[libcamera-devel] [v2 2/3] Documentation: media: Document read-only subdevice
Laurent Pinchart
laurent.pinchart at ideasonboard.com
Wed Apr 1 13:46:11 CEST 2020
Hi Hans,
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.
> 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.
> > +
> > +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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