Re: [PATCH] riscv: dts: spacemit: k3-pico-itx: Fix non-functional ethernet TX timing

From: Andrew Rembrandt <kernel@rembrandt.dev>
To: Inochi Amaoto <inochiama@gmail.com>
Cc: Yixun Lan <dlan@kernel.org>,
	linux-riscv@lists.infradead.org,  spacemit@lists.linux.dev,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] riscv: dts: spacemit: k3-pico-itx: Fix non-functional ethernet TX timing
Date: Tue, 9 Jun 2026 20:24:46 +0200	[thread overview]
Message-ID: <aihYftGNcpUd4UUR@mcmanus> (raw)
In-Reply-To: <aiemGajqcQeY9avz@inochi.infowork>

On Tue, Jun 09, 2026 at 01:37:56 pm Inochi Amaoto wrote:
> You should set tx-internal-delay-ps of the phy instead of
> the controller. This is a phy in "rgmii-id" mode.
Thanks for the review. I don't think that works on this board, but
please let me know if I'm missing something.

The PHY is a Realtek RTL8211F. Its driver only toggles the chip's fixed
~2ns delay on/off from phy-mode, and per the datasheet that's a hardware
limit, not a driver one -- the RTL8211F has no tunable delay, so a ps
value on the PHY node has nothing to act on.

The only configurable delay element on K3 is the SoC RGMII delay line,
and dwmac-spacemit programs it from tx-/rx-internal-delay-ps read off
the controller node (spacemit_dwmac_probe()), so that is where the
property has to live to take effect. This is the MAC "fine tuning ...
values expected here are small" case described in
Documentation/devicetree/bindings/net/ethernet-controller.yaml: the
RTL8211F supplies the fixed ~2ns via rgmii-id and the MAC adds a 400ps
trim on top. Ideally the PHY would own the delay, but as Andrew Lunn
noted reviewing dwmac-meson8b, that requires the PHY to implement a
configurable delay, which not all do.

The already-merged k1-bananapi-f3 does the same thing: eth1 combines
phy-mode = "rgmii-id" with tx-internal-delay-ps = <250> on the
controller node (via the K1 EMAC rather than dwmac, but the same DT
pattern).

Would you be OK keeping it on the controller given the above? For v2 I
can add an explicit rx-internal-delay-ps = <0> to match the other
SpacemiT boards (k1-bananapi-f3 included) and spell this reasoning out
in the commit message.

Thanks,
Andrew

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