linux-kernel - Re: [PATCH net-next v2 2/3] net: phy: bcm5481x: Implement MII-Lite mode

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Message-ID: <846f7b15-250f-4e76-99b3-f208768aab32@lunn.ch>
Date: Mon, 23 Jun 2025 20:08:00 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Kamil Horák - 2N <kamilh@...s.com>
Cc: florian.fainelli@...adcom.com, bcm-kernel-feedback-list@...adcom.com,
	hkallweit1@...il.com, linux@...linux.org.uk, davem@...emloft.net,
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
	krzk+dt@...nel.org, conor+dt@...nel.org, netdev@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	robh@...nel.org
Subject: Re: [PATCH net-next v2 2/3] net: phy: bcm5481x: Implement MII-Lite
 mode

On Mon, Jun 23, 2025 at 05:10:46PM +0200, Kamil Horák - 2N wrote:
> From: Kamil Horák (2N) <kamilh@...s.com>
> 
> The Broadcom bcm54810 and bcm54811 PHYs are capable to operate in
> simplified MII mode, without TXER, RXER, CRS and COL signals as defined
> for the MII. While the PHY can be strapped for MII mode, the selection
> between MII and MII-Lite must be done by software.
> The MII-Lite mode can be used with some Ethernet controllers, usually
> those used in automotive applications. The absence of COL signal
> makes half-duplex link modes impossible but does not interfere with
> BroadR-Reach link modes on Broadcom PHYs, because they are full-duplex
> only. The MII-Lite mode can be also used on an Ethernet controller with
> full MII interface by just leaving the input signals (RXER, CRS, COL)
> inactive.

So everybody seems to be a general agreement this needs more thought.

How does a MAC know its RXER, CRS, COL inputs are inactive? Are you
expecting boards use pull up resistors? Or is the MAC driver expected
to poke around in the PHY node and find this lite property? That would
not be accepted. A phy-mode make would this clear, but it does require
every MAC which could be connected to this PHY needs to also accept
this PHY mode. Which comes back to, are pull-ups enough, so the MAC
has no idea it is connected to a -lite PHY?

	Andrew