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Linux kernel SLIMbus support
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Overview
========

What is SLIMbus?
----------------
SLIMbus (Serial Low Power Interchip Media Bus) is a specification developed by
MIPI (Mobile Industry Processor Interface) alliance. The bus uses master/slave
configuration, and is a 2-wire multi-drop implementation (clock, and data).

Currently, SLIMbus is used to interface between application processors of SoCs
(System-on-Chip) and peripheral components (typically codec). SLIMbus uses
Time-Division-Multiplexing to accommodate multiple data channels, and
a control channel.

The control channel is used for various control functions such as bus
management, configuration and status updates. These messages can be unicast (e.g.
reading/writing device specific values), or multicast (e.g. data channel
reconfiguration sequence is a broadcast message announced to all devices)

A data channel is used for data-transfer between 2 SLIMbus devices. Data
channel uses dedicated ports on the device.

Hardware description:
---------------------
SLIMbus specification has different types of device classifications based on
their capabilities.
A manager device is responsible for enumeration, configuration, and dynamic
channel allocation. Every bus has 1 active manager.

A generic device is a device providing application functionality (e.g. codec).

Framer device is responsible for clocking the bus, and transmitting frame-sync
and framing information on the bus.

Each SLIMbus component has an interface device for monitoring physical layer.

Typically each SoC contains SLIMbus component having 1 manager, 1 framer device,
1 generic device (for data channel support), and 1 interface device.
External peripheral SLIMbus component usually has 1 generic device (for
functionality/data channel support), and an associated interface device.
The generic device's registers are mapped as 'value elements' so that they can
be written/read using SLIMbus control channel exchanging control/status type of
information.
In case there are multiple framer devices on the same bus, manager device is
responsible to select the active-framer for clocking the bus.

Per specification, SLIMbus uses "clock gears" to do power management based on
current frequency and bandwidth requirements. There are 10 clock gears and each
gear changes the SLIMbus frequency to be twice its previous gear.

Each device has a 6-byte enumeration-address and the manager assigns every
device with a 1-byte logical address after the devices report presence on the
bus.

Software description:
---------------------
There are 2 types of SLIMbus drivers:

slim_controller represents a 'controller' for SLIMbus. This driver should
implement duties needed by the SoC (manager device, associated
interface device for monitoring the layers and reporting errors, default
framer device).

slim_device represents the 'generic device/component' for SLIMbus, and a
slim_driver should implement driver for that slim_device.

Device notifications to the driver:
-----------------------------------
Since SLIMbus devices have mechanisms for reporting their presence, the
framework allows drivers to bind when corresponding devices report their
presence on the bus.
However, it is possible that the driver needs to be probed
first so that it can enable corresponding SLIMbus device (e.g. power it up and/or
take it out of reset). To support that behavior, the framework allows drivers
to probe first as well  (e.g. using standard DeviceTree compatibility field).
This creates the necessity for the driver to know when the device is functional
(i.e. reported present). device_up callback is used for that reason when the
device reports present and is assigned a logical address by the controller.

Similarly, SLIMbus devices 'report absent' when they go down. A 'device_down'
callback notifies the driver when the device reports absent and its logical
address assignment is invalidated by the controller.

Another notification "boot_device" is used to notify the slim_driver when
controller resets the bus. This notification allows the driver to take necessary
steps to boot the device so that it's functional after the bus has been reset.

Driver and Controller APIs:
--------------------------
.. kernel-doc:: include/linux/slimbus.h
   :internal:

.. kernel-doc:: drivers/slimbus/slimbus.h
   :internal:

.. kernel-doc:: drivers/slimbus/core.c
   :export:

Clock-pause:
------------
SLIMbus mandates that a reconfiguration sequence (known as clock-pause) be
broadcast to all active devices on the bus before the bus can enter low-power
mode. Controller uses this sequence when it decides to enter low-power mode so
that corresponding clocks and/or power-rails can be turned off to save power.
Clock-pause is exited by waking up framer device (if controller driver initiates
exiting low power mode), or by toggling the data line (if a slave device wants
to initiate it).

Clock-pause APIs:
~~~~~~~~~~~~~~~~~
.. kernel-doc:: drivers/slimbus/sched.c
   :export:

Messaging:
----------
The framework supports regmap and read/write apis to exchange control-information
with a SLIMbus device. APIs can be synchronous or asynchronous.
The header file <linux/slimbus.h> has more documentation about messaging APIs.

Messaging APIs:
~~~~~~~~~~~~~~~
.. kernel-doc:: drivers/slimbus/messaging.c
   :export: