From 3fb0ee8b3b79ee9c8fb7769bdf802bffeae7e085 Mon Sep 17 00:00:00 2001 From: Jacopo Mondi Date: Thu, 7 May 2020 17:12:49 +0200 Subject: media: Documentation: media: Document read-only subdevice Document a new kAPI function to register subdev device nodes in read only mode and for each affected ioctl report how access is restricted. Acked-by: Sakari Ailus Signed-off-by: Jacopo Mondi Signed-off-by: Hans Verkuil Signed-off-by: Mauro Carvalho Chehab --- Documentation/driver-api/media/v4l2-subdev.rst | 44 ++++++++++++++++++++++++++ 1 file changed, 44 insertions(+) (limited to 'Documentation/driver-api') diff --git a/Documentation/driver-api/media/v4l2-subdev.rst b/Documentation/driver-api/media/v4l2-subdev.rst index 41ccb3e5c707..6ced2381952a 100644 --- a/Documentation/driver-api/media/v4l2-subdev.rst +++ b/Documentation/driver-api/media/v4l2-subdev.rst @@ -332,6 +332,50 @@ Private ioctls All ioctls not in the above list are passed directly to the sub-device driver through the core::ioctl operation. +Read-only sub-device userspace API +---------------------------------- + +Bridge drivers that control their connected subdevices through direct calls to +the kernel API realized by :c:type:`v4l2_subdev_ops` structure do not usually +want userspace to be able to change the same parameters through the subdevice +device node and thus do not usually register any. + +It is sometimes useful to report to userspace the current subdevice +configuration through a read-only API, that does not permit applications to +change to the device parameters but allows interfacing to the subdevice device +node to inspect them. + +For instance, to implement cameras based on computational photography, userspace +needs to know the detailed camera sensor configuration (in terms of skipping, +binning, cropping and scaling) for each supported output resolution. To support +such use cases, bridge drivers may expose the subdevice operations to userspace +through a read-only API. + +To create a read-only device node for all the subdevices registered with the +``V4L2_SUBDEV_FL_HAS_DEVNODE`` set, the :c:type:`v4l2_device` driver should call +:c:func:`v4l2_device_register_ro_subdev_nodes`. + +Access to the following ioctls for userspace applications is restricted on +sub-device device nodes registered with +:c:func:`v4l2_device_register_ro_subdev_nodes`. + +``VIDIOC_SUBDEV_S_FMT``, +``VIDIOC_SUBDEV_S_CROP``, +``VIDIOC_SUBDEV_S_SELECTION``: + + These ioctls are only allowed on a read-only subdevice device node + for the :ref:`V4L2_SUBDEV_FORMAT_TRY ` + formats and selection rectangles. + +``VIDIOC_SUBDEV_S_FRAME_INTERVAL``, +``VIDIOC_SUBDEV_S_DV_TIMINGS``, +``VIDIOC_SUBDEV_S_STD``: + + These ioctls are not allowed on a read-only subdevice node. + +In case the ioctl is not allowed, or the format to modify is set to +``V4L2_SUBDEV_FORMAT_ACTIVE``, the core returns a negative error code and +the errno variable is set to ``-EPERM``. I2C sub-device drivers ---------------------- -- cgit v1.2.3