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

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Message-ID: <20250623175135.5ddee2ea@2a02-8440-d115-be0d-cec0-a2a1-bc3c-622e.rev.sfr.net>
Date: Mon, 23 Jun 2025 17:51:35 +0200
From: Maxime Chevallier <maxime.chevallier@...tlin.com>
To: Kamil Horák - 2N <kamilh@...s.com>
Cc: <florian.fainelli@...adcom.com>,
 <bcm-kernel-feedback-list@...adcom.com>, <andrew@...n.ch>,
 <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

Hi Kamil,

On Mon, 23 Jun 2025 17:10:46 +0200
Kamil Horák - 2N <kamilh@...s.com> 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.

I'm following-up to Andrew's suggestion of making it a dedicated
phy-mode. You say that this requires only phy-side configuration,
however you also say that with MII-lite, you can't do half-duplex.

Looking at the way we configure the MAC to PHY link, how can the MAC
driver know that HD isn't available if this is a phy-only property ?

Relying on the fact that the PHYs that use MII-Lite will only ever
setup a full-duplex link with the partner seems a bit fragile, when we
could indicate that this new MII-Lite mode only supports 10FD/100FD,
through this mapping code here :

https://elixir.bootlin.com/linux/v6.16-rc2/source/drivers/net/phy/phy_caps.c#L282

Besides that, given that this is a physically different MAC to PHY
interface (missing signals compared to MII), one could also argue that
this warrants a dedicated phy-mode.

Maxime