docs: Add PECI documentation

Add a brief overview of PECI and PECI wire interface.
The documentation also contains kernel-doc for PECI subsystem internals
and PECI CPU Driver API.

Reviewed-by: Pierre-Louis Bossart <[e-mail]>
Acked-by: Joel Stanley <[e-mail]>
Signed-off-by: Iwona Winiarska <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/index.rst

···
    scheduler/index
    mhi/index
    tty/index
+   peci/index
 
 Architecture-agnostic documentation
 -----------------------------------

Documentation/peci/index.rst

···
+.. SPDX-License-Identifier: GPL-2.0-only
+
+====================
+Linux PECI Subsystem
+====================
+
+.. toctree::
+
+   peci
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`

Documentation/peci/peci.rst

···
+.. SPDX-License-Identifier: GPL-2.0-only
+
+========
+Overview
+========
+
+The Platform Environment Control Interface (PECI) is a communication
+interface between Intel processor and management controllers
+(e.g. Baseboard Management Controller, BMC).
+PECI provides services that allow the management controller to
+configure, monitor and debug platform by accessing various registers.
+It defines a dedicated command protocol, where the management
+controller is acting as a PECI originator and the processor - as
+a PECI responder.
+PECI can be used in both single processor and multiple-processor based
+systems.
+
+NOTE:
+Intel PECI specification is not released as a dedicated document,
+instead it is a part of External Design Specification (EDS) for given
+Intel CPU. External Design Specifications are usually not publicly
+available.
+
+PECI Wire
+---------
+
+PECI Wire interface uses a single wire for self-clocking and data
+transfer. It does not require any additional control lines - the
+physical layer is a self-clocked one-wire bus signal that begins each
+bit with a driven, rising edge from an idle near zero volts. The
+duration of the signal driven high allows to determine whether the bit
+value is logic '0' or logic '1'. PECI Wire also includes variable data
+rate established with every message.
+
+For PECI Wire, each processor package will utilize unique, fixed
+addresses within a defined range and that address should
+have a fixed relationship with the processor socket ID - if one of the
+processors is removed, it does not affect addresses of remaining
+processors.
+
+PECI subsystem internals
+------------------------
+
+.. kernel-doc:: include/linux/peci.h
+.. kernel-doc:: drivers/peci/internal.h
+.. kernel-doc:: drivers/peci/core.c
+.. kernel-doc:: drivers/peci/request.c
+
+PECI CPU Driver API
+-------------------
+.. kernel-doc:: drivers/peci/cpu.c

MAINTAINERS

···
 L:	[e-mail] (moderated for non-subscribers)
 S:	Supported
 F:	Documentation/devicetree/bindings/peci/
+F:	Documentation/peci/
 F:	drivers/peci/
 F:	include/linux/peci-cpu.h
 F:	include/linux/peci.h