Handling an Extra Signal at PHY Reset

[Paul Kocialkowski <paul.kocialkowski@bootlin.com>]

Hi,

We are dealing with an Ethernet PHY (Marvell 88E1512) that comes with a
CONFIG pin that must be connected to one of the other pins of the PHY
to configure the LSB of the PHY address as well as I/O voltages (see
section 2.18.1 Hardware Configuration of the datasheet). It must be
connected "soon after reset" for the PHY to be correctly configured.

We have a switch for connecting the CONFIG pin to the other pin (LED0),
which needs to be controlled by Linux. The CONFIG pin seems to be used
for a PTP clock the rest of the time.

So we are wondering how to properly represent this case, especially on
the device-tree side.

The trick here is that this step is necessary before the PHY can be
discovered on the MDIO bus (and thus the PHY driver selected) so we
can't rely on the PHY driver to do this. Basically, it looks like we
need to handle this like the reset pin and describe it at the MDIO bus
level.

Here are some ideas for potential solutions:
- Allowing more than a single GPIO to be passed to the MDIO bus' reset-
gpios via device-tree and toggling all the passed GPIOs at once;

- Adding a new optional GPIO for the MDIO bus dedicated to controlling 
switches for such config switching, perhaps called "config-gpios"
(quite a narrow solution);

- Adding a broader power sequence description to the MDIO bus (a bit
like it's done with the mmc pwrseq descriptions) which would allow
specifying the toggle order/delays of various GPIOs (would probably be
the most extensive solution);

- Adding the extra GPIO control to the MAC description and toggling it
through bus->reset (probably the less invasive solution for the core
but not very satisfying from the description perspective, since this is
definitely not MAC-specific).

What do you think about how we could solve this issue?
Do you see other options that I missed here?

Cheers and thanks in advance,

Paul

-- 
Paul Kocialkowski, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com