[PATCH] arm64: dts: rockchip: use MAC TX delay for ROCK 4D

[Sebastian Reichel]

Hi,

On Thu, Jul 24, 2025 at 07:16:53PM +0100, Russell King (Oracle) wrote:
> On Thu, Jul 24, 2025 at 04:31:25PM +0200, Sebastian Reichel wrote:
> > According to the Ethernet controller device tree binding "rgmii-id"
> > means, that the PCB does not have extra long lines to add the required
> > delays. This is indeed the case for the ROCK 4D.
> > 
> > The problem is, that the Rockchip MAC Linux driver interprets the
> > interface type differently and abuses the information to configure
> > RX and TX delays in the MAC using (vendor) properties 'rx_delay' and
> > 'tx_delay'.
> > 
> > When Detlev Casanova upstreamed the ROCK 4D device tree, he used the
> > correct description for the board ("rgmii-id"). This results in no delays
> > being configured in the MAC. At the same time the PHY will provide
> > some delays.
> 
> So why can't the MAC be fixed to conform to the requirements that are
> well documented for RGMII?

Fixing the MAC driver without introducing regressions is not so
easy. It currently abuses the RGMII mode to detect if MAC delays
should be applied:

PHY_INTERFACE_MODE_RGMII => apply MAC delays
PHY_INTERFACE_MODE_RGMII_ID => do not apply MAC delays
PHY_INTERFACE_MODE_RGMII_RXID => apply TX MAC delay
PHY_INTERFACE_MODE_RGMII_TXID => apply RX MAC delay

As the delay values are sourced from DT, in theory in would be
possible to just always apply the MAC delays. The problem with
that is, that the default (missing DT properties) is not 0. So
this would result in regressions.

Greetings,

-- Sebastian
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