docs: driver-api: Add Surface Aggregator subsystem documentation

Add documentation for the Surface Aggregator subsystem and its client
drivers, giving an overview of the subsystem, its use-cases, its
internal structure and internal API, as well as its external API for
writing client drivers.
Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20201221183959.1186143-8-luzmaximilian@gmail.comSigned-off-by: Hans de Goede <hdegoede@redhat.com>

Documentation/driver-api/index.rst

@@ -99,6 +99,7 @@ available subsections can be seen below.
    rfkill
    serial/index
    sm501
+   surface_aggregator/index
    switchtec
    sync_file
    vfio-mediated-device

Documentation/driver-api/surface_aggregator/client-api.rst

+.. SPDX-License-Identifier: GPL-2.0+
+
+===============================
+Client Driver API Documentation
+===============================
+
+.. contents::
+    :depth: 2
+
+
+Serial Hub Communication
+========================
+
+.. kernel-doc:: include/linux/surface_aggregator/serial_hub.h
+
+.. kernel-doc:: drivers/platform/surface/aggregator/ssh_packet_layer.c
+    :export:
+
+
+Controller and Core Interface
+=============================
+
+.. kernel-doc:: include/linux/surface_aggregator/controller.h
+
+.. kernel-doc:: drivers/platform/surface/aggregator/controller.c
+    :export:
+
+.. kernel-doc:: drivers/platform/surface/aggregator/core.c
+    :export:
+
+
+Client Bus and Client Device API
+================================
+
+.. kernel-doc:: include/linux/surface_aggregator/device.h
+
+.. kernel-doc:: drivers/platform/surface/aggregator/bus.c
+    :export:

Documentation/driver-api/surface_aggregator/clients/index.rst

+.. SPDX-License-Identifier: GPL-2.0+
+
+===========================
+Client Driver Documentation
+===========================
+
+This is the documentation for client drivers themselves. Refer to
+:doc:`../client` for documentation on how to write client drivers.
+
+.. Place documentation for individual client drivers here.

Documentation/driver-api/surface_aggregator/index.rst

+.. SPDX-License-Identifier: GPL-2.0+
+
+=======================================
+Surface System Aggregator Module (SSAM)
+=======================================
+
+.. toctree::
+   :maxdepth: 2
+
+   overview
+   client
+   clients/index
+   ssh
+   internal
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`

Documentation/driver-api/surface_aggregator/internal-api.rst

+.. SPDX-License-Identifier: GPL-2.0+
+
+==========================
+Internal API Documentation
+==========================
+
+.. contents::
+    :depth: 2
+
+
+Packet Transport Layer
+======================
+
+.. kernel-doc:: drivers/platform/surface/aggregator/ssh_parser.h
+    :internal:
+
+.. kernel-doc:: drivers/platform/surface/aggregator/ssh_parser.c
+    :internal:
+
+.. kernel-doc:: drivers/platform/surface/aggregator/ssh_msgb.h
+    :internal:
+
+.. kernel-doc:: drivers/platform/surface/aggregator/ssh_packet_layer.h
+    :internal:
+
+.. kernel-doc:: drivers/platform/surface/aggregator/ssh_packet_layer.c
+    :internal:
+
+
+Request Transport Layer
+=======================
+
+.. kernel-doc:: drivers/platform/surface/aggregator/ssh_request_layer.h
+    :internal:
+
+.. kernel-doc:: drivers/platform/surface/aggregator/ssh_request_layer.c
+    :internal:
+
+
+Controller
+==========
+
+.. kernel-doc:: drivers/platform/surface/aggregator/controller.h
+    :internal:
+
+.. kernel-doc:: drivers/platform/surface/aggregator/controller.c
+    :internal:
+
+
+Client Device Bus
+=================
+
+.. kernel-doc:: drivers/platform/surface/aggregator/bus.c
+    :internal:
+
+
+Core
+====
+
+.. kernel-doc:: drivers/platform/surface/aggregator/core.c
+    :internal:
+
+
+Trace Helpers
+=============
+
+.. kernel-doc:: drivers/platform/surface/aggregator/trace.h

Documentation/driver-api/surface_aggregator/overview.rst

+.. 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.

MAINTAINERS

@@ -11818,6 +11818,7 @@ M:	Maximilian Luz <luzmaximilian@gmail.com>
 S:	Maintained
 W:	https://github.com/linux-surface/surface-aggregator-module
 C:	irc://chat.freenode.net/##linux-surface
+F:	Documentation/driver-api/surface_aggregator/
 F:	drivers/platform/surface/aggregator/
 F:	include/linux/surface_aggregator/