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 <sakari.ailus@linux.intel.com>
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

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 <v4l2-subdev-format-whence>`
+	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
 ----------------------

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
 ========

Documentation/userspace-api/media/v4l/vidioc-g-dv-timings.rst

@@ -57,6 +57,10 @@ pointer to the struct :c:type:`v4l2_dv_timings`
 structure as argument. If the ioctl is not supported or the timing
 values are not correct, the driver returns ``EINVAL`` error code.
 
+Calling ``VIDIOC_SUBDEV_S_DV_TIMINGS`` on a subdev device node that has been
+registered in read-only mode is not allowed. An error is returned and the errno
+variable is set to ``-EPERM``.
+
 The ``linux/v4l2-dv-timings.h`` header can be used to get the timings of
 the formats in the :ref:`cea861` and :ref:`vesadmt` standards. If
 the current input or output does not support DV timings (e.g. if
@@ -81,6 +85,8 @@ ENODATA
 EBUSY
     The device is busy and therefore can not change the timings.
 
+EPERM
+    ``VIDIOC_SUBDEV_S_DV_TIMINGS`` has been called on a read-only subdevice.
 
 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
 

Documentation/userspace-api/media/v4l/vidioc-g-std.rst

@@ -66,6 +66,9 @@ video timings (e.g. if :ref:`VIDIOC_ENUMINPUT`
 does not set the ``V4L2_IN_CAP_STD`` flag), then ``ENODATA`` error code is
 returned.
 
+Calling ``VIDIOC_SUBDEV_S_STD`` on a subdev device node that has been registered
+in read-only mode is not allowed. An error is returned and the errno variable is
+set to ``-EPERM``.
 
 Return Value
 ============
@@ -79,3 +82,6 @@ EINVAL
 
 ENODATA
     Standard video timings are not supported for this input or output.
+
+EPERM
+    ``VIDIOC_SUBDEV_S_STD`` has been called on a read-only subdevice.

Documentation/userspace-api/media/v4l/vidioc-subdev-g-crop.rst

@@ -73,6 +73,11 @@ crop rectangles and stored in the sub-device file handle. Two
 applications querying the same sub-device would thus not interact with
 each other.
 
+If the subdev device node has been registered in read-only mode, calls to
+``VIDIOC_SUBDEV_S_CROP`` are only valid if the ``which`` field is set to
+``V4L2_SUBDEV_FORMAT_TRY``, otherwise an error is returned and the errno
+variable is set to ``-EPERM``.
+
 Drivers must not return an error solely because the requested crop
 rectangle doesn't match the device capabilities. They must instead
 modify the rectangle to match what the hardware can provide. The
@@ -123,3 +128,7 @@ EINVAL
     references a non-existing pad, the ``which`` field references a
     non-existing format, or cropping is not supported on the given
     subdev pad.
+
+EPERM
+    The ``VIDIOC_SUBDEV_S_CROP`` ioctl has been called on a read-only subdevice
+    and the ``which`` field is set to ``V4L2_SUBDEV_FORMAT_ACTIVE``.

Documentation/userspace-api/media/v4l/vidioc-subdev-g-fmt.rst

@@ -78,6 +78,11 @@ current links configuration or sub-device controls value. For instance,
 a low-pass noise filter might crop pixels at the frame boundaries,
 modifying its output frame size.
 
+If the subdev device node has been registered in read-only mode, calls to
+``VIDIOC_SUBDEV_S_FMT`` are only valid if the ``which`` field is set to
+``V4L2_SUBDEV_FORMAT_TRY``, otherwise an error is returned and the errno
+variable is set to ``-EPERM``.
+
 Drivers must not return an error solely because the requested format
 doesn't match the device capabilities. They must instead modify the
 format to match what the hardware can provide. The modified format
@@ -146,6 +151,9 @@ EINVAL
     ``pad`` references a non-existing pad, or the ``which`` field
     references a non-existing format.
 
+EPERM
+    The ``VIDIOC_SUBDEV_S_FMT`` ioctl has been called on a read-only subdevice
+    and the ``which`` field is set to ``V4L2_SUBDEV_FORMAT_ACTIVE``.
 
 ============
 

Documentation/userspace-api/media/v4l/vidioc-subdev-g-frame-interval.rst

@@ -65,6 +65,10 @@ struct
 contains the current frame interval as would be returned by a
 ``VIDIOC_SUBDEV_G_FRAME_INTERVAL`` call.
 
+Calling ``VIDIOC_SUBDEV_S_FRAME_INTERVAL`` on a subdev device node that has been
+registered in read-only mode is not allowed. An error is returned and the errno
+variable is set to ``-EPERM``.
+
 Drivers must not return an error solely because the requested interval
 doesn't match the device capabilities. They must instead modify the
 interval to match what the hardware can provide. The modified interval
@@ -118,3 +122,7 @@ EINVAL
     :c:type:`v4l2_subdev_frame_interval`
     ``pad`` references a non-existing pad, or the pad doesn't support
     frame intervals.
+
+EPERM
+    The ``VIDIOC_SUBDEV_S_FRAME_INTERVAL`` ioctl has been called on a read-only
+    subdevice.

Documentation/userspace-api/media/v4l/vidioc-subdev-g-selection.rst

@@ -53,6 +53,10 @@ function of the crop API, and more, are supported by the selections API.
 See :ref:`subdev` for more information on how each selection target
 affects the image processing pipeline inside the subdevice.
 
+If the subdev device node has been registered in read-only mode, calls to
+``VIDIOC_SUBDEV_S_SELECTION`` are only valid if the ``which`` field is set to
+``V4L2_SUBDEV_FORMAT_TRY``, otherwise an error is returned and the errno
+variable is set to ``-EPERM``.
 
 Types of selection targets
 --------------------------
@@ -123,3 +127,7 @@ EINVAL
     ``pad`` references a non-existing pad, the ``which`` field
     references a non-existing format, or the selection target is not
     supported on the given subdev pad.
+
+EPERM
+    The ``VIDIOC_SUBDEV_S_SELECTION`` ioctl has been called on a read-only
+    subdevice and the ``which`` field is set to ``V4L2_SUBDEV_FORMAT_ACTIVE``.