.. 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``
.. |SID| replace:: ``SID``
.. |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_itu_t(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 between 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|
     - |u8|
     - Target ID for commands/messages.

   * - |SID|
     - |u8|
     - Source ID for commands/messages.

   * - |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``) and source ID (``SID``). These two fields indicate where a message
originates from (``SID``) and what the intended target of the message is
(``TID``). Note that a response to a specific request therefore has the source
and target IDs swapped when compared to the original request (i.e. the request
target is the response source and the request source is the response target).
See (:c:type:`enum ssh_request_id <ssh_request_id>`) for possible values of
both.

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. While
this may not always hold in reality, you can think of different target/source
IDs indicating different physical ECs with potentially different feature sets.


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.
