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.

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 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 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:

Overview of Status/Error Categories.
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:

Overview of Error Codes.
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:

Latch Status Codes.
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.
Base State Codes.
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 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 struct sdtx_event. Specific event types can be identified by their event code, described in enum 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:

Overview of DTX events.
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.

Detachment Cancel Event Payload
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 struct sdtx_base_info with its layout presented below:

Base-Connection-Change Event Payload
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:

Latch-Status-Change Event Payload
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:

Device-Mode-Change Event Payload
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

The following IOCTLs are provided:

Overview of DTX IOCTLs
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 struct 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

enum sdtx_device_mode

Mode describing how (and if) the clipboard is attached to the base of the device.

Constants

SDTX_DEVICE_MODE_TABLET
The clipboard is detached from the base and the device operates as tablet.
SDTX_DEVICE_MODE_LAPTOP
The clipboard is attached normally to the base and the device operates as laptop.
SDTX_DEVICE_MODE_STUDIO
The clipboard is attached to the base in reverse. The device operates as tablet with keyboard and touchpad deactivated, however, the base battery and, if present in the specific device model, dGPU are available to the system.
struct sdtx_event

Event provided by reading from the DTX device file.

Definition

struct sdtx_event {
  __u16 length;
  __u16 code;
  __u8 data[];
};

Members

length
Length of the event payload, in bytes.
code
Event code, detailing what type of event this is.
data
Payload of the event, containing length bytes.

Description

See enum sdtx_event_code for currently valid event codes.

enum sdtx_event_code

Code describing the type of an event.

Constants

SDTX_EVENT_REQUEST
Detachment request event type.
SDTX_EVENT_CANCEL
Cancel detachment process event type.
SDTX_EVENT_BASE_CONNECTION
Base/clipboard connection change event type.
SDTX_EVENT_LATCH_STATUS
Latch status change event type.
SDTX_EVENT_DEVICE_MODE
Device mode change event type.

Description

Used in struct sdtx_event to describe the type of the event. Further event codes are reserved for future use. Any event parser should be able to gracefully handle unknown events, i.e. by simply skipping them.

Consult the DTX user-space interface documentation for details regarding the individual event types.

struct sdtx_base_info

Describes if and what type of base is connected.

Definition

struct sdtx_base_info {
  __u16 state;
  __u16 base_id;
};

Members

state
The state of the connection. Valid values are SDTX_BASE_DETACHED, SDTX_BASE_ATTACHED, and SDTX_DETACH_NOT_FEASIBLE (in case a base is attached but low clipboard battery prevents detachment). Other values are currently reserved.
base_id
The type of base connected. Zero if no base is connected.

API Users

A user-space daemon utilizing this API can be found at https://github.com/linux-surface/surface-dtx-daemon.