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 .. SPDX-License-Identifier: GPL-2.0+
 
 .. |u8| replace:: :c:type:`u8 <u8>`
 .. |u16| replace:: :c:type:`u16 <u16>`
 .. |TYPE| replace:: ``TYPE``
 .. |LEN| replace:: ``LEN``
 .. |SEQ| replace:: ``SEQ``
 .. |SYN| replace:: ``SYN``
 .. |NAK| replace:: ``NAK``
 .. |ACK| replace:: ``ACK``
 .. |DATA| replace:: ``DATA``
 .. |DATA_SEQ| replace:: ``DATA_SEQ``
 .. |DATA_NSQ| replace:: ``DATA_NSQ``
 .. |TC| replace:: ``TC``
 .. |TID| replace:: ``TID``
 .. |IID| replace:: ``IID``
 .. |RQID| replace:: ``RQID``
 .. |CID| replace:: ``CID``
 
 ===========================
 Surface Serial Hub Protocol
 ===========================
 
 The Surface Serial Hub (SSH) is the central communication interface for the
 embedded Surface Aggregator Module controller (SAM or EC), found on newer
 Surface generations. We will refer to this protocol and interface as
 SAM-over-SSH, as opposed to SAM-over-HID for the older generations.
 
 On Surface devices with SAM-over-SSH, SAM is connected to the host via UART
 and defined in ACPI as device with ID ``MSHW0084``. On these devices,
 significant functionality is provided via SAM, including access to battery
 and power information and events, thermal read-outs and events, and many
 more. For Surface Laptops, keyboard input is handled via HID directed
 through SAM, on the Surface Laptop 3 and Surface Book 3 this also includes
 touchpad input.
 
 Note that the standard disclaimer for this subsystem also applies to this
 document: All of this has been reverse-engineered and may thus be erroneous
 and/or incomplete.
 
 All CRCs used in the following are two-byte ``crc_ccitt_false(0xffff, ...)``.
 All multi-byte values are little-endian, there is no implicit padding between
 values.
 
 
 SSH Packet Protocol: Definitions
 ================================
 
 The fundamental communication unit of the SSH protocol is a frame
 (:c:type:`struct ssh_frame <ssh_frame>`). A frame consists of the following
 fields, packed together and in order:
 
 .. flat-table:: SSH Frame
    :widths: 1 1 4
    :header-rows: 1
 
    * - Field
      - Type
      - Description
 
    * - |TYPE|
      - |u8|
      - Type identifier of the frame.
 
    * - |LEN|
      - |u16|
      - Length of the payload associated with the frame.
 
    * - |SEQ|
      - |u8|
      - Sequence ID (see explanation below).
 
 Each frame structure is followed by a CRC over this structure. The CRC over
 the frame structure (|TYPE|, |LEN|, and |SEQ| fields) is placed directly
 after the frame structure and before the payload. The payload is followed by
 its own CRC (over all payload bytes). If the payload is not present (i.e.
 the frame has ``LEN=0``), the CRC of the payload is still present and will
 evaluate to ``0xffff``. The |LEN| field does not include any of the CRCs, it
 equals the number of bytes inbetween the CRC of the frame and the CRC of the
 payload.
 
 Additionally, the following fixed two-byte sequences are used:
 
 .. flat-table:: SSH Byte Sequences
    :widths: 1 1 4
    :header-rows: 1
 
    * - Name
      - Value
      - Description
 
    * - |SYN|
      - ``[0xAA, 0x55]``
      - Synchronization bytes.
 
 A message consists of |SYN|, followed by the frame (|TYPE|, |LEN|, |SEQ| and
 CRC) and, if specified in the frame (i.e. ``LEN > 0``), payload bytes,
 followed finally, regardless if the payload is present, the payload CRC. The
 messages corresponding to an exchange are, in part, identified by having the
 same sequence ID (|SEQ|), stored inside the frame (more on this in the next
 section). The sequence ID is a wrapping counter.
 
 A frame can have the following types
 (:c:type:`enum ssh_frame_type <ssh_frame_type>`):
 
 .. flat-table:: SSH Frame Types
    :widths: 1 1 4
    :header-rows: 1
 
    * - Name
      - Value
      - Short Description
 
    * - |NAK|
      - ``0x04``
      - Sent on error in previously received message.
 
    * - |ACK|
      - ``0x40``
      - Sent to acknowledge receival of |DATA| frame.
 
    * - |DATA_SEQ|
      - ``0x80``
      - Sent to transfer data. Sequenced.
 
    * - |DATA_NSQ|
      - ``0x00``
      - Same as |DATA_SEQ|, but does not need to be ACKed.
 
 Both |NAK|- and |ACK|-type frames are used to control flow of messages and
 thus do not carry a payload. |DATA_SEQ|- and |DATA_NSQ|-type frames on the
 other hand must carry a payload. The flow sequence and interaction of
 different frame types will be described in more depth in the next section.
 
 
 SSH Packet Protocol: Flow Sequence
 ==================================
 
 Each exchange begins with |SYN|, followed by a |DATA_SEQ|- or
 |DATA_NSQ|-type frame, followed by its CRC, payload, and payload CRC. In
 case of a |DATA_NSQ|-type frame, the exchange is then finished. In case of a
 |DATA_SEQ|-type frame, the receiving party has to acknowledge receival of
 the frame by responding with a message containing an |ACK|-type frame with
 the same sequence ID of the |DATA| frame. In other words, the sequence ID of
 the |ACK| frame specifies the |DATA| frame to be acknowledged. In case of an
 error, e.g. an invalid CRC, the receiving party responds with a message
 containing an |NAK|-type frame. As the sequence ID of the previous data
 frame, for which an error is indicated via the |NAK| frame, cannot be relied
 upon, the sequence ID of the |NAK| frame should not be used and is set to
 zero. After receival of an |NAK| frame, the sending party should re-send all
 outstanding (non-ACKed) messages.
 
 Sequence IDs are not synchronized between the two parties, meaning that they
 are managed independently for each party. Identifying the messages
 corresponding to a single exchange thus relies on the sequence ID as well as
 the type of the message, and the context. Specifically, the sequence ID is
 used to associate an ``ACK`` with its ``DATA_SEQ``-type frame, but not
 ``DATA_SEQ``- or ``DATA_NSQ``-type frames with other ``DATA``- type frames.
 
 An example exchange might look like this:
 
 ::
 
     tx: -- SYN FRAME(D) CRC(F) PAYLOAD CRC(P) -----------------------------
     rx: ------------------------------------- SYN FRAME(A) CRC(F) CRC(P) --
 
 where both frames have the same sequence ID (``SEQ``). Here, ``FRAME(D)``
 indicates a |DATA_SEQ|-type frame, ``FRAME(A)`` an ``ACK``-type frame,
 ``CRC(F)`` the CRC over the previous frame, ``CRC(P)`` the CRC over the
 previous payload. In case of an error, the exchange would look like this:
 
 ::
 
     tx: -- SYN FRAME(D) CRC(F) PAYLOAD CRC(P) -----------------------------
     rx: ------------------------------------- SYN FRAME(N) CRC(F) CRC(P) --
 
 upon which the sender should re-send the message. ``FRAME(N)`` indicates an
 |NAK|-type frame. Note that the sequence ID of the |NAK|-type frame is fixed
 to zero. For |DATA_NSQ|-type frames, both exchanges are the same:
 
 ::
 
     tx: -- SYN FRAME(DATA_NSQ) CRC(F) PAYLOAD CRC(P) ----------------------
     rx: -------------------------------------------------------------------
 
 Here, an error can be detected, but not corrected or indicated to the
 sending party. These exchanges are symmetric, i.e. switching ``rx`` and
 ``tx`` results again in a valid exchange. Currently, no longer exchanges are
 known.
 
 
 Commands: Requests, Responses, and Events
 =========================================
 
 Commands are sent as payload inside a data frame. Currently, this is the
 only known payload type of |DATA| frames, with a payload-type value of
 ``0x80`` (:c:type:`SSH_PLD_TYPE_CMD <ssh_payload_type>`).
 
 The command-type payload (:c:type:`struct ssh_command <ssh_command>`)
 consists of an eight-byte command structure, followed by optional and
 variable length command data. The length of this optional data is derived
 from the frame payload length given in the corresponding frame, i.e. it is
 ``frame.len - sizeof(struct ssh_command)``. The command struct contains the
 following fields, packed together and in order:
 
 .. flat-table:: SSH Command
    :widths: 1 1 4
    :header-rows: 1
 
    * - Field
      - Type
      - Description
 
    * - |TYPE|
      - |u8|
      - Type of the payload. For commands always ``0x80``.
 
    * - |TC|
      - |u8|
      - Target category.
 
    * - |TID| (out)
      - |u8|
      - Target ID for outgoing (host to EC) commands.
 
    * - |TID| (in)
      - |u8|
      - Target ID for incoming (EC to host) commands.
 
    * - |IID|
      - |u8|
      - Instance ID.
 
    * - |RQID|
      - |u16|
      - Request ID.
 
    * - |CID|
      - |u8|
      - Command ID.
 
 The command struct and data, in general, does not contain any failure
 detection mechanism (e.g. CRCs), this is solely done on the frame level.
 
 Command-type payloads are used by the host to send commands and requests to
 the EC as well as by the EC to send responses and events back to the host.
 We differentiate between requests (sent by the host), responses (sent by the
 EC in response to a request), and events (sent by the EC without a preceding
 request).
 
 Commands and events are uniquely identified by their target category
 (``TC``) and command ID (``CID``). The target category specifies a general
 category for the command (e.g. system in general, vs. battery and AC, vs.
 temperature, and so on), while the command ID specifies the command inside
 that category. Only the combination of |TC| + |CID| is unique. Additionally,
 commands have an instance ID (``IID``), which is used to differentiate
 between different sub-devices. For example ``TC=3`` ``CID=1`` is a
 request to get the temperature on a thermal sensor, where |IID| specifies
 the respective sensor. If the instance ID is not used, it should be set to
 zero. If instance IDs are used, they, in general, start with a value of one,
 whereas zero may be used for instance independent queries, if applicable. A
 response to a request should have the same target category, command ID, and
 instance ID as the corresponding request.
 
 Responses are matched to their corresponding request via the request ID
 (``RQID``) field. This is a 16 bit wrapping counter similar to the sequence
 ID on the frames. Note that the sequence ID of the frames for a
 request-response pair does not match. Only the request ID has to match.
 Frame-protocol wise these are two separate exchanges, and may even be
 separated, e.g. by an event being sent after the request but before the
 response. Not all commands produce a response, and this is not detectable by
 |TC| + |CID|. It is the responsibility of the issuing party to wait for a
 response (or signal this to the communication framework, as is done in
 SAN/ACPI via the ``SNC`` flag).
 
 Events are identified by unique and reserved request IDs. These IDs should
 not be used by the host when sending a new request. They are used on the
 host to, first, detect events and, second, match them with a registered
 event handler. Request IDs for events are chosen by the host and directed to
 the EC when setting up and enabling an event source (via the
 enable-event-source request). The EC then uses the specified request ID for
 events sent from the respective source. Note that an event should still be
 identified by its target category, command ID, and, if applicable, instance
 ID, as a single event source can send multiple different event types. In
 general, however, a single target category should map to a single reserved
 event request ID.
 
 Furthermore, requests, responses, and events have an associated target ID
 (``TID``). This target ID is split into output (host to EC) and input (EC to
 host) fields, with the respecting other field (e.g. output field on incoming
 messages) set to zero. Two ``TID`` values are known: Primary (``0x01``) and
 secondary (``0x02``). In general, the response to a request should have the
 same ``TID`` value, however, the field (output vs. input) should be used in
 accordance to the direction in which the response is sent (i.e. on the input
 field, as responses are generally sent from the EC to the host).
 
 Note that, even though requests and events should be uniquely identifiable
 by target category and command ID alone, the EC may require specific
 target ID and instance ID values to accept a command. A command that is
 accepted for ``TID=1``, for example, may not be accepted for ``TID=2``
 and vice versa.
 
 
 Limitations and Observations
 ============================
 
 The protocol can, in theory, handle up to ``U8_MAX`` frames in parallel,
 with up to ``U16_MAX`` pending requests (neglecting request IDs reserved for
 events). In practice, however, this is more limited. From our testing
 (although via a python and thus a user-space program), it seems that the EC
 can handle up to four requests (mostly) reliably in parallel at a certain
 time. With five or more requests in parallel, consistent discarding of
 commands (ACKed frame but no command response) has been observed. For five
 simultaneous commands, this reproducibly resulted in one command being
 dropped and four commands being handled.
 
 However, it has also been noted that, even with three requests in parallel,
 occasional frame drops happen. Apart from this, with a limit of three
 pending requests, no dropped commands (i.e. command being dropped but frame
 carrying command being ACKed) have been observed. In any case, frames (and
 possibly also commands) should be re-sent by the host if a certain timeout
 is exceeded. This is done by the EC for frames with a timeout of one second,
 up to two re-tries (i.e. three transmissions in total). The limit of
 re-tries also applies to received NAKs, and, in a worst case scenario, can
 lead to entire messages being dropped.
 
 While this also seems to work fine for pending data frames as long as no
 transmission failures occur, implementation and handling of these seems to
 depend on the assumption that there is only one non-acknowledged data frame.
 In particular, the detection of repeated frames relies on the last sequence
 number. This means that, if a frame that has been successfully received by
 the EC is sent again, e.g. due to the host not receiving an |ACK|, the EC
 will only detect this if it has the sequence ID of the last frame received
 by the EC. As an example: Sending two frames with ``SEQ=0`` and ``SEQ=1``
 followed by a repetition of ``SEQ=0`` will not detect the second ``SEQ=0``
 frame as such, and thus execute the command in this frame each time it has
 been received, i.e. twice in this example. Sending ``SEQ=0``, ``SEQ=1`` and
 then repeating ``SEQ=1`` will detect the second ``SEQ=1`` as repetition of
 the first one and ignore it, thus executing the contained command only once.
 
 In conclusion, this suggests a limit of at most one pending un-ACKed frame
 (per party, effectively leading to synchronous communication regarding
 frames) and at most three pending commands. The limit to synchronous frame
 transfers seems to be consistent with behavior observed on Windows.

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