Re: [PATCH 4/5] arm64: dts: allwinner: a527: add EMAC0 to Radxa A5E board

[Andre Przywara <andre.przywara@arm.com>]

On Wed, 23 Apr 2025 18:58:37 +0200
Andrew Lunn <andrew@lunn.ch> wrote:

Hi,

> > +&emac0 {
> > +	phy-mode = "rgmii";  
> 
> Does the PCB have extra long clock lines in order to provide the
> needed 2ns delay? I guess not, so this should be rgmii-id.

That's a good point, and it probably true.

> 
> > +	phy-handle = <&ext_rgmii_phy>;
> > +
> > +	allwinner,tx-delay-ps = <300>;
> > +	allwinner,rx-delay-ps = <400>;  
> 
> These are rather low delays, since the standard requires 2ns. Anyway,
> once you change phy-mode, you probably don't need these.

Those go on top of the main 2ns delay, I guess to accommodate some skew
between the RX and TX lines, or to account for extra some PCB delay
between clock and data? The vendor BSP kernels/DTs program those board
specific values, so we have been following suit for a while, for the
previous SoCs as well.
I just tried, it also works with some variations of those values, but
setting tx-delay to 0 stops communication.

Cheers,
Andre