Re: [PATCH 2/3] arm64: dts: socfpga: agilex5: Add SoCDK TSN Config2 board

[Andrew Lunn <andrew@lunn.ch>]

On Tue, Jun 30, 2026 at 02:39:50PM +0000, Nazle Asmade, Muhammad Nazim Amirul wrote:
> On 30/6/2026 9:58 pm, Andrew Lunn wrote:
> >> + * gmac1 is the TSN port. The MAC operates in GMII mode internally
> >> + * while the PHY-side interface is RGMII, so mac-mode and phy-mode differ.
> >> + */
> >> +&gmac1 {
> >> +	status = "okay";
> >> +	phy-mode = "rgmii"; /* TX/RX clock delays provided by Agilex5 I/O hardware */
> > Could you provide more details about this. I want to understand the
> > big picture.
> > 
> > Normally we talk about the PCB providing the delays. This sounds like
> > it is the FPGA? So i need convincing this is correct.
> Hi Andrew,
> 
> Thanks for your quick review and yes, it is the FPGA — specifically a 
> soft IP block in the FPGA fabric that implements the RGMII clock delays 
> and is configured before Linux boots via the FPGA bitstream. The driver 
> must not add additional delays on top.

So it depends on how the converter block is described, but ....

From a big picture, MAC and PHY pair, it is the MAC which
implements the delays.

https://elixir.bootlin.com/linux/v6.15/source/Documentation/devicetree/bindings/net/ethernet-controller.yaml#L346

# There are a small number of cases where the MAC has hard coded
# delays which cannot be disabled. The 'phy-mode' only describes the
# PCB.  The inability to disable the delays in the MAC does not change
# the meaning of 'phy-mode'. It does however mean that a 'phy-mode' of
# 'rgmii' is now invalid, it cannot be supported, since both the PCB
# and the MAC and PHY adding delays cannot result in a functional
# link. Thus the MAC should report a fatal error for any modes which
# cannot be supported. When the MAC implements the delay, it must
# ensure that the PHY does not also implement the same delay. So it
# must modify the phy-mode it passes to the PHY, removing the delay it
# has added. Failure to remove the delay will result in a
# non-functioning link.

    Andrew

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