[libcamera-devel] [v2 2/3] Documentation: media: Document read-only subdevice
Jacopo Mondi
jacopo at jmondi.org
Mon Apr 6 13:07:32 CEST 2020
Hi
On Sat, Apr 04, 2020 at 04:35:45AM +0300, Laurent Pinchart wrote:
> 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 ?
I would, for the moment, make v4l2_device_register_ro_subdev_nodes and
v4l2_device_register_subdev_nodes dependendent on
VIDEO_V4L2_SUBDEV_API, so that they're only available to drivers
actually wanting userspace access. Then when moving everything to
MEDIA_CONTROLLER, they can be moved to.
However, this is not clear to me
https://elixir.bootlin.com/linux/latest/source/drivers/media/v4l2-core/v4l2-subdev.c#L338
What is the reason to have some of the subdev ioctls protected by
VIDEO_V4L2_SUBDEV_API, while the v4l2-control related ones are not ?
Thanks
j
>
> > >>> +
> > >>> +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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