[PATCH 1/4] Documentation: media: Document read-only subdevice

[Jacopo Mondi <jacopo@jmondi.org>]

Document a new kapi function to register subdev device nodes in read only
mode and for each affected ioctl report how access is restricted.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
---
 Documentation/media/kapi/v4l2-subdev.rst      | 38 +++++++++++++++++++
 .../media/uapi/v4l/vidioc-g-dv-timings.rst    |  6 +++
 Documentation/media/uapi/v4l/vidioc-g-std.rst |  6 +++
 .../media/uapi/v4l/vidioc-subdev-g-crop.rst   |  9 +++++
 .../media/uapi/v4l/vidioc-subdev-g-fmt.rst    |  8 ++++
 .../v4l/vidioc-subdev-g-frame-interval.rst    |  8 ++++
 .../uapi/v4l/vidioc-subdev-g-selection.rst    |  8 ++++
 7 files changed, 83 insertions(+)

diff --git a/Documentation/media/kapi/v4l2-subdev.rst b/Documentation/media/kapi/v4l2-subdev.rst
index 29e07e23f888..26edb4178eea 100644
--- a/Documentation/media/kapi/v4l2-subdev.rst
+++ b/Documentation/media/kapi/v4l2-subdev.rst
@@ -327,6 +327,44 @@ 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.
+
+Although, it is sometime useful to report to userspace the current subdevice
+configuration through a read-only API, that do not allow any change to the
+device parameters but allows userspace applications to interface to the
+subdevice device node and inspect them. To register a read-only device node for
+all the registered sub-devices marked with ``V4L2_SUBDEV_FL_HAS_DEVNODE``
+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_FRAME_INTERVAL``
+``VIDIOC_SUBDEV_S_SELECTION``
+``VIDIOC_SUBDEV_S_DV_TIMINGS``
+``VIDIOC_SUBDEV_S_CROP``
+``VIDIOC_SUBDEV_S_STD``
+
+``VIDIOC_SUBDEV_S_FMT``, ``VIDIOC_SUBDEV_S_SELECTION`` and
+``VIDIOC_SUBDEV_S_CROP`` are only allowed on a read-only subdevice device node
+only if the format to modify is set to ``V4L2_SUBDEV_FORMAT_TRY`` (
+:ref:`v4l2_subdev_format_whence <v4l2-subdev-format-whence>`).
+
+``VIDIOC_SUBDEV_S_FRAME_INTERVAL``, ``VIDIOC_S_DV_TIMINGS`` and
+``VIDIOC_SUBDEV_S_STD`` are always disallowed on a read-only subdevice node.
+
+In case the ioclt is not allowed at all or the format to modify is set to
+``V4L2_SUBDEV_FORMAT_ACTIVE``, the core returns a negative error code and
+sets the errno variable to ``-EPERM``.
 
 I2C sub-device drivers
 ----------------------
diff --git a/Documentation/media/uapi/v4l/vidioc-g-dv-timings.rst b/Documentation/media/uapi/v4l/vidioc-g-dv-timings.rst
index 5712bd48e687..435d955aaf85 100644
--- a/Documentation/media/uapi/v4l/vidioc-g-dv-timings.rst
+++ b/Documentation/media/uapi/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}|
 
diff --git a/Documentation/media/uapi/v4l/vidioc-g-std.rst b/Documentation/media/uapi/v4l/vidioc-g-std.rst
index e633e42e3910..e220b38b859f 100644
--- a/Documentation/media/uapi/v4l/vidioc-g-std.rst
+++ b/Documentation/media/uapi/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.
diff --git a/Documentation/media/uapi/v4l/vidioc-subdev-g-crop.rst b/Documentation/media/uapi/v4l/vidioc-subdev-g-crop.rst
index 632ee053accc..62f5d9870ca7 100644
--- a/Documentation/media/uapi/v4l/vidioc-subdev-g-crop.rst
+++ b/Documentation/media/uapi/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``.
diff --git a/Documentation/media/uapi/v4l/vidioc-subdev-g-fmt.rst b/Documentation/media/uapi/v4l/vidioc-subdev-g-fmt.rst
index 472577bd1745..3a2f64bb00e7 100644
--- a/Documentation/media/uapi/v4l/vidioc-subdev-g-fmt.rst
+++ b/Documentation/media/uapi/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``.
 
 ============
 
diff --git a/Documentation/media/uapi/v4l/vidioc-subdev-g-frame-interval.rst b/Documentation/media/uapi/v4l/vidioc-subdev-g-frame-interval.rst
index 4b1b4bc78bfe..34aa39096e3d 100644
--- a/Documentation/media/uapi/v4l/vidioc-subdev-g-frame-interval.rst
+++ b/Documentation/media/uapi/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.
diff --git a/Documentation/media/uapi/v4l/vidioc-subdev-g-selection.rst b/Documentation/media/uapi/v4l/vidioc-subdev-g-selection.rst
index fc73d27e6d74..abd046cef612 100644
--- a/Documentation/media/uapi/v4l/vidioc-subdev-g-selection.rst
+++ b/Documentation/media/uapi/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``.
-- 
2.25.1