linux-kernel - Re: [PATCH] net: stmmac: dwmac-sophgo: Add phy interface filter

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Message-ID: <370d13b7-bba8-449d-9050-e0719d20b57c@lunn.ch>
Date: Sun, 19 Oct 2025 02:04:02 +0200
From: Andrew Lunn <andrew@...n.ch>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Inochi Amaoto <inochiama@...il.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Chen Wang <unicorn_wang@...look.com>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>,
	Alexandre Torgue <alexandre.torgue@...s.st.com>,
	Han Gao <rabenda.cn@...il.com>, Icenowy Zheng <uwu@...nowy.me>,
	Vivian Wang <wangruikang@...as.ac.cn>, Yao Zi <ziyao@...root.org>,
	netdev@...r.kernel.org, sophgo@...ts.linux.dev,
	linux-stm32@...md-mailman.stormreply.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Yixun Lan <dlan@...too.org>, Longbin Li <looong.bin@...il.com>
Subject: Re: [PATCH] net: stmmac: dwmac-sophgo: Add phy interface filter

> "rgmii-id" doesn't mean "there is a delay _somewhere_ in the system".
> It's supposed to mean that the PHY should add delays on both tx and
> rx paths.

When passed to the PHY it means that.

However, DT describes the hardware, the PCB. "rgmii-id" means the PCB
does not provide the delays. So the MAC/PHY combination needs to add
the delays. We normally have the PHY provide the delays, so the
phy-mode is normally passed straight to the PHY. However, if the MAC
is adding a delay, which it is in this case, in one direction and
cannot be turned off, the value passed to the PHY needs to reflect
this, to avoid double delays.

And because the MAC delay cannot be turned off, it means there are PCB
designs which don't work, double delays. So it would be nice not to
list them in the binding.

	Andrew