linux-kernel - Re: [PATCH net-next] Revert "net: ethernet: ti: am65-cpsw: fixup PHY mode for fixed RGMII TX delay"

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Message-ID: <84588371-ddae-453e-8de9-2527c5e15740@lunn.ch>
Date: Wed, 30 Jul 2025 16:27:52 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Matthias Schiffer <matthias.schiffer@...tq-group.com>
Cc: Michael Walle <mwalle@...nel.org>, Nishanth Menon <nm@...com>,
	Vignesh Raghavendra <vigneshr@...com>,
	Tero Kristo <kristo@...nel.org>,
	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>,
	Roger Quadros <rogerq@...nel.org>, Simon Horman <horms@...nel.org>,
	Siddharth Vadapalli <s-vadapalli@...com>,
	Maxime Chevallier <maxime.chevallier@...tlin.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux@...tq-group.com
Subject: Re: [PATCH net-next] Revert "net: ethernet: ti: am65-cpsw: fixup PHY
 mode for fixed RGMII TX delay"

> I can confirm that the undocumented/reserved bit switches the MAC-side TX delay
> on and off on the J722S/AM67A.

Thanks.

> I have not checked if there is anything wrong with the undelayed
> mode that might explain why TI doesn't want to support it, but
> traffic appears to flow through the interface without issue if I
> disable the MAC-side and enable the PHY-side delay.

I cannot say this is true for TI, but i've often had vendors say that
they want the MAC to do the delay so you can use a PHY which does not
implement delays. However, every single RGMII PHY driver in Linux
supports all four RGMII modes. So it is a bit of a pointless argument.

And MAC vendors want to make full use of the hardware they have, so
naturally want to do the delay in the MAC because they can.

TI is a bit unusual in this, in that they force the delay on. So that
adds a little bit of weight towards maybe there being a design issue
with it turned off.

	Andrew