[RFC PATCH 4/7] Documentation: Add Linux Motion Control documentation

David Jander
2025-02-27T16:28:50Z

Add general- and UAPI documentation for the Linux Motion Control
subsystem.

Signed-off-by: David Jander <david@protonic.nl>
---
Documentation/motion/index.rst | 18 +
Documentation/motion/motion-uapi.rst | 555 +++++++++++++++++++++++++++
Documentation/subsystem-apis.rst | 1 +
3 files changed, 574 insertions(+)
create mode 100644 Documentation/motion/index.rst
create mode 100644 Documentation/motion/motion-uapi.rst

diff --git a/Documentation/motion/index.rst b/Documentation/motion/index.rst
new file mode 100644
index 000000000000..eee1c180478d
--- /dev/null
+++ b/Documentation/motion/index.rst
@@ -0,0 +1,18 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==============================
+Linux Motion Control Subsystem
+==============================
+
+.. toctree::
+ :maxdepth: 1
+
+ motion-uapi
+ motion-drivers
+
+.. only:: subproject and html
+
+ Indices
+ =======
+
+ * :ref:`genindex`
diff --git a/Documentation/motion/motion-uapi.rst b/Documentation/motion/motion-uapi.rst
new file mode 100644
index 000000000000..f57d62bcacb3
--- /dev/null
+++ b/Documentation/motion/motion-uapi.rst
@@ -0,0 +1,555 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===================================
+Linux Motion Control Subsystem UAPI
+===================================
+
+The User-space API for the Linux Motion Control (LMC) subsystem consists of
+a device node ioctl() interface and driver-specific sysfs attributes. The
+devnodes use dynamically allocated MAJOR numbers and are named /dev/motionX
+by default, where X is the ordinal of the device in probe order starting at
+zero. The entries in /sys/class/motion/... can be used to further identify
+each device.
+
+If CONFIG_IIO is defined, then also a IIO trigger device is created, that
+can be accessed via /sys/bus/iio/devices/iiotriggerX and the usual IIO API.
+
+Motion Control Background
+=========================
+
+A motion controller device can control one or more actuators (motors), that
+can move individually or synchronously. To accommodate for that, the API
+has two distinct ways of specifying the actuator channel a specific function
+applies to. Some structs take a single channel number as an unsigned integer,
+while others apply to a set of channels represented in the form of a bitmask.
+A "motion" in general can be any type of mechanical movement or displacement.
+This can be carried out by any sort of motor or linear actuator. Types of
+motors can be (but not limited to) brushed- or brushless DC motors, induction
+motors, switched-reluctance motors, stepper motors, etc...
+The mechanical movement can be coupled directly to a position sensor, such as
+a linear encoder for example. Some actuators (such as stepper motors) can be
+absolutely positioned without the need of a sensor.
+A mechanical movement (motion) can be described by a (multi-dimensional-)
+position and its two common time-derivatives, speed and acceleration.
+Depending on the motion controller's capabilities, these can be specified in
+more or less detail. For example a motor connected to a simple on-off switch
+can only be conrolled at zero or maximum speed. If an encoder is present,
+position control becomes possible. A stepper motor controller OTOH, can
+often precisely control all three, position, speed and acceleration, and
+sometimes even higher order time-derivatives, such as jerk.
+
+.. _motion-profiles:
+
+Motion profiles
+===============
+
+Some motion controllers have means of specifying a motion (or acceleration)
+profile. This is generally represented by a curve of speed versus time. The
+most common types of such a curve are triangular, trapezoidal, dual-slope and
+S-curve (or an 8-point approximation thereof).
+A triangular profile is in essence a special case of the trapezoidal profile,
+where constant maximum speed is never reached. LMC supports the following 3
+types of motion profiles:
+
+1. Trapezoidal
+--------------
+
+A trapezoidal profile is comprised of 3 speed values: Start-, maximum- and
+stop speed. 2 acceleration values determine the start and ending slopes of
+the trapezoid. Start- and stop speed need not be zero, nor equal. Specific
+constraints may depend on the underlying conroller, but often speed below
+a certain value is hard to attain for example with a stepper motor controller
+if the interval time between steps is limited. In addition to the 3 speed
+and 2 acceleration values, a value for minimum constant speed time can also
+be specified. This time value determines the minimum time the movement needs
+to have a constant speed after acceleration, before deceleration begins.
+
+2. Dual-slope
+-------------
+
+A dual-slope profile, is similar to a trapezoidal profile, but both the
+acceleration and deceleration phase is split in two sections. The transition
+point is determined by a 4th speed value which lies between max(Vs, Ve)
+and Vmax, where Vs, Vmax and Ve are the trapezoidal profile values for
+Start-, maximum- and stop speed respectively. Since there are two segments
+to each acceleration phase, this profile has 4 acceleration values.
+
+3. S-curve (8-point) profile
+----------------------------
+
+An 8-point profile (approximation of an S-curve) takes the dual-slope profile
+one step further by adding a second intermediate speed value, which results
+in acceleration and deceleration phases split into 3 segments each. This
+profile type thus has 5 speed values and 6 acceleration values.
+
+Units
+=====
+
+The LMC subsystem measures all time values in nanoseconds, in the format
+mot_time_t (signed 64-bit integer). This is similar to the current
+imoplementation of ktime_t, so conversion to/from mot_time_t is simple.
+Internally ktime_t is used, but since ktime_t cannot be interfaced to
+user-space directly, the mot_time_t type is defined for use in the user-
+space API, but it currently is equivalent to ktime_t.
+
+All position, speed and acceleration values are assumed in machine-units by
+the kernel.
+Machine-units are defined by the respective motion controller hardware.
+For example for a stepper motor controller with 1/256 microstepping,
+one unit of distance is equivalent to one microstep. Each driver can specify
+scaling factors to convert units in user-space if needed. Since speed and
+acceleration values have an implied time component, these conversion factors
+also have an implied time conversion from machine-time units to nanoseconds.
+These conversion factors are each specified by a rational number represented
+by a numerator and a denominator. Both are unsigned 32bit integers.
+
+Machine units for position, speed and acceleration are signed 32-bit integers,
+but have a type definition as pos_raw_t, speed_raw_t and accel_raw_t
+respectively.
+
+Convention of signed position, speed and acceleration:
+Movement of one channel is unidimensional, meaning position can be above or
+below the origin (positive or negative respecively). Consequently, given
+a positive position, a positive speed represents a movement further away
+from the origin (position 0), while a negative speed value represents a
+movement towards the origin. The opposite is valid when starting from a
+negative position value.
+Analogous to what speed does to position, is what acceletation does to speed:
+Given positive speed, positive acceleration increments the speed, and given
+"negative" speed, negative acceleration decrements the speed (increments its
+absolute value).
+For movement profiles, the convention is that profile (acceleration-, speed-)
+values are strictly positive. The direction of movement is solely determined
+by the relative position (i.e. "positive" or "negative" displacement).
+
+
+IOCTL based interface description
+=================================
+
+API capabilities interrogation
+------------------------------
+
+``ioctl(fd, MOT_IOCTL_APIVER, null)``
+ Function return value is the LMC API version number, or -1 on error.
+
+``ioctl(fd, MOT_IOCTL_GET_CAPA, struct mot_capabilities *capa)``
+ Returns a struct mot_capabilities filled in.
+
+Basic motion control
+--------------------
+
+``ioctl(fd, MOT_IOCTL_GET_STATUS, struct mot_status *st)``
+ Takes a struct mot_status with the channel parameter filled in and
+ specifying the channel number for which to retrieve the status.
+ Returns the current speed and position (if supported) in the same
+ struct.
+
+``ioctl(fd, MOT_IOCTL_BASIC_RUN, struct mot_speed_duration *s)``
+ Takes a struct mot_speed_duration with the channel number, the speed and
+ optionally either a non-zero value of duration or a non-zero value of
+ distance. Note that one of these two values *must* be zero, otherwise
+ an error is returned. If both are zero, then a movement is started
+ with speed abs(speed) and direction the sign of speed for an indefinite
+ amount of time. If duration is non-zero, then the movement is stopped
+ at most after duration (in nanoseconds).
+
+``ioctl(fd, MOT_IOCTL_BASIC_STOP, u32 chmsk)``
+ Takes an unsigned 32-bit integer argument chmsk, which represents a bit-mask
+ of channels. All current movements of all specified channels are stopped at
+ once.
+
+Feedback control
+----------------
+
+``ioctl(fd, MOT_IOCTL_CONFIG_INPUT, struct mot_input *inp)``
+ Takes a struct mot_input with an index number, a function value and a channel
+ bit-mask among other flags. This configures one of the internal or external
+ feedback inputs of the motion controller for a certain function to act on a
+ set of channels as specified by the chmask bit-mask. A common type of feedback
+ input for example are end-stop switches on a 3D printer. If the function
+ parameter is set to MOT_INP_FUNC_NONE (0) then the specified input is
+ deconfigured.
+
+Profile-based control
+---------------------
+
+``ioctl(fd, MOT_IOCTL_SET_PROFILE, struct mot_profile *p)``
+ Adjust a motion profile for the specified slot index to the values supplied
+ by this struct. (MOT_MAX_PROFILES-1) is the highest accepted value for index.
+ The only values allowed for na (number of acceleration values) are 2, 4 or 6.
+ The only values allowed for nv (number of speed values) are 3, 4 or 5. A
+ specific driver can restrict these parameters even further. This is
+ communicated in the struct mot_capabilities data.
+
+``ioctl(fd, MOT_IOCTL_GET_PROFILE, struct mot_profile *p)``
+ Takes the index from the provided struct mot_profile and returns a the same
+ struct with the profile values filled in for that index. Returns an erro if
+ the index does not point to a valid profile.
+
+``ioctl(fd, MOT_IOCTL_START, struct mot_start *s)``
+ Takes a struct mot_start, that programs a motion to start immediately or
+ triggered by an event. The motion can be time-based if the duration parameter
+ is non-zero - in this case also the direction parameter is taken into
+ account - or distance-base. In the latter case the duration value must be
+ zero and in the former case, the distance value must be zero. The parameter
+ index specifies a motion profile to use for this movement.
+
+``ioctl(fd, MOT_IOCTL_START, struct mot_start *s)``
+ Takes a struct mot_stop. This struct contains a channel mask parameter chmask,
+ which contains a bit-mask of all channels this command applis to. All motions
+ of selected channels are stopped or prepared to be stopped by an event,
+ following the deceleration path of the selected motion profile that started
+ each respective motion or immediately, depending on the function of the
+ trigger input that has been armed for each respective channel (if applicable).
+
+Unimplemented future functionality
+----------------------------------
+
+``ioctl(fd, MOT_IOCTL_TORQUE_LIMITS, struct mot_torque *t)``
+ Analogous to motion profiles, torque limit curves can augment a motion profile
+ with a time-sectioned or position-sectioned profile of torque limit values.
+ Background for torque limit curves: In some cases, it is desired to limit the
+ torque (or force) of a movement depending on the position. For example when
+ hitting an end-stop. In the case of a sliding door for example, one might
+ require higher torque initially, while limiting torque during the reminder of
+ the movement, in order to detect stalls due to a person standing in the way
+ of the door without causing injuries. Torque control isn't always possible,
+ and sometimes only in a limited fashion. For example, on AC motors, the
+ controller could vary the gain of the V/f curve to reduce or increase the
+ stall or slip limit. Stepper- or BLDC motor could vary the run current limit,
+ or stall detect threshold.
+
+``ioctl(fd, MOT_IOCTL_VECTOR_START, struct mot_vector *v)``
+ Takes a struct mot_vector, that defines a multi-channel coordinated multi-
+ dimensional linear movement along an n-dimensional space vector following a
+ motion profile and optional torque curve. The chmask bit-mask specifies the
+ channels that participate in this movement. The vector dist[] has the same
+ size as the number of bits set in chmask and specifies the nth coordinate
+ of the vector. This would be the prime application for controlling 3D
+ printers, where tight coordination of several axes is required.
+
+Constants used in the ioctl API
+-------------------------------
+
+The following constants are used in different field of the structs provided
+to or returned by the ioctl API:
+
+.. code-block:: C
+
+ /* Trigger inputs and End Stop functions */
+ enum {
+ MOT_INP_FUNC_NONE = 0,
+ MOT_INP_FUNC_STOP,
+ MOT_INP_FUNC_STOP_POS,
+ MOT_INP_FUNC_STOP_NEG,
+ MOT_INP_FUNC_DECEL,
+ MOT_INP_FUNC_DECEL_POS,
+ MOT_INP_FUNC_DECEL_NEG,
+ MOT_INP_FUNC_START,
+ MOT_INP_FUNC_SIGNAL,
+ MOT_INP_FUNC_LAST
+ };
+
+These constants are used in for the field function of struct mot_input.
+This field specicies the function that a specified trigger input or end-stop
+should have on the selected channels. FUNC_STOP means immediately set the
+speed to zero, not following an acceleration curve, whereas FUNC_DECEL means
+stopping by folowwing the deceleration slope(s) specified in the motion
+profile that started the motion. _NEG and _POS restrict the action of the
+trigger to act on backwards (_NEG, into negative distance) or forwards (_POS,
+into positive distance) motion. If this is specified and the controller
+supports this, this will avoid a situation in which an actuator that has
+two end-stops for example at each extreme of the actuation range will stop
+a movement to "the left" when the "right" end-stop is accidentally triggered.
+MOT_INP_FUNC_SIGNAL will only generate an event to user-space or the IIO
+trigger and not affect the motion directly.
+
+.. code-block:: C
+
+ /* Config trigger input edge */
+ #define MOT_EDGE_RISING 0
+ #define MOT_EDGE_FALLING 1
+
+These constants are used for the edge parameter in ioctl structs. It specifies
+the trigger input to be high-active (MOT_EDGE_RISING) or low-active
+(MOT_EDGE_FALLING).
+
+.. code-block:: C
+
+ /* Start/Stop conditions */
+ enum {
+ MOT_WHEN_IMMEDIATE = 0,
+ MOT_WHEN_INT_TRIGGER,
+ MOT_WHEN_EXT_TRIGGER,
+ MOT_WHEN_NEXT,
+ MOT_WHEN_LAST
+ };
+
+Each parameter called "when" in the ioctl structs can potentially take one
+of the MOT_WHEN_xxx values. INT_TRIGGER will execute the corresponding action
+on an internal trigger signal, while EXT_TRIGGER will execute the
+corresponding action on an external trigger signal (GPIO specified in fwnode).
+MOT_WHEN_NEXT makes it possible to prepare a new motion that will start when
+the currently active motion ends. By listening to poll() events of type
+MOT_EVENT_TARGET and then sending the next motion start command with
+MOT_WHEN_NEXT, it is possible to produce an uninterrupted stream of
+consecutive movements. In that case MOT_EVENT_TARGET correspondes to the
+end of the preceding motion that ended when the current "next" motion is
+started, freeing the slot for the new "next" motion after that.
+
+.. code-block:: C
+
+ /* Event types */
+ enum {
+ MOT_EVENT_NONE = 0,
+ MOT_EVENT_TARGET,
+ MOT_EVENT_STOP,
+ MOT_EVENT_INPUT,
+ MOT_EVENT_STALL,
+ MOT_EVENT_ERROR,
+ MOT_EVENT_LAST
+ };
+
+These are constants for the event field of struct mot_event. See below for
+a description of events ingeneral. The type of events reported to user space
+are "target reached" (MOT_EVENT_TARGET), "stopped by internal trigger"
+(MOT_EVENT_STOP), "external trigger" (MOT_EVENT_INPUT) or different fault
+conditions (stall or generic error event). MOT_EVENT_STALL can be produced by
+some motion controllers that can react to motor stalls in a natural way, for
+example in the case of force-based obstacle- or end stop detection.
+
+.. code-block:: C
+
+ #define MOT_DIRECTION_LEFT 0
+ #define MOT_DIRECTION_RIGHT 1
+
+These constants are used for specifying direction of movement. LEFT in this
+case means decreasing position value, while RIGHT is increasing position
+value.
+
+Structs used in the ioctl API
+-----------------------------
+
+.. code-block:: C
+
+ #define MOT_FEATURE_SPEED BIT(0)
+ #define MOT_FEATURE_ACCEL BIT(1)
+ #define MOT_FEATURE_ENCODER BIT(2)
+ #define MOT_FEATURE_PROFILE BIT(3)
+ #define MOT_FEATURE_VECTOR BIT(4)
+
+ enum motion_device_type {
+ MOT_TYPE_DC_MOTOR,
+ MOT_TYPE_AC_MOTOR,
+ MOT_TYPE_STEPPER,
+ MOT_TYPE_BLDC,
+ MOT_TYPE_SRM,
+ MOT_TYPE_LINEAR,
+ MOT_TYPE_LAST
+ };
+
+ struct mot_capabilities {
+ __u32 features;
+ __u8 type;
+ __u8 num_channels;
+ __u8 num_int_triggers;
+ __u8 num_ext_triggers;
+ __u8 max_profiles;
+ __u8 max_vpoints;
+ __u8 max_apoints;
+ __u8 reserved1;
+ __u32 subdiv;
+ __u32 speed_conv_mul;
+ __u32 speed_conv_div;
+ __u32 accel_conv_mul;
+ __u32 accel_conv_div;
+ __u32 reserved2;
+ };
+
+The field ``features`` is a bit-mask of MOT_FEATURE_XX flags. The feature
+SPEED means that the motion controller supports adjustable speed. All but the
+most simple (on/off switch) controllers will have this bit set.
+ACCEL means that the motion controller supports specifying acceleration
+values. Not that this is not sufficient to indicate that motion profiles can
+be used.
+ENCODER meaans that the motion controller has a built-in or fixed connected
+position encoder. If this bit is set, the position values returned by
+MOT_IOCTL_GET_STATUS can be assumed to be accurate. I.e. slip and/or skipped
+steps are properly taken into account.
+PROFILE means that the motion controller supports setting motion profiles.
+How many profiles, and how many speed and/or acceleration values are supported
+is inticated by the fields ``max_profiles``, ``max_vpoints`` and
+``max_apoints``.
+The field ``type`` can take any of the values MOT_TYPE_XX and is only
+informative. The fields ``num_channels``, ``num_int_triggers`` and
+``num_ext_triggers`` specify the supported number of channels, internal
+triggers and configured external triggers respectively.
+The field ``subdiv`` can have a different meaning, depending on the type of
+motion controller. For example for stepper motors, this typically indicates
+the microstepping divider. If this number is not 1, this means that the value
+of distance divided by ``subdiv`` will give the amount of machine-natural
+mechanical units of distance (whole steps in case of a stepper motor).
+The 4 different ``_conv_`` fields specify two rational conversion factors for
+speed and acceleration respectively. All unit conversions of speed and
+acceleration are done in user-space. The kernel only provides these numbers
+to user-space as part of physical characteristics of the motion controller.
+If the driver does not specify these values, or they lack a defined meaning,
+all four of these fields will have a value of 1, so no zero-division can
+happen and the conversion is just 1:1.
+
+.. code-block:: C
+
+ struct mot_speed_duration {
+ __u32 channel;
+ speed_raw_t speed;
+ mot_time_t duration;
+ pos_raw_t distance;
+ __u32 reserved[3];
+ };
+
+This struct is used int the ioctl MOT_IOCTL_BASIC_RUN to start a new motion.
+There are different ways of using BASIC_RUN:
+
+ 1. Specify only ``channel`` and ``speed``. All other fields are zero. This
+ starts a new motion for indefinte amount of time.
+
+ 2. Additionally to 1. specify non-zero ``duration``. Starts a timed motion
+ that will stop after the specified time.
+
+ 3. Additionally to 1. specify non-zero ``distance``. Starts a motion until
+ the position specified by distance from starting point is reached.
+
+Non zero values for both ``duration`` and ``distance`` will result in an
+error (-EINVAL).
+
+.. code-block:: C
+
+ struct mot_status {
+ __u32 channel;
+ pos_raw_t position;
+ speed_raw_t speed;
+ __u32 local_inputs;
+ };
+
+This struct is used for the ioctl MOT_IOCTL_GET_STATUS. The caller needs to
+set the field ``channel`` to the channel number it wants to request status
+information from. After a successful call, the field ``speed`` will contain
+the current speed of movement of the channel, and ``position`` will contain
+the actual position if this is supported by the hardware. The field
+``local_inputs`` contains a bit-mask of the state of all internal trigger
+inputs.
+
+.. code-block:: C
+
+ struct mot_input {
+ __u32 index;
+ __u8 external;
+ __u8 edge;
+ __u8 reserved[2];
+ __u32 function;
+ channel_mask_t chmask;
+ };
+
+This struct is used for configuring internal or external trigger inputs. The
+input ``index`` is specific to the flag ``external``. This means that internal
+and external trigger inputs can have the same ``index`` value. The exact
+``index`` value for internal inputs is determined by the driver and should be
+enumerated from 0 onwards. For external GPIO intputs the index number is the
+order in which they appear in the fwnode (DT property "motion,gpios").
+``edge`` indicates the polarity of the input electrical signal,
+MOT_EDGE_FALLING or MOT_EDGE_RISING. Please note, that any GPIO polarity is
+also applied. So if a GPIO input is GPIO_ACTIVE_LOW and ``edge`` is
+MOT_EDGE_FALLING, the resulting trigger will occur on the rising edge of the
+electrical input signal.
+``function`` takes any of the MOT_INP_FUNC_XX values mentioned above.
+``chmask`` is a bit-mask of all motion channels this input will affect.
+
+.. code-block:: C
+
+ struct mot_profile {
+ __u32 index;
+ mot_time_t tvmax;
+ mot_time_t tvzero;
+ __u8 na;
+ __u8 nv;
+ __u8 reserved[2];
+ accel_raw_t acc[MOT_MAX_ACCELPTS];
+ speed_raw_t vel[MOT_MAX_SPEEDPTS];
+ };
+
+Used to define a motion profile. See :ref:`motion-profiles` for a general
+explanation of motion profiles. ``nv`` and ``na`` are the number of valid
+entries in the ``velp[]`` and ``acc[]`` arrays respectively. ``tvmax`` is the
+minimum amount of time the constant (maximum) speed needs to be maintained
+after ending the acceleration phase, and before beginning the deceleration
+phase. ``tvzero`` is the minimum amount of time the speed needs to be 0,
+before a new motion is started in the same direction as the preceding motion.
+In other words, analogous to ``tvmax``, a new acceleration with opposite sign
+of the preceding deceleration. This time value is not applied if the next
+motion is into the opposite direction, since there will be no change in the
+sign of the resulting acceleration.
+
+.. code-block:: C
+
+ struct mot_start {
+ __u32 channel;
+ __u8 direction;
+ __u8 index;
+ __u8 when;
+ __u8 reserved1;
+ mot_time_t duration;
+ pos_raw_t distance;
+ __u32 reserved2;
+ };
+
+Used for the MOT_IOCTL_START ioctl. The ``index`` parameter is the index
+number of the motion profile that will be used for the motion. ``when``
+determines when (under which condition) the motion is started, and takes
+the value of any of the MOT_WHEN_XX constants. Like in the case of struct
+mot_speed_duration, here also ``duration`` and ``distance`` are mutually
+exclusive, meaning that one of both must always be zero.
+
+.. code-block:: C
+
+ struct mot_stop {
+ channel_mask_t chmask;
+ __u8 when;
+ __u8 reserved[3];
+ };
+
+This struct is used to schedule a deceleration of a running motion of all
+channels specified by ``chmask``, by following the deceleration part of their
+respecive motion profiles. ``when`` takes any of the MOT_WHEN_XX constants and
+determines the condition that triggers the deceleration.
+
+Event handling with (e)poll/select
+----------------------------------
+
+When user-space opens the devnode for reading, (e)poll() can be used to wait
+for motion events, using the (E)POLLIN flag. These events can then be read by
+calling read() on the file-descriptor with a buffer size equal to the size of
+struct mot_event. Make sure the file is opened in non-blocking mode for
+for reliable event processing. read() will return -EAGAIN in this case.
+
+.. code-block:: C
+
+ struct mot_event {
+ __u32 channel;
+ __u8 event;
+ __u8 reserved1[3];
+ pos_raw_t position;
+ speed_raw_t speed;
+ mot_time_t timestamp;
+ __u32 input_index;
+ __u32 reserved2;
+ };
+
+``event`` can take any of the MOT_EVENT_XX constants, and is used to determine
+the type of event. The values of ``position``, ``speed`` and ``timestamp`` are
+the corresponding motion position, speed and time at which the event ocurred.
+Not all drivers support reporting a position value. If they don't that field
+will always be zero. ``input_index`` is the index number of the external input
+(in the case of a MOT_EVENT_INPUT event), or the index number of the internal
+input (in the case of a MOT_EVENT_STOP event), that caused the event. For
+other event types than MOT_EVENT_INPUT or MOT_EVENT_STOP, this field has no
+meaning and will be zero.
diff --git a/Documentation/subsystem-apis.rst b/Documentation/subsystem-apis.rst
index b52ad5b969d4..0707b297f150 100644
--- a/Documentation/subsystem-apis.rst
+++ b/Documentation/subsystem-apis.rst
@@ -90,3 +90,4 @@ Other subsystems
peci/index
wmi/index
tee/index
+ motion/index
--
2.47.2