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media: Documentation: media: Document read-only subdevice

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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>
This commit is contained in:
Jacopo Mondi
2020-05-07 17:12:49 +02:00
committed by Mauro Carvalho Chehab
parent 18200e9e24
commit 3fb0ee8b3b
8 changed files with 94 additions and 0 deletions
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44
Documentation/driver-api/media/v4l2-subdev.rst
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@@ -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
----------------------