linux-kernel - Re: [PATCH net v2] dt-bindings: net: ethernet-controller: Add informative text about RGMII delays

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Message-ID: <f82a86d3-6e06-4f24-beb5-68231383e635@lunn.ch>
Date: Wed, 11 Jun 2025 17:28:39 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Icenowy Zheng <uwu@...nowy.me>
Cc: "Russell King (Oracle)" <linux@...linux.org.uk>,
	Rob Herring <robh@...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>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Chaoyi Chen <chaoyi.chen@...k-chips.com>,
	Matthias Schiffer <matthias.schiffer@...tq-group.com>,
	Heiner Kallweit <hkallweit1@...il.com>, netdev@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v2] dt-bindings: net: ethernet-controller: Add
 informative text about RGMII delays

> Well in fact I have an additional question: when the MAC has any extra
> [tr]x-internal-delay-ps property, what's the threshold of MAC
> triggering patching phy mode? (The property might be only used for a
> slight a few hundred ps delay for tweak instead of the full 2ns one)

Maybe you should read the text.

The text says:

  In the MAC node, the Device Tree properties 'rx-internal-delay-ps'
  and 'tx-internal-delay-ps' should be used to indicate fine tuning
  performed by the MAC. The values expected here are small. A value of
  2000ps, i.e 2ns, and a phy-mode of 'rgmii' will not be accepted by
  Reviewers.

So a few hundred ps delay is fine. The MAC is not providing the 2ns
delay, the PHY needs to do that, so you don't mask the value.

> > > Well I can't find the reason of phy-mode being so designed except
> > > for
> > > leaky abstraction from phylib.
> > 
> > I have no idea what that sentence means, sorry.
> 
> Well, I mean the existence of rgmii-* modes is coupled with the
> internal of phylib, did I get it right?

This is the external API of phylib, it has nothing to do with the
internals of phylib.

/**
 * phy_attach - attach a network device to a particular PHY device
 * @dev: network device to attach
 * @bus_id: Bus ID of PHY device to attach
 * @interface: PHY device's interface
 *
 * Description: Same as phy_attach_direct() except that a PHY bus_id
 *     string is passed instead of a pointer to a struct phy_device.
 */
struct phy_device *phy_attach(struct net_device *dev, const char *bus_id,
			      phy_interface_t interface)

interface tells the PHY how it should configure its interface.

If you follow the guidelines, the PHY adds the delay if needed, you
get interface == phy-mode. However, interface and phy-mode are
different things. phy-mode describes the hardware, the PCB. interface
tells the PHY what to do. There are legitimate cases where
interface != phy-mode.

	Andrew