Merge branch 'net-pse-pd-add-realtek-pse-mcu-support'

Jonas Jelonek says:

====================
net: pse-pd: add Realtek PSE MCU support

This series adds a PSE-PD driver for the microcontroller (MCU) that
fronts the PSE silicon on a range of managed switches, together with its
DT binding.

Hardware model
==============

These boards do not expose the PSE chips to the host directly. A small
microcontroller sits on an I2C/SMBus or UART bus and manages one or more
PSE chips behind it; the host CPU only ever talks to that MCU, using a
fixed 12-byte request/response protocol with a trailing checksum. The
PSE silicon never appears on the bus.

Two generations of the protocol exist, both Realtek's: an older one on
boards with Broadcom PSE silicon (BCM59111, BCM59121) and a newer one
used with Realtek's own PSE silicon (RTL8238B, RTL8239, RTL8239C). They
diverge in opcode numbering and a few response layouts; the driver
abstracts that behind a per-dialect opcode table and parser hooks,
selected by the compatible. The specific PSE chip behind the MCU is
detected at runtime and only influences per-chip constants (power scaling
and the per-port cap).

The compatibles
===============

The protocol compatibles name two generations of the Realtek protocol,
with the I2C framing folded in:

  realtek,pse-mcu-gen1        gen1, UART
  realtek,pse-mcu-gen1-smbus  gen1, I2C/SMBus
  realtek,pse-mcu-gen2        gen2, UART
  realtek,pse-mcu-gen2-smbus  gen2, I2C/SMBus
  realtek,pse-mcu-gen2-i2c    gen2, raw I2C

and each board carries a device-specific compatible that falls back to one
of these, e.g.

  compatible = "zyxel,xs1930-12hp-pse", "realtek,pse-mcu-gen2-smbus";

The naming is the part most likely to raise questions, so the reasoning up
front (the binding documents it too):

  - The node describes the MCU together with its Realtek firmware, not a
    PSE chip and not the microcontroller silicon. The PSE chips sit behind
    the MCU, never appear on the bus, and are reported by the MCU and
    detected at runtime; the microcontroller itself is a general-purpose
    part (GigaDevice, Nuvoton, ...) that varies across boards. What is
    fixed and Realtek's is the firmware and its host protocol - hence the
    'realtek' prefix.

  - gen1 and gen2 are two generations of that protocol, both Realtek's:
    gen1 on older boards fronting Broadcom PSE silicon, gen2 the altered
    protocol used once Realtek shipped their own PSE silicon. The
    generation is fixed per board and is all the driver needs at DT-parse
    time, so the compatible encodes it.

  - On I2C the MCU firmware expects one of two framings - SMBus or raw
    I2C - which is a genuine programming-model difference, so it is part
    of the compatible ('-smbus' / '-i2c'). A UART attachment carries no
    framing suffix; the transport is given structurally by the parent
    'serial' node.

  - Each board additionally carries a device-specific compatible that
    falls back to the protocol one. The driver only ever binds on the
    protocol compatible; the device-specific string keeps the binding
    specific and reserves a place for a future per-board quirk without
    having to retrofit device trees already deployed in the field.

Testing
=======

 - Linksys LGS328MPCv2     (RTL8238B, I2C)
 - Zyxel GS1900-10HP A1    (BCM59121, UART)
 - Zyxel GS1900-10HP B1    (RTL8238B, UART)
 - Zyxel GS1920-24HPv2     (BCM59121, SMBus)
 - Zyxel XMG1915-10EP      (RTL8239C, UART)
 - Zyxel XS1930-12HP       (RTL8239, SMBus)
====================

Link: https://patch.msgid.link/20260813222036.873930-1-jelonek.jonas@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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