[PATCH v3 1/4] dt-bindings: net: ethernet-controller: Correct the definition of phy-mode

[Andrew Lunn]

On Tue, Jan 21, 2025 at 02:08:40PM +0100, Maxime Chevallier wrote:
> On Tue, 21 Jan 2025 15:54:53 +0800
> Yijie Yang <quic_yijiyang at quicinc.com> wrote:
> 
> > Correct the definition of 'phy-mode' to reflect that RX and TX delays are
> > added by the board, not the MAC, to prevent confusion and ensure accurate
> > documentation.
> 
> That's not entirely correct though. The purpose of the RGMII variants
> (TXID, RXID, ID) are mostly to know whether or not the PHY must add
> internal delays. That would be when the MAC can't AND there's no PCB
> delay traces. Some MAC can insert delays.
> 
> There's documentation here as well on that point :
> 
> https://elixir.bootlin.com/linux/v6.13-rc3/source/Documentation/networking/phy.rst#L82

This is part of the problem. This describes
PHY_INTERFACE_MODE_RGMII_*, and the value passed to phylib. The
documentation even says:

   The values of phy_interface_t must be understood from the
   perspective of the PHY device itself,

But the value in DT is about the board as a whole, it describes the
hardware. Software gets to decide if the MAC or the PHY adds the
delays, if the board does not provide the delay.

> So, MACs may insert delays. A modification for the doc, if needed,
> would rather be :
> 
> -      # RX and TX delays are added by the MAC when required
> +      # RX and TX delays are added by the MAC or PCB traces when required

>From the perspective of the board, this is wrong. 'rgmii' means the
board provides the delays.

There is a parallel discussion going on, about how aspeed have also
got there implementation wrong. See:

https://lore.kernel.org/netdev/0ee94fd3-d099-4d82-9ba8-eb1939450cc3@lunn.ch/

and the test of that thread.

	Andrew