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 Documentation/driver-api/surface_aggregator/ssh.rst for a 77more technical protocol documentation and 78Documentation/driver-api/surface_aggregator/internal.rst for an 79overview of the internal driver architecture. 80