netdev - Re: [PATCH 3/4] arm64: dts: socfpga: agilex5: enable gmac2 on the Agilex5 dev kit

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Message-ID: <86e1e04a-3242-482c-adb0-dde7375561c1@lunn.ch>
Date: Mon, 14 Jul 2025 20:52:17 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Matthew Gerlach <matthew.gerlach@...era.com>
Cc: andrew+netdev@...n.ch, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, robh@...nel.org,
	krzk+dt@...nel.org, conor+dt@...nel.org, mcoquelin.stm32@...il.com,
	alexandre.torgue@...s.st.com, dinguyen@...nel.org,
	maxime.chevallier@...tlin.com, richardcochran@...il.com,
	netdev@...r.kernel.org, devicetree@...r.kernel.org,
	linux-stm32@...md-mailman.stormreply.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] arm64: dts: socfpga: agilex5: enable gmac2 on the
 Agilex5 dev kit

On Mon, Jul 14, 2025 at 11:09:33AM -0700, Matthew Gerlach wrote:
> 
> 
> On 7/14/25 10:25 AM, Andrew Lunn wrote:
> > > +&gmac2 {
> > > +	status = "okay";
> > > +	phy-mode = "rgmii";	/* Delays implemented by the IO ring of the Agilex5 SOCFPGA. */
> > 
> > Please could you explain in more details what this means.
> > 
> > The normal meaning for 'rgmii' is that the PCB implements the delay. I
> > just want to fully understand what this IO ring is, and if it is part
> > of the PCB.
> 
> The IO ring is the logic in the Agilex5 that controls the pins on the chip.
> It is this logic that sits between the MAC IP in the Agilex5 and the pins
> connected to the PCB that is inserting the necessary delays. Technically the
> PCB is not implementing the delays, but the "wires" between the MAC and the
> external pins of the Agilex5 are implementing the delay. It seems to me that
> "rgmii" is a more accurate description of the hardware than "rgmii-id" in
> this case.

Is this delay hard coded, physically impossible to be disabled? A
syntheses option? Can it be changed at run time? Is the IO ring under
the control of a pinctrl driver? Can i use the standard 'skew-delay'
DT property to control this delay?

For silicon, if the delay cannot be removed, we have MAC drivers masks
the phy-mode to indicate it has implemented the delay. The MAC driver
should also return -EINVAL for any other RGMII mode than rgmii-id,
because that is the only RGMII mode which is possible.

Since this is an FPGA, it is a bit more complex, so i want to fully
understand what is going on, what the different options are.

	Andrew