docs: driver-api: Add Surface DTX driver documentation

Add documentation for the user-space interface of the Surface DTX
(detachment system) driver, used on Microsoft Surface Book series
devices.

Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Link: https://lore.kernel.org/r/20210308184819.437438-4-luzmaximilian@gmail.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

2 files changed, 719 insertions, 0 deletions

diff --git a/Documentation/driver-api/surface_aggregator/clients/dtx.rst b/Documentation/driver-api/surface_aggregator/clients/dtx.rst
new file mode 100644
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@@ -0,0 +1,718 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+.. |__u16| replace:: :c:type:`__u16 <__u16>`
+.. |sdtx_event| replace:: :c:type:`struct sdtx_event <sdtx_event>`
+.. |sdtx_event_code| replace:: :c:type:`enum sdtx_event_code <sdtx_event_code>`
+.. |sdtx_base_info| replace:: :c:type:`struct sdtx_base_info <sdtx_base_info>`
+.. |sdtx_device_mode| replace:: :c:type:`struct sdtx_device_mode <sdtx_device_mode>`
+
+======================================================
+User-Space DTX (Clipboard Detachment System) Interface
+======================================================
+
+The ``surface_dtx`` driver is responsible for proper clipboard detachment
+and re-attachment handling. To this end, it provides the ``/dev/surface/dtx``
+device file, through which it can interface with a user-space daemon. This
+daemon is then ultimately responsible for determining and taking necessary
+actions, such as unmounting devices attached to the base,
+unloading/reloading the graphics-driver, user-notifications, etc.
+
+There are two basic communication principles used in this driver: Commands
+(in other parts of the documentation also referred to as requests) and
+events. Commands are sent to the EC and may have a different implications in
+different contexts. Events are sent by the EC upon some internal state
+change. Commands are always driver-initiated, whereas events are always
+initiated by the EC.
+
+.. contents::
+
+Nomenclature
+============
+
+* **Clipboard:**
+  The detachable upper part of the Surface Book, housing the screen and CPU.
+
+* **Base:**
+  The lower part of the Surface Book from which the clipboard can be
+  detached, optionally (model dependent) housing the discrete GPU (dGPU).
+
+* **Latch:**
+  The mechanism keeping the clipboard attached to the base in normal
+  operation and allowing it to be detached when requested.
+
+* **Silently ignored commands:**
+  The command is accepted by the EC as a valid command and acknowledged
+  (following the standard communication protocol), but the EC does not act
+  upon it, i.e. ignores it.e upper part of the
+
+
+Detachment Process
+==================
+
+Warning: This part of the documentation is based on reverse engineering and
+testing and thus may contain errors or be incomplete.
+
+Latch States
+------------
+
+The latch mechanism has two major states: *open* and *closed*. In the
+*closed* state (default), the clipboard is secured to the base, whereas in
+the *open* state, the clipboard can be removed by a user.
+
+The latch can additionally be locked and, correspondingly, unlocked, which
+can influence the detachment procedure. Specifically, this locking mechanism
+is intended to prevent the dGPU, positioned in the base of the device, from
+being hot-unplugged while in use. More details can be found in the
+documentation for the detachment procedure below. By default, the latch is
+unlocked.
+
+Detachment Procedure
+--------------------
+
+Note that the detachment process is governed fully by the EC. The
+``surface_dtx`` driver only relays events from the EC to user-space and
+commands from user-space to the EC, i.e. it does not influence this process.
+
+The detachment process is started with the user pressing the *detach* button
+on the base of the device or executing the ``SDTX_IOCTL_LATCH_REQUEST`` IOCTL.
+Following that:
+
+1. The EC turns on the indicator led on the detach-button, sends a
+   *detach-request* event (``SDTX_EVENT_REQUEST``), and awaits further
+   instructions/commands. In case the latch is unlocked, the led will flash
+   green. If the latch has been locked, the led will be solid red
+
+2. The event is, via the ``surface_dtx`` driver, relayed to user-space, where
+   an appropriate user-space daemon can handle it and send instructions back
+   to the EC via IOCTLs provided by this driver.
+
+3. The EC waits for instructions from user-space and acts according to them.
+   If the EC does not receive any instructions in a given period, it will
+   time out and continue as follows:
+
+   - If the latch is unlocked, the EC will open the latch and the clipboard
+     can be detached from the base. This is the exact behavior as without
+     this driver or any user-space daemon. See the ``SDTX_IOCTL_LATCH_CONFIRM``
+     description below for more details on the follow-up behavior of the EC.
+
+   - If the latch is locked, the EC will *not* open the latch, meaning the
+     clipboard cannot be detached from the base. Furthermore, the EC sends
+     an cancel event (``SDTX_EVENT_CANCEL``) detailing this with the cancel
+     reason ``SDTX_DETACH_TIMEDOUT`` (see :ref:`events` for details).
+
+Valid responses by a user-space daemon to a detachment request event are:
+
+- Execute ``SDTX_IOCTL_LATCH_REQUEST``. This will immediately abort the
+  detachment process. Furthermore, the EC will send a detach-request event,
+  similar to the user pressing the detach-button to cancel said process (see
+  below).
+
+- Execute ``SDTX_IOCTL_LATCH_CONFIRM``. This will cause the EC to open the
+  latch, after which the user can separate clipboard and base.
+
+  As this changes the latch state, a *latch-status* event
+  (``SDTX_EVENT_LATCH_STATUS``) will be sent once the latch has been opened
+  successfully. If the EC fails to open the latch, e.g. due to hardware
+  error or low battery, a latch-cancel event (``SDTX_EVENT_CANCEL``) will be
+  sent with the cancel reason indicating the specific failure.
+
+  If the latch is currently locked, the latch will automatically be
+  unlocked before it is opened.
+
+- Execute ``SDTX_IOCTL_LATCH_HEARTBEAT``. This will reset the internal timeout.
+  No other actions will be performed, i.e. the detachment process will neither
+  be completed nor canceled, and the EC will still be waiting for further
+  responses.
+
+- Execute ``SDTX_IOCTL_LATCH_CANCEL``. This will abort the detachment process,
+  similar to ``SDTX_IOCTL_LATCH_REQUEST``, described above, or the button
+  press, described below. A *generic request* event (``SDTX_EVENT_REQUEST``)
+  is send in response to this. In contrast to those, however, this command
+  does not trigger a new detachment process if none is currently in
+  progress.
+
+- Do nothing. The detachment process eventually times out as described in
+  point 3.
+
+See :ref:`ioctls` for more details on these responses.
+
+It is important to note that, if the user presses the detach button at any
+point when a detachment operation is in progress (i.e. after the EC has sent
+the initial *detach-request* event (``SDTX_EVENT_REQUEST``) and before it
+received the corresponding response concluding the process), the detachment
+process is canceled on the EC-level and an identical event is being sent.
+Thus a *detach-request* event, by itself, does not signal the start of the
+detachment process.
+
+The detachment process may further be canceled by the EC due to hardware
+failures or a low clipboard battery. This is done via a cancel event
+(``SDTX_EVENT_CANCEL``) with the corresponding cancel reason.
+
+
+User-Space Interface Documentation
+==================================
+
+Error Codes and Status Values
+-----------------------------
+
+Error and status codes are divided into different categories, which can be
+used to determine if the status code is an error, and, if it is, the
+severity and type of that error. The current categories are:
+
+.. flat-table:: Overview of Status/Error Categories.
+   :widths: 2 1 3
+   :header-rows: 1
+
+   * - Name
+     - Value
+     - Short Description
+
+   * - ``STATUS``
+     - ``0x0000``
+     - Non-error status codes.
+
+   * - ``RUNTIME_ERROR``
+     - ``0x1000``
+     - Non-critical runtime errors.
+
+   * - ``HARDWARE_ERROR``
+     - ``0x2000``
+     - Critical hardware failures.
+
+   * - ``UNKNOWN``
+     - ``0xF000``
+     - Unknown error codes.
+
+Other categories are reserved for future use. The ``SDTX_CATEGORY()`` macro
+can be used to determine the category of any status value. The
+``SDTX_SUCCESS()`` macro can be used to check if the status value is a
+success value (``SDTX_CATEGORY_STATUS``) or if it indicates a failure.
+
+Unknown status or error codes sent by the EC are assigned to the ``UNKNOWN``
+category by the driver and may be implemented via their own code in the
+future.
+
+Currently used error codes are:
+
+.. flat-table:: Overview of Error Codes.
+   :widths: 2 1 1 3
+   :header-rows: 1
+
+   * - Name
+     - Category
+     - Value
+     - Short Description
+
+   * - ``SDTX_DETACH_NOT_FEASIBLE``
+     - ``RUNTIME``
+     - ``0x1001``
+     - Detachment not feasible due to low clipboard battery.
+
+   * - ``SDTX_DETACH_TIMEDOUT``
+     - ``RUNTIME``
+     - ``0x1002``
+     - Detachment process timed out while the latch was locked.
+
+   * - ``SDTX_ERR_FAILED_TO_OPEN``
+     - ``HARDWARE``
+     - ``0x2001``
+     - Failed to open latch.
+
+   * - ``SDTX_ERR_FAILED_TO_REMAIN_OPEN``
+     - ``HARDWARE``
+     - ``0x2002``
+     - Failed to keep latch open.
+
+   * - ``SDTX_ERR_FAILED_TO_CLOSE``
+     - ``HARDWARE``
+     - ``0x2003``
+     - Failed to close latch.
+
+Other error codes are reserved for future use. Non-error status codes may
+overlap and are generally only unique within their use-case:
+
+.. flat-table:: Latch Status Codes.
+   :widths: 2 1 1 3
+   :header-rows: 1
+
+   * - Name
+     - Category
+     - Value
+     - Short Description
+
+   * - ``SDTX_LATCH_CLOSED``
+     - ``STATUS``
+     - ``0x0000``
+     - Latch is closed/has been closed.
+
+   * - ``SDTX_LATCH_OPENED``
+     - ``STATUS``
+     - ``0x0001``
+     - Latch is open/has been opened.
+
+.. flat-table:: Base State Codes.
+   :widths: 2 1 1 3
+   :header-rows: 1
+
+   * - Name
+     - Category
+     - Value
+     - Short Description
+
+   * - ``SDTX_BASE_DETACHED``
+     - ``STATUS``
+     - ``0x0000``
+     - Base has been detached/is not present.
+
+   * - ``SDTX_BASE_ATTACHED``
+     - ``STATUS``
+     - ``0x0001``
+     - Base has been attached/is present.
+
+Again, other codes are reserved for future use.
+
+.. _events:
+
+Events
+------
+
+Events can be received by reading from the device file. They are disabled by
+default and have to be enabled by executing ``SDTX_IOCTL_EVENTS_ENABLE``
+first. All events follow the layout prescribed by |sdtx_event|. Specific
+event types can be identified by their event code, described in
+|sdtx_event_code|. Note that other event codes are reserved for future use,
+thus an event parser must be able to handle any unknown/unsupported event
+types gracefully, by relying on the payload length given in the event header.
+
+Currently provided event types are:
+
+.. flat-table:: Overview of DTX events.
+   :widths: 2 1 1 3
+   :header-rows: 1
+
+   * - Name
+     - Code
+     - Payload
+     - Short Description
+
+   * - ``SDTX_EVENT_REQUEST``
+     - ``1``
+     - ``0`` bytes
+     - Detachment process initiated/aborted.
+
+   * - ``SDTX_EVENT_CANCEL``
+     - ``2``
+     - ``2`` bytes
+     - EC canceled detachment process.
+
+   * - ``SDTX_EVENT_BASE_CONNECTION``
+     - ``3``
+     - ``4`` bytes
+     - Base connection state changed.
+
+   * - ``SDTX_EVENT_LATCH_STATUS``
+     - ``4``
+     - ``2`` bytes
+     - Latch status changed.
+
+   * - ``SDTX_EVENT_DEVICE_MODE``
+     - ``5``
+     - ``2`` bytes
+     - Device mode changed.
+
+Individual events in more detail:
+
+``SDTX_EVENT_REQUEST``
+^^^^^^^^^^^^^^^^^^^^^^
+
+Sent when a detachment process is started or, if in progress, aborted by the
+user, either via a detach button press or a detach request
+(``SDTX_IOCTL_LATCH_REQUEST``) being sent from user-space.
+
+Does not have any payload.
+
+``SDTX_EVENT_CANCEL``
+^^^^^^^^^^^^^^^^^^^^^
+
+Sent when a detachment process is canceled by the EC due to unfulfilled
+preconditions (e.g. clipboard battery too low to detach) or hardware
+failure. The reason for cancellation is given in the event payload detailed
+below and can be one of
+
+* ``SDTX_DETACH_TIMEDOUT``: Detachment timed out while the latch was locked.
+  The latch has neither been opened nor unlocked.
+
+* ``SDTX_DETACH_NOT_FEASIBLE``: Detachment not feasible due to low clipboard
+  battery.
+
+* ``SDTX_ERR_FAILED_TO_OPEN``: Could not open the latch (hardware failure).
+
+* ``SDTX_ERR_FAILED_TO_REMAIN_OPEN``: Could not keep the latch open (hardware
+  failure).
+
+* ``SDTX_ERR_FAILED_TO_CLOSE``: Could not close the latch (hardware failure).
+
+Other error codes in this context are reserved for future use.
+
+These codes can be classified via the ``SDTX_CATEGORY()`` macro to discern
+between critical hardware errors (``SDTX_CATEGORY_HARDWARE_ERROR``) or
+runtime errors (``SDTX_CATEGORY_RUNTIME_ERROR``), the latter of which may
+happen during normal operation if certain preconditions for detachment are
+not given.
+
+.. flat-table:: Detachment Cancel Event Payload
+   :widths: 1 1 4
+   :header-rows: 1
+
+   * - Field
+     - Type
+     - Description
+
+   * - ``reason``
+     - |__u16|
+     - Reason for cancellation.
+
+``SDTX_EVENT_BASE_CONNECTION``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Sent when the base connection state has changed, i.e. when the base has been
+attached, detached, or detachment has become infeasible due to low clipboard
+battery. The new state and, if a base is connected, ID of the base is
+provided as payload of type |sdtx_base_info| with its layout presented
+below:
+
+.. flat-table:: Base-Connection-Change Event Payload
+   :widths: 1 1 4
+   :header-rows: 1
+
+   * - Field
+     - Type
+     - Description
+
+   * - ``state``
+     - |__u16|
+     - Base connection state.
+
+   * - ``base_id``
+     - |__u16|
+     - Type of base connected (zero if none).
+
+Possible values for ``state`` are:
+
+* ``SDTX_BASE_DETACHED``,
+* ``SDTX_BASE_ATTACHED``, and
+* ``SDTX_DETACH_NOT_FEASIBLE``.
+
+Other values are reserved for future use.
+
+``SDTX_EVENT_LATCH_STATUS``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Sent when the latch status has changed, i.e. when the latch has been opened,
+closed, or an error occurred. The current status is provided as payload:
+
+.. flat-table:: Latch-Status-Change Event Payload
+   :widths: 1 1 4
+   :header-rows: 1
+
+   * - Field
+     - Type
+     - Description
+
+   * - ``status``
+     - |__u16|
+     - Latch status.
+
+Possible values for ``status`` are:
+
+* ``SDTX_LATCH_CLOSED``,
+* ``SDTX_LATCH_OPENED``,
+* ``SDTX_ERR_FAILED_TO_OPEN``,
+* ``SDTX_ERR_FAILED_TO_REMAIN_OPEN``, and
+* ``SDTX_ERR_FAILED_TO_CLOSE``.
+
+Other values are reserved for future use.
+
+``SDTX_EVENT_DEVICE_MODE``
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Sent when the device mode has changed. The new device mode is provided as
+payload:
+
+.. flat-table:: Device-Mode-Change Event Payload
+   :widths: 1 1 4
+   :header-rows: 1
+
+   * - Field
+     - Type
+     - Description
+
+   * - ``mode``
+     - |__u16|
+     - Device operation mode.
+
+Possible values for ``mode`` are:
+
+* ``SDTX_DEVICE_MODE_TABLET``,
+* ``SDTX_DEVICE_MODE_LAPTOP``, and
+* ``SDTX_DEVICE_MODE_STUDIO``.
+
+Other values are reserved for future use.
+
+.. _ioctls:
+
+IOCTLs
+------
+
+The following IOCTLs are provided:
+
+.. flat-table:: Overview of DTX IOCTLs
+   :widths: 1 1 1 1 4
+   :header-rows: 1
+
+   * - Type
+     - Number
+     - Direction
+     - Name
+     - Description
+
+   * - ``0xA5``
+     - ``0x21``
+     - ``-``
+     - ``EVENTS_ENABLE``
+     - Enable events for the current file descriptor.
+
+   * - ``0xA5``
+     - ``0x22``
+     - ``-``
+     - ``EVENTS_DISABLE``
+     - Disable events for the current file descriptor.
+
+   * - ``0xA5``
+     - ``0x23``
+     - ``-``
+     - ``LATCH_LOCK``
+     - Lock the latch.
+
+   * - ``0xA5``
+     - ``0x24``
+     - ``-``
+     - ``LATCH_UNLOCK``
+     - Unlock the latch.
+
+   * - ``0xA5``
+     - ``0x25``
+     - ``-``
+     - ``LATCH_REQUEST``
+     - Request clipboard detachment.
+
+   * - ``0xA5``
+     - ``0x26``
+     - ``-``
+     - ``LATCH_CONFIRM``
+     - Confirm clipboard detachment request.
+
+   * - ``0xA5``
+     - ``0x27``
+     - ``-``
+     - ``LATCH_HEARTBEAT``
+     - Send heartbeat signal to EC.
+
+   * - ``0xA5``
+     - ``0x28``
+     - ``-``
+     - ``LATCH_CANCEL``
+     - Cancel detachment process.
+
+   * - ``0xA5``
+     - ``0x29``
+     - ``R``
+     - ``GET_BASE_INFO``
+     - Get current base/connection information.
+
+   * - ``0xA5``
+     - ``0x2A``
+     - ``R``
+     - ``GET_DEVICE_MODE``
+     - Get current device operation mode.
+
+   * - ``0xA5``
+     - ``0x2B``
+     - ``R``
+     - ``GET_LATCH_STATUS``
+     - Get current device latch status.
+
+``SDTX_IOCTL_EVENTS_ENABLE``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Defined as ``_IO(0xA5, 0x22)``.
+
+Enable events for the current file descriptor. Events can be obtained by
+reading from the device, if enabled. Events are disabled by default.
+
+``SDTX_IOCTL_EVENTS_DISABLE``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Defined as ``_IO(0xA5, 0x22)``.
+
+Disable events for the current file descriptor. Events can be obtained by
+reading from the device, if enabled. Events are disabled by default.
+
+``SDTX_IOCTL_LATCH_LOCK``
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Defined as ``_IO(0xA5, 0x23)``.
+
+Locks the latch, causing the detachment procedure to abort without opening
+the latch on timeout. The latch is unlocked by default. This command will be
+silently ignored if the latch is already locked.
+
+``SDTX_IOCTL_LATCH_UNLOCK``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Defined as ``_IO(0xA5, 0x24)``.
+
+Unlocks the latch, causing the detachment procedure to open the latch on
+timeout. The latch is unlocked by default. This command will not open the
+latch when sent during an ongoing detachment process. It will be silently
+ignored if the latch is already unlocked.
+
+``SDTX_IOCTL_LATCH_REQUEST``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Defined as ``_IO(0xA5, 0x25)``.
+
+Generic latch request. Behavior depends on the context: If no
+detachment-process is active, detachment is requested. Otherwise the
+currently active detachment-process will be aborted.
+
+If a detachment process is canceled by this operation, a generic detachment
+request event (``SDTX_EVENT_REQUEST``) will be sent.
+
+This essentially behaves the same as a detachment button press.
+
+``SDTX_IOCTL_LATCH_CONFIRM``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Defined as ``_IO(0xA5, 0x26)``.
+
+Acknowledges and confirms a latch request. If sent during an ongoing
+detachment process, this command causes the latch to be opened immediately.
+The latch will also be opened if it has been locked. In this case, the latch
+lock is reset to the unlocked state.
+
+This command will be silently ignored if there is currently no detachment
+procedure in progress.
+
+``SDTX_IOCTL_LATCH_HEARTBEAT``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Defined as ``_IO(0xA5, 0x27)``.
+
+Sends a heartbeat, essentially resetting the detachment timeout. This
+command can be used to keep the detachment process alive while work required
+for the detachment to succeed is still in progress.
+
+This command will be silently ignored if there is currently no detachment
+procedure in progress.
+
+``SDTX_IOCTL_LATCH_CANCEL``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Defined as ``_IO(0xA5, 0x28)``.
+
+Cancels detachment in progress (if any). If a detachment process is canceled
+by this operation, a generic detachment request event
+(``SDTX_EVENT_REQUEST``) will be sent.
+
+This command will be silently ignored if there is currently no detachment
+procedure in progress.
+
+``SDTX_IOCTL_GET_BASE_INFO``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Defined as ``_IOR(0xA5, 0x29, struct sdtx_base_info)``.
+
+Get the current base connection state (i.e. attached/detached) and the type
+of the base connected to the clipboard. This is command essentially provides
+a way to query the information provided by the base connection change event
+(``SDTX_EVENT_BASE_CONNECTION``).
+
+Possible values for ``struct sdtx_base_info.state`` are:
+
+* ``SDTX_BASE_DETACHED``,
+* ``SDTX_BASE_ATTACHED``, and
+* ``SDTX_DETACH_NOT_FEASIBLE``.
+
+Other values are reserved for future use.
+
+``SDTX_IOCTL_GET_DEVICE_MODE``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Defined as ``_IOR(0xA5, 0x2A, __u16)``.
+
+Returns the device operation mode, indicating if and how the base is
+attached to the clipboard. This is command essentially provides a way to
+query the information provided by the device mode change event
+(``SDTX_EVENT_DEVICE_MODE``).
+
+Returned values are:
+
+* ``SDTX_DEVICE_MODE_LAPTOP``
+* ``SDTX_DEVICE_MODE_TABLET``
+* ``SDTX_DEVICE_MODE_STUDIO``
+
+See |sdtx_device_mode| for details. Other values are reserved for future
+use.
+
+
+``SDTX_IOCTL_GET_LATCH_STATUS``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Defined as ``_IOR(0xA5, 0x2B, __u16)``.
+
+Get the current latch status or (presumably) the last error encountered when
+trying to open/close the latch. This is command essentially provides a way
+to query the information provided by the latch status change event
+(``SDTX_EVENT_LATCH_STATUS``).
+
+Returned values are:
+
+* ``SDTX_LATCH_CLOSED``,
+* ``SDTX_LATCH_OPENED``,
+* ``SDTX_ERR_FAILED_TO_OPEN``,
+* ``SDTX_ERR_FAILED_TO_REMAIN_OPEN``, and
+* ``SDTX_ERR_FAILED_TO_CLOSE``.
+
+Other values are reserved for future use.
+
+A Note on Base IDs
+------------------
+
+Base types/IDs provided via ``SDTX_EVENT_BASE_CONNECTION`` or
+``SDTX_IOCTL_GET_BASE_INFO`` are directly forwarded from the EC in the lower
+byte of the combined |__u16| value, with the driver storing the EC type from
+which this ID comes in the high byte (without this, base IDs over different
+types of ECs may be overlapping).
+
+The ``SDTX_DEVICE_TYPE()`` macro can be used to determine the EC device
+type. This can be one of
+
+* ``SDTX_DEVICE_TYPE_HID``, for Surface Aggregator Module over HID, and
+
+* ``SDTX_DEVICE_TYPE_SSH``, for Surface Aggregator Module over Surface Serial
+  Hub.
+
+Note that currently only the ``SSH`` type EC is supported, however ``HID``
+type is reserved for future use.
+
+Structures and Enums
+--------------------
+
+.. kernel-doc:: include/uapi/linux/surface_aggregator/dtx.h
+
+API Users
+=========
+
+A user-space daemon utilizing this API can be found at
+https://github.com/linux-surface/surface-dtx-daemon.
diff --git a/Documentation/driver-api/surface_aggregator/clients/index.rst b/Documentation/driver-api/surface_aggregator/clients/index.rst
index 3ccabce23271..98ea9946b8a2 100644
--- a/Documentation/driver-api/surface_aggregator/clients/index.rst
+++ b/Documentation/driver-api/surface_aggregator/clients/index.rst
@@ -11,6 +11,7 @@ This is the documentation for client drivers themselves. Refer to
    :maxdepth: 1
 
    cdev
+   dtx
    san
 
 .. only::  subproject and html