netdev - Re: [net-next,PATCH 3/3] net: phy: realtek: Add property to enable SSC

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Message-ID: <acbbef86-9f8a-45fd-8643-24c8efcbbb01@mailbox.org>
Date: Wed, 3 Dec 2025 21:17:24 +0100
From: Marek Vasut <marek.vasut@...lbox.org>
To: Vladimir Oltean <vladimir.oltean@....com>,
 Ivan Galkin <ivan.galkin@...s.com>,
 Christophe Roullier <christophe.roullier@...s.st.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
 Aleksander Jan Bajkowski <olek2@...pl>, Andrew Lunn <andrew@...n.ch>,
 Conor Dooley <conor+dt@...nel.org>, Eric Dumazet <edumazet@...gle.com>,
 Florian Fainelli <f.fainelli@...il.com>,
 Heiner Kallweit <hkallweit1@...il.com>, Jakub Kicinski <kuba@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Michael Klein <michael@...sekall.de>, Paolo Abeni <pabeni@...hat.com>,
 Rob Herring <robh@...nel.org>, Russell King <linux@...linux.org.uk>,
 devicetree@...r.kernel.org
Subject: Re: [net-next,PATCH 3/3] net: phy: realtek: Add property to enable
 SSC

On 12/3/25 10:42 AM, Vladimir Oltean wrote:

Hello Vladimir,

>> +       ret = phy_write_paged(phydev, RTL8211F_SSC_PAGE, RTL8211F_SSC_RXC, 0x5f00);
>> +       if (ret < 0) {
>> +               dev_err(dev, "RXC SCC configuration failed: %pe\n", ERR_PTR(ret));
>> +               return ret;
>> +       }
> 
> I'm going to show a bit of lack of knowledge, but I'm thinking in the context
> of stmmac (user of phylink_config :: mac_requires_rxc), which I don't exactly
> know what it requires it for. Does it use the RGMII RXC as a system clock?

I believe dwmac (stmmac) uses RXC to drive at least (part of) its DMA.

+CC Christophe

> If so, I guess intentionally introducing jitter (via the spread spectrum
> feature) would be disastrous for it. In that case we should seriously consider
> separating the "spread spectrum for CLKOUT" and "spread spectrum for RGMII"
> device tree control properties.

I can split this into realtek,clkout-ssc-enable and realtek,rxc-ssc-enable.

Note that I use this exact configuration with both STM32MP13xx and 
STM32MP25xx RGMII / stmmac .

-- 
Best regards,
Marek Vasut