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@xxxxxxxxxxxxxxx>
Signed-off-by: Jacopo Mondi <jacopo@xxxxxxxxxx>
---
.../driver-api/media/v4l2-subdev.rst | 44 +++++++++++++++++++
.../userspace-api/media/v4l/dev-subdev.rst | 5 +++
.../media/v4l/vidioc-g-dv-timings.rst | 6 +++
.../userspace-api/media/v4l/vidioc-g-std.rst | 6 +++
.../media/v4l/vidioc-subdev-g-crop.rst | 9 ++++
.../media/v4l/vidioc-subdev-g-fmt.rst | 8 ++++
.../v4l/vidioc-subdev-g-frame-interval.rst | 8 ++++
.../media/v4l/vidioc-subdev-g-selection.rst | 8 ++++
8 files changed, 94 insertions(+)
diff --git a/Documentation/driver-api/media/v4l2-subdev.rst b/Documentation/driver-api/media/v4l2-subdev.rst
index 41ccb3e5c7075..6ced2381952ab 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 <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
----------------------
diff --git a/Documentation/userspace-api/media/v4l/dev-subdev.rst b/Documentation/userspace-api/media/v4l/dev-subdev.rst
index 0c1a5f50ee218..134d2fb909fa4 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
========
diff --git a/Documentation/userspace-api/media/v4l/vidioc-g-dv-timings.rst b/Documentation/userspace-api/media/v4l/vidioc-g-dv-timings.rst
index 84806a893cb7b..9a035a4ea0f05 100644
--- a/Documentation/userspace-api/media/v4l/vidioc-g-dv-timings.rst
+++ b/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}|
diff --git a/Documentation/userspace-api/media/v4l/vidioc-g-std.rst b/Documentation/userspace-api/media/v4l/vidioc-g-std.rst
index b0bdb719d4050..6d8cb7f29ac62 100644
--- a/Documentation/userspace-api/media/v4l/vidioc-g-std.rst
+++ b/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.
diff --git a/Documentation/userspace-api/media/v4l/vidioc-subdev-g-crop.rst b/Documentation/userspace-api/media/v4l/vidioc-subdev-g-crop.rst
index 8d9fc13015a61..615e3efdf9356 100644
--- a/Documentation/userspace-api/media/v4l/vidioc-subdev-g-crop.rst
+++ b/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``.
diff --git a/Documentation/userspace-api/media/v4l/vidioc-subdev-g-fmt.rst b/Documentation/userspace-api/media/v4l/vidioc-subdev-g-fmt.rst
index 69d60e18664ba..909ee9f908673 100644
--- a/Documentation/userspace-api/media/v4l/vidioc-subdev-g-fmt.rst
+++ b/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``.
============
diff --git a/Documentation/userspace-api/media/v4l/vidioc-subdev-g-frame-interval.rst b/Documentation/userspace-api/media/v4l/vidioc-subdev-g-frame-interval.rst
index b61baaf116240..51e1bff797f01 100644
--- a/Documentation/userspace-api/media/v4l/vidioc-subdev-g-frame-interval.rst
+++ b/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.
diff --git a/Documentation/userspace-api/media/v4l/vidioc-subdev-g-selection.rst b/Documentation/userspace-api/media/v4l/vidioc-subdev-g-selection.rst
index 981c95df2dec0..06c9553ac48f5 100644
--- a/Documentation/userspace-api/media/v4l/vidioc-subdev-g-selection.rst
+++ b/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``.
--
2.26.1
