Merging upstream version 6.10.3.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.. SPDX-License-Identifier: GPL-2.0
+
+PSE Power Interface (PSE PI) Documentation
+==========================================
+
+The Power Sourcing Equipment Power Interface (PSE PI) plays a pivotal role in
+the architecture of Power over Ethernet (PoE) systems. It is essentially a
+blueprint that outlines how one or multiple power sources are connected to the
+eight-pin modular jack, commonly known as the Ethernet RJ45 port. This
+connection scheme is crucial for enabling the delivery of power alongside data
+over Ethernet cables.
+
+Documentation and Standards
+---------------------------
+
+The IEEE 802.3 standard provides detailed documentation on the PSE PI.
+Specifically:
+
+- Section "33.2.3 PI pin assignments" covers the pin assignments for PoE
+  systems that utilize two pairs for power delivery.
+- Section "145.2.4 PSE PI" addresses the configuration for PoE systems that
+  deliver power over all four pairs of an Ethernet cable.
+
+PSE PI and Single Pair Ethernet
+-------------------------------
+
+Single Pair Ethernet (SPE) represents a different approach to Ethernet
+connectivity, utilizing just one pair of conductors for both data and power
+transmission. Unlike the configurations detailed in the PSE PI for standard
+Ethernet, which can involve multiple power sourcing arrangements across four or
+two pairs of wires, SPE operates on a simpler model due to its single-pair
+design. As a result, the complexities of choosing between alternative pin
+assignments for power delivery, as described in the PSE PI for multi-pair
+Ethernet, are not applicable to SPE.
+
+Understanding PSE PI
+--------------------
+
+The Power Sourcing Equipment Power Interface (PSE PI) is a framework defining
+how Power Sourcing Equipment (PSE) delivers power to Powered Devices (PDs) over
+Ethernet cables. It details two main configurations for power delivery, known
+as Alternative A and Alternative B, which are distinguished not only by their
+method of power transmission but also by the implications for polarity and data
+transmission direction.
+
+Alternative A and B Overview
+----------------------------
+
+- **Alternative A:** Utilizes RJ45 conductors 1, 2, 3 and 6. In either case of
+  networks 10/100BaseT or 1G/2G/5G/10GBaseT, the pairs used are carrying data.
+  The power delivery's polarity in this alternative can vary based on the MDI
+  (Medium Dependent Interface) or MDI-X (Medium Dependent Interface Crossover)
+  configuration.
+
+- **Alternative B:** Utilizes RJ45 conductors 4, 5, 7 and 8. In case of
+  10/100BaseT network the pairs used are spare pairs without data and are less
+  influenced by data transmission direction. This is not the case for
+  1G/2G/5G/10GBaseT network. Alternative B includes two configurations with
+  different polarities, known as variant X and variant S, to accommodate
+  different network requirements and device specifications.
+
+Table 145-3 PSE Pinout Alternatives
+-----------------------------------
+
+The following table outlines the pin configurations for both Alternative A and
+Alternative B.
+
++------------+-------------------+-----------------+-----------------+-----------------+
+| Conductor  | Alternative A     | Alternative A   | Alternative B   | Alternative B   |
+|            |    (MDI-X)        |      (MDI)      |        (X)      |        (S)      |
++============+===================+=================+=================+=================+
+| 1          | Negative V        | Positive V      | -               | -               |
++------------+-------------------+-----------------+-----------------+-----------------+
+| 2          | Negative V        | Positive V      | -               | -               |
++------------+-------------------+-----------------+-----------------+-----------------+
+| 3          | Positive V        | Negative V      | -               | -               |
++------------+-------------------+-----------------+-----------------+-----------------+
+| 4          | -                 | -               | Negative V      | Positive V      |
++------------+-------------------+-----------------+-----------------+-----------------+
+| 5          | -                 | -               | Negative V      | Positive V      |
++------------+-------------------+-----------------+-----------------+-----------------+
+| 6          | Positive V        | Negative V      | -               | -               |
++------------+-------------------+-----------------+-----------------+-----------------+
+| 7          | -                 | -               | Positive V      | Negative V      |
++------------+-------------------+-----------------+-----------------+-----------------+
+| 8          | -                 | -               | Positive V      | Negative V      |
++------------+-------------------+-----------------+-----------------+-----------------+
+
+.. note::
+    - "Positive V" and "Negative V" indicate the voltage polarity for each pin.
+    - "-" indicates that the pin is not used for power delivery in that
+      specific configuration.
+
+PSE PI compatibilities
+----------------------
+
+The following table outlines the compatibility between the pinout alternative
+and the 1000/2.5G/5G/10GBaseT in the PSE 2 pairs connection.
+
++---------+---------------+---------------------+-----------------------+
+| Variant | Alternative   | Power Feeding Type  | Compatibility with    |
+|         | (A/B)         | (Direct/Phantom)    | 1000/2.5G/5G/10GBaseT |
++=========+===============+=====================+=======================+
+| 1       | A             | Phantom             | Yes                   |
++---------+---------------+---------------------+-----------------------+
+| 2       | B             | Phantom             | Yes                   |
++---------+---------------+---------------------+-----------------------+
+| 3       | B             | Direct              | No                    |
++---------+---------------+---------------------+-----------------------+
+
+.. note::
+    - "Direct" indicate a variant where the power is injected directly to pairs
+       without using magnetics in case of spare pairs.
+    - "Phantom" indicate power path over coils/magnetics as it is done for
+       Alternative A variant.
+
+In case of PSE 4 pairs, a PSE supporting only 10/100BaseT (which mean Direct
+Power on pinout Alternative B) is not compatible with a 4 pairs
+1000/2.5G/5G/10GBaseT.
+
+PSE Power Interface (PSE PI) Connection Diagram
+-----------------------------------------------
+
+The diagram below illustrates the connection architecture between the RJ45
+port, the Ethernet PHY (Physical Layer), and the PSE PI (Power Sourcing
+Equipment Power Interface), demonstrating how power and data are delivered
+simultaneously through an Ethernet cable. The RJ45 port serves as the physical
+interface for these connections, with each of its eight pins connected to both
+the Ethernet PHY for data transmission and the PSE PI for power delivery.
+
+.. code-block::
+
+    +--------------------------+
+    |                          |
+    |          RJ45 Port       |
+    |                          |
+    +--+--+--+--+--+--+--+--+--+                +-------------+
+      1| 2| 3| 4| 5| 6| 7| 8|                   |             |
+       |  |  |  |  |  |  |  o-------------------+             |
+       |  |  |  |  |  |  o--|-------------------+             +<--- PSE 1
+       |  |  |  |  |  o--|--|-------------------+             |
+       |  |  |  |  o--|--|--|-------------------+             |
+       |  |  |  o--|--|--|--|-------------------+  PSE PI     |
+       |  |  o--|--|--|--|--|-------------------+             |
+       |  o--|--|--|--|--|--|-------------------+             +<--- PSE 2 (optional)
+       o--|--|--|--|--|--|--|-------------------+             |
+       |  |  |  |  |  |  |  |                   |             |
+    +--+--+--+--+--+--+--+--+--+                +-------------+
+    |                          |
+    |       Ethernet PHY       |
+    |                          |
+    +--------------------------+
+
+Simple PSE PI Configuration for Alternative A
+---------------------------------------------
+
+The diagram below illustrates a straightforward PSE PI (Power Sourcing
+Equipment Power Interface) configuration designed to support the Alternative A
+setup for Power over Ethernet (PoE). This implementation is tailored to provide
+power delivery through the data-carrying pairs of an Ethernet cable, suitable
+for either MDI or MDI-X configurations, albeit supporting one variation at a
+time.
+
+.. code-block::
+
+         +-------------+
+         |    PSE PI   |
+ 8  -----+                             +-------------+
+ 7  -----+                    Rail 1   |
+ 6  -----+------+----------------------+
+ 5  -----+      |                      |
+ 4  -----+      |             Rail 2   |  PSE 1
+ 3  -----+------/         +------------+
+ 2  -----+--+-------------/            |
+ 1  -----+--/                          +-------------+
+         |
+         +-------------+
+
+In this configuration:
+
+- Pins 1 and 2, as well as pins 3 and 6, are utilized for power delivery in
+  addition to data transmission. This aligns with the standard wiring for
+  10/100BaseT Ethernet networks where these pairs are used for data.
+- Rail 1 and Rail 2 represent the positive and negative voltage rails, with
+  Rail 1 connected to pins 1 and 2, and Rail 2 connected to pins 3 and 6.
+  More advanced PSE PI configurations may include integrated or external
+  switches to change the polarity of the voltage rails, allowing for
+  compatibility with both MDI and MDI-X configurations.
+
+More complex PSE PI configurations may include additional components, to support
+Alternative B, or to provide additional features such as power management, or
+additional power delivery capabilities such as 2-pair or 4-pair power delivery.
+
+.. code-block::
+
+         +-------------+
+         |    PSE PI   |
+         |        +---+
+ 8  -----+--------+   |                 +-------------+
+ 7  -----+--------+   |       Rail 1   |
+ 6  -----+--------+   +-----------------+
+ 5  -----+--------+   |                |
+ 4  -----+--------+   |       Rail 2   |  PSE 1
+ 3  -----+--------+   +----------------+
+ 2  -----+--------+   |                |
+ 1  -----+--------+   |                 +-------------+
+         |        +---+
+         +-------------+
+
+Device Tree Configuration: Describing PSE PI Configurations
+-----------------------------------------------------------
+
+The necessity for a separate PSE PI node in the device tree is influenced by
+the intricacy of the Power over Ethernet (PoE) system's setup. Here are
+descriptions of both simple and complex PSE PI configurations to illustrate
+this decision-making process:
+
+**Simple PSE PI Configuration:**
+In a straightforward scenario, the PSE PI setup involves a direct, one-to-one
+connection between a single PSE controller and an Ethernet port. This setup
+typically supports basic PoE functionality without the need for dynamic
+configuration or management of multiple power delivery modes. For such simple
+configurations, detailing the PSE PI within the existing PSE controller's node
+may suffice, as the system does not encompass additional complexity that
+warrants a separate node. The primary focus here is on the clear and direct
+association of power delivery to a specific Ethernet port.
+
+**Complex PSE PI Configuration:**
+Contrastingly, a complex PSE PI setup may encompass multiple PSE controllers or
+auxiliary circuits that collectively manage power delivery to one Ethernet
+port. Such configurations might support a range of PoE standards and require
+the capability to dynamically configure power delivery based on the operational
+mode (e.g., PoE2 versus PoE4) or specific requirements of connected devices. In
+these instances, a dedicated PSE PI node becomes essential for accurately
+documenting the system architecture. This node would serve to detail the
+interactions between different PSE controllers, the support for various PoE
+modes, and any additional logic required to coordinate power delivery across
+the network infrastructure.
+
+**Guidance:**
+
+For simple PSE setups, including PSE PI information in the PSE controller node
+might suffice due to the straightforward nature of these systems. However,
+complex configurations, involving multiple components or advanced PoE features,
+benefit from a dedicated PSE PI node. This method adheres to IEEE 802.3
+specifications, improving documentation clarity and ensuring accurate
+representation of the PoE system's complexity.
+
+PSE PI Node: Essential Information
+----------------------------------
+
+The PSE PI (Power Sourcing Equipment Power Interface) node in a device tree can
+include several key pieces of information critical for defining the power
+delivery capabilities and configurations of a PoE (Power over Ethernet) system.
+Below is a list of such information, along with explanations for their
+necessity and reasons why they might not be found within a PSE controller node:
+
+1. **Powered Pairs Configuration**
+
+   - *Description:* Identifies the pairs used for power delivery in the
+     Ethernet cable.
+   - *Necessity:* Essential to ensure the correct pairs are powered according
+     to the board's design.
+   - *PSE Controller Node:* Typically lacks details on physical pair usage,
+     focusing on power regulation.
+
+2. **Polarity of Powered Pairs**
+
+   - *Description:* Specifies the polarity (positive or negative) for each
+     powered pair.
+   - *Necessity:* Critical for safe and effective power transmission to PDs.
+   - *PSE Controller Node:* Polarity management may exceed the standard
+     functionalities of PSE controllers.
+
+3. **PSE Cells Association**
+
+   - *Description:* Details the association of PSE cells with Ethernet ports or
+     pairs in multi-cell configurations.
+   - *Necessity:* Allows for optimized power resource allocation in complex
+     systems.
+   - *PSE Controller Node:* Controllers may not manage cell associations
+     directly, focusing instead on power flow regulation.
+
+4. **Support for PoE Standards**
+
+   - *Description:* Lists the PoE standards and configurations supported by the
+     system.
+   - *Necessity:* Ensures system compatibility with various PDs and adherence
+     to industry standards.
+   - *PSE Controller Node:* Specific capabilities may depend on the overall PSE
+     PI design rather than the controller alone. Multiple PSE cells per PI
+     do not necessarily imply support for multiple PoE standards.
+
+5. **Protection Mechanisms**
+
+   - *Description:* Outlines additional protection mechanisms, such as
+     overcurrent protection and thermal management.
+   - *Necessity:* Provides extra safety and stability, complementing PSE
+     controller protections.
+   - *PSE Controller Node:* Some protections may be implemented via
+     board-specific hardware or algorithms external to the controller.