Hi Jacopo,
Some comments below:
On 3/27/20 11:35 PM, Jacopo Mondi wrote:
> Document a new kapi function to register subdev device nodes in read only
kAPI
> mode and for each affected ioctl report how access is restricted.
>
> Signed-off-by: Jacopo Mondi <jacopo@xxxxxxxxxx>
> ---
> Documentation/media/kapi/v4l2-subdev.rst | 44 +++++++++++++++++++
> Documentation/media/uapi/v4l/dev-subdev.rst | 5 +++
> .../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 ++++
> 8 files changed, 94 insertions(+)
>
> diff --git a/Documentation/media/kapi/v4l2-subdev.rst b/Documentation/media/kapi/v4l2-subdev.rst
> index 41ccb3e5c707..6506a673e6a1 100644
> --- a/Documentation/media/kapi/v4l2-subdev.rst
> +++ b/Documentation/media/kapi/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`.
Should we add something about creating a /dev/media device as well? It's basically
required for this functionality.
I think it might be a good idea to put v4l2_device_register_ro_subdev_nodes()
under #ifdef CONFIG_MEDIA_CONTROLLER so that this config *has* to be set in order
to be able to call this function. Or possibly have an explicit test in
__v4l2_device_register_subdev_nodes for the presence of a media device if
read_only is true.
> +
> +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 ioclt 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/media/uapi/v4l/dev-subdev.rst b/Documentation/media/uapi/v4l/dev-subdev.rst
> index 029bb2d9928a..6082f9c2f8f4 100644
> --- a/Documentation/media/uapi/v4l/dev-subdev.rst
> +++ b/Documentation/media/uapi/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 don't modify the device state. In such a case the sub-devices
don't -> do not
("don't" is a bit too informal)
> +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/media/uapi/v4l/vidioc-g-dv-timings.rst b/Documentation/media/uapi/v4l/vidioc-g-dv-timings.rst
> index e36dd2622857..611f94e4510a 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
mode calls -> mode, calls
> +``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
ditto.
> +``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
ditto
> +``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``.
>
Regards,
Hans
