1.. SPDX-License-Identifier: GPL-2.0+
2
3========
4Overview
5========
6
7The Surface/System Aggregator Module (SAM, SSAM) is an (arguably *the*)
8embedded controller (EC) on Microsoft Surface devices. It has been originally
9introduced on 4th generation devices (Surface Pro 4, Surface Book 1), but
10its responsibilities and feature-set have since been expanded significantly
11with the following generations.
12
13
14Features and Integration
15========================
16
17Not much is currently known about SAM on 4th generation devices (Surface Pro
184, Surface Book 1), due to the use of a different communication interface
19between host and EC (as detailed below). On 5th (Surface Pro 2017, Surface
20Book 2, Surface Laptop 1) and later generation devices, SAM is responsible
21for providing battery information (both current status and static values,
22such as maximum capacity etc.), as well as an assortment of temperature
23sensors (e.g. skin temperature) and cooling/performance-mode setting to the
24host. On the Surface Book 2, specifically, it additionally provides an
25interface for properly handling clipboard detachment (i.e. separating the
26display part from the keyboard part of the device), on the Surface Laptop 1
27and 2 it is required for keyboard HID input. This HID subsystem has been
28restructured for 7th generation devices and on those, specifically Surface
29Laptop 3 and Surface Book 3, is responsible for all major HID input (i.e.
30keyboard and touchpad).
31
32While features have not changed much on a coarse level since the 5th
33generation, internal interfaces have undergone some rather large changes. On
345th and 6th generation devices, both battery and temperature information is
35exposed to ACPI via a shim driver (referred to as Surface ACPI Notify, or
36SAN), translating ACPI generic serial bus write-/read-accesses to SAM
37requests. On 7th generation devices, this additional layer is gone and these
38devices require a driver hooking directly into the SAM interface. Equally,
39on newer generations, less devices are declared in ACPI, making them a bit
40harder to discover and requiring us to hard-code a sort of device registry.
41Due to this, a SSAM bus and subsystem with client devices
42(:c:type:`struct ssam_device <ssam_device>`) has been implemented.
43
44
45Communication
46=============
47
48The type of communication interface between host and EC depends on the
49generation of the Surface device. On 4th generation devices, host and EC
50communicate via HID, specifically using a HID-over-I2C device, whereas on
515th and later generations, communication takes place via a USART serial
52device. In accordance to the drivers found on other operating systems, we
53refer to the serial device and its driver as Surface Serial Hub (SSH). When
54needed, we differentiate between both types of SAM by referring to them as
55SAM-over-SSH and SAM-over-HID.
56
57Currently, this subsystem only supports SAM-over-SSH. The SSH communication
58interface is described in more detail below. The HID interface has not been
59reverse engineered yet and it is, at the moment, unclear how many (and
60which) concepts of the SSH interface detailed below can be transferred to
61it.
62
63Surface Serial Hub
64------------------
65
66As already elaborated above, the Surface Serial Hub (SSH) is the
67communication interface for SAM on 5th- and all later-generation Surface
68devices. On the highest level, communication can be separated into two main
69types: Requests, messages sent from host to EC that may trigger a direct
70response from the EC (explicitly associated with the request), and events
71(sometimes also referred to as notifications), sent from EC to host without
72being a direct response to a previous request. We may also refer to requests
73without response as commands. In general, events need to be enabled via one
74of multiple dedicated requests before they are sent by the EC.
75
76See :doc:`ssh` for a more technical protocol documentation and
77:doc:`internal` for an overview of the internal driver architecture.
78