linux-kernel - Re: [PATCH v3 2/2] net: dsa: mxl-gsw1xx: Support R(G)MII slew rate configuration

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Message-ID: <ac648a7e6883e68026f67ae0544b544614006d8f.camel@siemens.com>
Date: Mon, 5 Jan 2026 21:00:27 +0000
From: "Sverdlin, Alexander" <alexander.sverdlin@...mens.com>
To: "olteanv@...il.com" <olteanv@...il.com>
CC: "hauke@...ke-m.de" <hauke@...ke-m.de>, "andrew@...n.ch" <andrew@...n.ch>,
	"davem@...emloft.net" <davem@...emloft.net>, "devicetree@...r.kernel.org"
	<devicetree@...r.kernel.org>, "robh@...nel.org" <robh@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"krzk+dt@...nel.org" <krzk+dt@...nel.org>, "pabeni@...hat.com"
	<pabeni@...hat.com>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"edumazet@...gle.com" <edumazet@...gle.com>, "kuba@...nel.org"
	<kuba@...nel.org>, "daniel@...rotopia.org" <daniel@...rotopia.org>,
	"conor+dt@...nel.org" <conor+dt@...nel.org>
Subject: Re: [PATCH v3 2/2] net: dsa: mxl-gsw1xx: Support R(G)MII slew rate
 configuration

Hi Vladimir,

On Mon, 2026-01-05 at 21:30 +0200, Vladimir Oltean wrote:
> > +	return regmap_update_bits(gsw1xx_priv->shell, GSW1XX_SHELL_RGMII_SLEW_CFG,
> > +				  RGMII_SLEW_CFG_DRV_TXD | RGMII_SLEW_CFG_DRV_TXC,
> > +				  (RGMII_SLEW_CFG_DRV_TXD | RGMII_SLEW_CFG_DRV_TXC) * rate);
> 
> I don't have a particularly strong EE background, but my understanding
> is this:
> 
> RGMII MACs provide individual slew rate configuration for TXD[3:0] and
> for TX_CLK because normally, you'd want to focus on the TX_CLK slew rate
> (in the sense of reducing EMI) more than on the TXD[3:0] slew rate.
> This is for 2 reasons:
> (1) the EMI noise produced by TX_CLK is in a much narrower spectrum
>     (runs at fixed 125/25/2.5 MHz) than TXD[3:0] (pseudo-random data).
> (2) reducing the slew rate for TXD[3:0] risks introducing inter-symbol
>     interference, risk which does not exist for TX_CLK
> 
> Your dt-binding does not permit configuring the slew rates separately,
> even though the hardware permits that. Was it intentional?

thanks for the hint! This is definitely something I need to discuss with HW
colleagues and get back to you!

-- 
Alexander Sverdlin
Siemens AG
www.siemens.com