Re: [PATCH net-next v5 1/4] dt-bindings: net: pse-pd: add bindings for Realtek/Broadcom PSE MCU

[Conor Dooley <conor@kernel.org>]

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On Tue, Jul 07, 2026 at 10:50:21PM +0200, Jonas Jelonek wrote:
> Hi Conor,
> 
> On 07.07.26 19:25, Conor Dooley wrote:
> > On Mon, Jul 06, 2026 at 10:30:00PM +0200, Jonas Jelonek wrote:
> >> [...]
> >> The protocol and firmware on the MCU, most likely the whole "solution",
> >> is from Realtek. The setup is always the same on most Realtek-based
> >> switches (saying most because a few counterexamples use completely
> >> different setups, not even Broadcom or Realtek PSE silicon). The host
> >> interface is always the same (except for I2C vs. SMBus vs. UART, which
> >> is likely just a config in the MCU firmware). Therefore "realtek," is the
> >> right prefix for all of these.
> >>
> >> Broadcom is not really involved here except for their PSE silicon being
> >> used. Maybe Realtek modeled their MCU host protocol after the one that
> >> Broadcom PSE silicon uses as host interface, but this is rather guessing.
> >>
> >> Maybe a historical view might help. Older RTL83xx-based switches with
> >> PoE shipped with this setup using Broadcom PSE silicon. From what I know,
> >> at this point Realtek didn't design their own PSE silicon. They used the
> >> Broadcom silicon, put a MCU as a manager in front of it with their firmware
> >> and a host protocol based on what Broadcom PSE itself uses. At some
> >> point Realtek started to design their own PSE silicon which then was
> >> used in newer switches instead of Broadcom PSE.
> > Right, in that case it does make sense to use a realtek prefix, since
> > the software and mcu solution is all theirs.
> >
> >> [...]
> >> Only one at a time is used, but not combined in any way. All switches
> >> I've seen so far always have a single management MCU for PoE, not
> >> multiple. Thus, only a single variant is used. Which variant is used
> >> likely depends on the board vendor which then tells Realtek "I want your
> >> PoE solution, I can attach it via (I2C/SMBus/UART)". At least for UART vs.
> >> I2C/SMBus there are sometimes valid reasons to use UART over the other.
> >>
> >> There is only a single switch (from Linksys) where the MCU expects raw
> >> I2C messages. SMBus transaction fail actually. But I don't see the reason
> >> why Linksys did it that way. The reason can't be that the MCU is attached
> >> on a bit-banged I2C because another switch uses SMBus transaction on
> >> a bit-banged I2C.
> > Reading this, it feels like you "should" have compatibles that uniquely
> > identify the protocol used. 
> 
> Ok, I hope I put this together correctly. A concrete proposal:
> 
> "realtek,pse-mcu-gen1"                        (Protocol Gen 1, UART)
> "realtek,pse-mcu-gen1-smbus"            (Protocol Gen 1, SMBus)
> "realtek,pse-mcu-gen2"                        (Protocol Gen 2, UART)
> "realtek,pse-mcu-gen2-i2c"                  (Protocol Gen 2, raw I2C)
> "realtek,pse-mcu-gen2-smbus"            (Protocol Gen 2, SMBus)
> 
> This uniquely identifies the protocol used: first generation and second
> generation. As Rob mentioned before [1], this also pulls in the raw I2C
> vs. SMBus framing in contrast to having it in a property. The framing
> suffix appears only on I2C attachments because it doesn't apply to
> UART transport, and this is given by the parent serial@ node.
> 
> Though I'm still open for suggestions regarding the protocol
> identification if "-gen1"/"-gen2" is not acceptable.

This seems reasonable enough.

> > Looking at the devices below, it seems like it
> > would be possible to use compatibles based on the switches themselves, e.g.
> > zyxel,xs1930-pse etc. If there are other devices that use the same
> > protocol, they could fall back to the ones below.
> >
> > It'd be good to have the net developers weigh in though, as to whether
> > using compatibles based on the switches is suitable.
> 
> I'd lean against, but happy to defer to you and the net maintainers. The
> node describes the MCU with its Realtek firmware — the firmware/protocol
> defines the device. Everything that differs between instances on the
> controller level would be captured by the compatibles proposed above, so
> a board compatible would encode nothing there the gen+framing string
> doesn't.
> 
> Observed variation lives on another level. For instance, some boards have
> heterogeneous per-port caps (e.g. 16 ports at 60W, 8 ports at 30W). This
> is clearly something that should be expressed per-pse-pi, not in a
> switch-specific compatible.
> 
> It would also be an exception to the other PSE-PD bindings. They describe
> controllers used across many switches too, yet none encode the

The difference is those cases (for what few pse-psd bindings there are)
the compatibles correspond to individual devices. Here you have
compatibles you're going to use to cover multiple devices (with device
corresponding to a combination of mcu/firmware/hardware behind the mcu).
That lack of a 1:1 mapping is why I'm asking for something different from
you than you see with the existing pse-pd devices. The switch the device
is integrated on seems to be the only thing that reasonably makes sense
to use.


> switch/enclosure. Board-specific compatibles might still be added later in
> case a device really has a variation or quirk that genuinely needs its own
> compatible.

And in doing so, have to retrofit that compatible to all devicetrees
that use it. This is one of the reasons that we generally demand
device-specific compatibles.

You could add switch-specific compatibles that fall back to the ones you
provide above, with the driver only using the ones above unless
something crops up in the future?

Cheers,
Conor.

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