linux-kernel - Re: [PATCH net-next v4 4/4] net: ftgmac100: Add RGMII delay support for AST2600

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Message-ID: <4f0e4aa0-166d-4a7f-b91e-42dbc6b832e5@lunn.ch>
Date: Tue, 2 Dec 2025 14:36:11 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Jacky Chou <jacky_chou@...eedtech.com>
Cc: Tao Ren <rentao.bupt@...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>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Po-Yu Chuang <ratbert@...aday-tech.com>,
	Joel Stanley <joel@....id.au>,
	Andrew Jeffery <andrew@...econstruct.com.au>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	"linux-aspeed@...ts.ozlabs.org" <linux-aspeed@...ts.ozlabs.org>,
	"taoren@...a.com" <taoren@...a.com>
Subject: Re: [PATCH net-next v4 4/4] net: ftgmac100: Add RGMII delay support
 for AST2600

> Hi Andrew,
> 
> I miss one condition is using fixed-link property.
> In ftgmac100, there are RGMII, NC-SI and fixed-link property.
> On RGMII, we have solution on dedicated PHY, but there is an issue on fixed-link
> property.
> 
> Example on dedicated PHY.
> The driver can pass the "rgmii-id" to tell PHY driver to enable the internal delay on
> PHY side. Therefore, we can force to disable RGMII delay on MAC side.
> But there is not any driver when using fixed-link property, which means
> no body can tell the outside device, like switch or MAC-to-MAC, to enable the internal
> delay on them. Also mean the phy-mode in fixed-link case is not used.
> 
> Therefore, could we ignore the RGMII delay on MAC side when the ftgmac100 driver gets
> the fixed-link property? Just keep the original delay value?

MAC to MAC is one of the edge cases where phy-mode is not great. What
is generally accepted it to use rx-internal-delay-ps and
tx-internal-delay-ps with 2ns and a comment explaining it is a MAC to
MAC link, with this MAC adding the delays.

I suggest you see how messy it is to implement this for 2600. If the
code looks horrible, keep the bootloader delays. For 2700 you have a
clean slate, you can implement all this correctly from the start.

	Andrew