Re: [PATCH net-next 2/2] net: sfp: manage receiver and transmitter regulators

[Andrew Lunn]

> For SFP my understanding is that SFP has a physical specification which
> includes power inputs and that these supplies are being requested by the
> devices that consume them.  If some part of that is not the case then it
> sounds like the bindings aren't describing the hardware (or at least are
> a bit unclear about how they're doing so) and should be revised.  The
> series doesn't seem to do anything at all with the supply side either,
> I'm guessing there are some SFP controllers with integrated power
> provisioning.

There is not really an SFP controller.

SFPs really break up into two parts, because they are
hot-pluggable. There is a cage, which is mounted on the board, and a
module which is inserted into the cage. The cage is passive.

https://en.wikipedia.org/wiki/Small_Form-factor_Pluggable gives a
reasonable overview.

The cage provides the module with power, 3.3v, max 300mA for Rx and
the same for TX. Something must supply the cage, and most designs just
connect the cage to the board power rails. The example give in
Multisource Agreement does exactly that, with some capacitors and
inducters to limit surge on hot plug.

This is the first board since 2017, when support for SFPs was added,
which can actually control the power supplies. We cannot make
regulators mandatory without breaking backwards compatibility.

So for me, the patch is good as it is now, the regulators are
optional.

   Andrew