linux-kernel - Re: [PATCH] arm64: dts: rockchip: Change ethernet TXD timing delay value

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Message-ID: <4d25c642-d457-4cb7-8bc1-587edd4f1d75@lunn.ch>
Date: Tue, 19 Aug 2025 18:03:47 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Dongjin Kim <tobetter@...il.com>
Cc: Rob Herring <robh+dt@...nel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
	Heiko Stuebner <heiko@...ech.de>, devicetree@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-rockchip@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: dts: rockchip: Change ethernet TXD timing delay
 value

On Tue, Aug 19, 2025 at 01:50:17PM +0900, Dongjin Kim wrote:
> Very poor network performance is observed on the ODROID-M1 Board
> (REV 1.0 20220613) when running iperf3 (under 200Mbits/sec), and
> this problem can be resolved after changing the value of TXD
> timing to 0x35 from 0x4f.

How did you decide on 0x35?

~/linux/arch/arm64$ grep -hr "tx_delay = " | sort | uniq -c
      4 	tx_delay = <0x10>;
      1 	tx_delay = <0x1a>;
      1 	tx_delay = <0x21>;
      2 	tx_delay = <0x22>;
      4 	tx_delay = <0x24>;
      2 	tx_delay = <0x26>;
     14 	tx_delay = <0x28>;
      1 	tx_delay = <0x2a>;
      6 	tx_delay = <0x30>;
      1 	tx_delay = <0x3a>;
      3 	tx_delay = <0x3c>;
      2 	tx_delay = <0x42>;
      5 	tx_delay = <0x43>;
      2 	tx_delay = <0x44>;
      1 	tx_delay = <0x46>;
      6 	tx_delay = <0x4f>;

The numbers are all over the place, but it is clear that 0x28 is the
most popular for some reason. But since this delay values are magic,
i've no idea why.

     Andrew