linux-kernel - Re: [PATCH 1/8] net: phy: realtek: rtl8221: allow to configure SERDES mode

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Message-ID: <ZFzWhn7yXazjjmzq@pidgin.makrotopia.org>
Date:   Thu, 11 May 2023 13:50:30 +0200
From:   Daniel Golle <daniel@...rotopia.org>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     netdev@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
Subject: Re: [PATCH 1/8] net: phy: realtek: rtl8221: allow to configure
 SERDES mode

On Thu, May 11, 2023 at 02:38:19AM +0200, Andrew Lunn wrote:
> > +#define RTL8221B_SERDES_OPTION_MODE_2500BASEX_SGMII	0
> > +#define RTL8221B_SERDES_OPTION_MODE_HISGMII_SGMII	1
> > +#define RTL8221B_SERDES_OPTION_MODE_2500BASEX		2
> 
> So what is 2500BASEX_SGMII? You cannot run SGMII at 2.5G, because
> there is no way to repeat a symbol 2.5 times so that a 1G stream takes
> up 2.5G bandwidth. The SGMII signalling also does not work at 2.5G.

*_MODE_2500BASEX_SGMII means that the PHY will dynamically switch
interface mode between 2500Base-X for 2500M links and SGMII for
everything else.

Setting  *_MODE_2500BASEX in contrast to that enabled rate-adapter mode
and always uses 2500Base-X no matter what the speed of the link on the
TP interface is.

I will add a comment explaining that.