[PATCH v2 3/4] arm64: dts: freescale: Add support for the Variscite i.MX8MP DART8MCustomBoard

[Laurent Pinchart]

Hi Ahmad,

On Mon, Nov 27, 2023 at 07:07:16AM +0100, Ahmad Fatoum wrote:
> On 25.10.23 18:50, Laurent Pinchart wrote:
> > The DT8MCustomBoard is a carrier board for DART i.MX8-based modules.
> > This device tree file adds support for the DT8MCustomBoard v2.0 with a
> > connected DART-MX8M-PLUS module.
> > 
> > Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > ---
> 
> 
> > +&eqos {
> > +	mdio {
> > +		ethphy1: ethernet-phy at 1 {
> > +			compatible = "ethernet-phy-ieee802.3-c22";
> > +			reg = <1>;
> > +			eee-broken-1000t;
> > +			reset-gpios = <&gpio_exp_2 0 GPIO_ACTIVE_LOW>;
> > +			reset-assert-us = <10000>;
> > +			reset-deassert-us = <20000>;
> 
> Ouch. So you have a PHY at address 0 (broadcast address) and a PHY at address
> 1 on the same bus? I think it's worth a comment whether broadcast for this
> PHY here is disabled by strapping or in the bootloader.

This particular PHY is an ADI1300. The schematics indicates:

LEDs - active HIGH, address 00001b
SOM PHY MDIO address 00000b

I tried to investigate, and I haven't found any mention of address 0
being a broadcast address in the ADIN1300 documentation. Trying to dig a
bit more, I've read clause 22 of the IEEE 802.3ak specification and
found this:

22.2.4.5.5 PHYAD (PHY Address)

The PHY Address is five bits, allowing 32 unique PHY addresses. The
first PHY address bit transmitted and received is the MSB of the
address. A PHY that is connected to the station management entity via
the mechanical interface defined in 22.6 shall always respond to
transactions addressed to PHY Address zero <00000>. A station management
entity that is attached to multiple PHYs must have prior knowledge of
the appropriate PHY Address for each PHY.

Section 22.6 defines a 40-pin physical connector, which is not
applicable here. I've also found
https://ieee802.org/3/10G_study/email/msg03514.html which states

"People have made all kinds of wild assumptions about the way MDIO/MDC
work in the past. Some people actually believe that PHYADD <00000> is a
broadcast address."

-- 
Regards,

Laurent Pinchart