.. SPDX-License-Identifier: GPL-2.0+

========
Overview
========

The Surface/System Aggregator Module (SAM, SSAM) is an (arguably *the*)
embedded controller (EC) on Microsoft Surface devices. It has been originally
introduced on 4th generation devices (Surface Pro 4, Surface Book 1), but
its responsibilities and feature-set have since been expanded significantly
with the following generations.


Features and Integration
========================

Not much is currently known about SAM on 4th generation devices (Surface Pro
4, Surface Book 1), due to the use of a different communication interface
between host and EC (as detailed below). On 5th (Surface Pro 2017, Surface
Book 2, Surface Laptop 1) and later generation devices, SAM is responsible
for providing battery information (both current status and static values,
such as maximum capacity etc.), as well as an assortment of temperature
sensors (e.g. skin temperature) and cooling/performance-mode setting to the
host. On the Surface Book 2, specifically, it additionally provides an
interface for properly handling clipboard detachment (i.e. separating the
display part from the keyboard part of the device), on the Surface Laptop 1
and 2 it is required for keyboard HID input. This HID subsystem has been
restructured for 7th generation devices and on those, specifically Surface
Laptop 3 and Surface Book 3, is responsible for all major HID input (i.e.
keyboard and touchpad).

While features have not changed much on a coarse level since the 5th
generation, internal interfaces have undergone some rather large changes. On
5th and 6th generation devices, both battery and temperature information is
exposed to ACPI via a shim driver (referred to as Surface ACPI Notify, or
SAN), translating ACPI generic serial bus write-/read-accesses to SAM
requests. On 7th generation devices, this additional layer is gone and these
devices require a driver hooking directly into the SAM interface. Equally,
on newer generations, less devices are declared in ACPI, making them a bit
harder to discover and requiring us to hard-code a sort of device registry.
Due to this, a SSAM bus and subsystem with client devices
(:c:type:`struct ssam_device <ssam_device>`) has been implemented.


Communication
=============

The type of communication interface between host and EC depends on the
generation of the Surface device. On 4th generation devices, host and EC
communicate via HID, specifically using a HID-over-I2C device, whereas on
5th and later generations, communication takes place via a USART serial
device. In accordance to the drivers found on other operating systems, we
refer to the serial device and its driver as Surface Serial Hub (SSH). When
needed, we differentiate between both types of SAM by referring to them as
SAM-over-SSH and SAM-over-HID.

Currently, this subsystem only supports SAM-over-SSH. The SSH communication
interface is described in more detail below. The HID interface has not been
reverse engineered yet and it is, at the moment, unclear how many (and
which) concepts of the SSH interface detailed below can be transferred to
it.

Surface Serial Hub
------------------

As already elaborated above, the Surface Serial Hub (SSH) is the
communication interface for SAM on 5th- and all later-generation Surface
devices. On the highest level, communication can be separated into two main
types: Requests, messages sent from host to EC that may trigger a direct
response from the EC (explicitly associated with the request), and events
(sometimes also referred to as notifications), sent from EC to host without
being a direct response to a previous request. We may also refer to requests
without response as commands. In general, events need to be enabled via one
of multiple dedicated requests before they are sent by the EC.

See :doc:`ssh` for a more technical protocol documentation and
:doc:`internal` for an overview of the internal driver architecture.
