose
+operations that do not modify the device state. In such a case the sub-devices
+are referred to as ``read-only`` in the rest of this documentation, and the
+related restrictions are documented in individual ioctls.
+
Controls
========