.. SPDX-License-Identifier: GPL-2.0

Generic System Interconnect Subsystem
=====================================

Introduction
------------

This framework is designed to provide a standard kernel interface to control
the settings of the interconnects on an SoC. These settings can be throughput,
latency and priority between multiple interconnected devices or functional
blocks. This can be controlled dynamically in order to save power or provide
maximum performance.

The interconnect bus is hardware with configurable parameters, which can be
set on a data path according to the requests received from various drivers.
An example of interconnect buses are the interconnects between various
components or functional blocks in chipsets. There can be multiple interconnects
on an SoC that can be multi-tiered.

Below is a simplified diagram of a real-world SoC interconnect bus topology.

::

 +----------------+    +----------------+
 | HW Accelerator |--->|      M NoC     |<---------------+
 +----------------+    +----------------+                |
                         |      |                    +------------+
  +-----+  +-------------+      V       +------+     |            |
  | DDR |  |                +--------+  | PCIe |     |            |
  +-----+  |                | Slaves |  +------+     |            |
    ^ ^    |                +--------+     |         |   C NoC    |
    | |    V                               V         |            |
 +------------------+   +------------------------+   |            |   +-----+
 |                  |-->|                        |-->|            |-->| CPU |
 |                  |-->|                        |<--|            |   +-----+
 |     Mem NoC      |   |         S NoC          |   +------------+
 |                  |<--|                        |---------+    |
 |                  |<--|                        |<------+ |    |   +--------+
 +------------------+   +------------------------+       | |    +-->| Slaves |
   ^  ^    ^    ^          ^                             | |        +--------+
   |  |    |    |          |                             | V
 +------+  |  +-----+   +-----+  +---------+   +----------------+   +--------+
 | CPUs |  |  | GPU |   | DSP |  | Masters |-->|       P NoC    |-->| Slaves |
 +------+  |  +-----+   +-----+  +---------+   +----------------+   +--------+
           |
       +-------+
       | Modem |
       +-------+

Terminology
-----------

Interconnect provider is the software definition of the interconnect hardware.
The interconnect providers on the above diagram are M NoC, S NoC, C NoC, P NoC
and Mem NoC.

Interconnect node is the software definition of the interconnect hardware
port. Each interconnect provider consists of multiple interconnect nodes,
which are connected to other SoC components including other interconnect
providers. The point on the diagram where the CPUs connect to the memory is
called an interconnect node, which belongs to the Mem NoC interconnect provider.

Interconnect endpoints are the first or the last element of the path. Every
endpoint is a node, but not every node is an endpoint.

Interconnect path is everything between two endpoints including all the nodes
that have to be traversed to reach from a source to destination node. It may
include multiple master-slave pairs across several interconnect providers.

Interconnect consumers are the entities which make use of the data paths exposed
by the providers. The consumers send requests to providers requesting various
throughput, latency and priority. Usually the consumers are device drivers, that
send request based on their needs. An example for a consumer is a video decoder
that supports various formats and image sizes.

U-Boot Implementation
---------------------

The implementation is derived from the Linux 6.17 Interconnect implementation,
adapted to use the U-Boot Driver Model. Under Linux the nodes are allocated
via `idr_alloc()`, while under U-Boot they are created as `icc_node` devices
which are children of the provider device. This provides the same lifetime
by using a robust and ready to use mechanism, simplifying the implementation.

Under Linux, the nodes link is done by always allocating a new `icc_node` when
creating a link, and when the link with the associated ID is registered
it is associated to the new provider. Under U-Boot only the nodes from a provider
are created at bind time, and when the node graph is traversed to calculate
a path the link ID is looked dynamically amongst the node devices. This
may take longer at the gain of time when registering nodes a bind time.

Since U-Boot Driver Model does on-demand device probe, the nodes and provider
devices are also probed when a path is determined and removed when the path
is deleted.

A test suite is present in `test/dm/interconnect.c` using a test driver
`sandbox-interconnect` to exercise those U-Boot specific aspects while making
sure the graph traversal and calculation are accurate.

Interconnect consumers API
--------------------------

Interconnect consumers are the clients which use the interconnect APIs to
get paths between endpoints and set their bandwidth/latency/QoS requirements
for these interconnect paths.

.. kernel-doc:: include/interconnect.h

Interconnect uclass providers API
---------------------------------

Interconnect provider is an entity that implements methods to initialize and
configure interconnect bus hardware. The interconnect provider drivers should
be registered a interconnect uclass drivers.

.. kernel-doc:: include/interconnect-uclass.h
