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

[Andrew Lunn <andrew@lunn.ch>]

> > # 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
> >
> > ---
> > pw-bot: cr
> Hi Andrew,
> 
> The delays are provided by the FPGA GMII-to-RGMII converter soft IP,
> which is hardcoded in the FPGA bitstream and cannot be disabled or
> modified from the driver side.
> 
> Using phy-mode = "rgmii" is intentional here — it prevents the PHY from
> adding its own internal delays on top, since the FPGA converter already
> provides the full required delay. This is consistent with how all other
> Agilex5 SoCDK board variants are described, as seen in commit
> c5637e5ceb4b ("arm64: dts: socfpga: agilex5: Fix phy-mode to rgmii as HW
> provides clock delay") already in Dinh Nguyen's tree, which applies the
> same rationale across all Agilex5 boards.

I've become more insistent that designs get this correct. So i don't
care too much about past systems. Many vendors are having to fix up
their drivers and DT in order to make new boards consistent.

You can look at your system as the FPGA being the MAC, and the PHY is
the PHY. The PCB is not providing the delay, the MAC is. This exactly
fits the description above.

     Andrew