[libcamera-devel] [v2 2/3] Documentation: media: Document read-only subdevice
Laurent Pinchart
laurent.pinchart at ideasonboard.com
Sat Apr 4 03:35:45 CEST 2020
Hi Hans,
On Wed, Apr 01, 2020 at 01:54:13PM +0200, Hans Verkuil wrote:
> On 4/1/20 1:46 PM, Laurent Pinchart wrote:
> > On Wed, Apr 01, 2020 at 01:19:00PM +0200, Hans Verkuil wrote:
> >> 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 at jmondi.org>
> >>> ---
> >>> 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'm not opposed to that, but I don't think this should be specific to
> > the read-only API, as it's a shared requirement with thr R/W API. The
> > previous section, "V4L2 sub-device userspace API", doesn't mention media
> > devices.
>
> True.
>
> >> 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.
> >
> > VIDEO_V4L2_SUBDEV_API depends on MEDIA_CONTROLLER, but only part of that
> > actually depends on MEDIA_CONTROLLER in the code. I'd vote to simplify
> > this and make MEDIA_CONTROLLER a requirement for any userspace access
> > from userspace.
>
> I agree.
Does that mean we should rework it as a prerequisite for this series, as
part of the series, or separately ?
> >>> +
> >>> +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,
Laurent Pinchart
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