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Date:   Thu, 11 May 2023 13:50:30 +0200
From:   Daniel Golle <daniel@makrotopia.org>
To:     Andrew Lunn <andrew@lunn.ch>
Cc:     netdev@vger.kernel.org, linux-mediatek@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Heiner Kallweit <hkallweit1@gmail.com>,
        Russell King <linux@armlinux.org.uk>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>
Subject: Re: [PATCH 1/8] net: phy: realtek: rtl8221: allow to configure
 SERDES mode
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References: <cover.1683756691.git.daniel@makrotopia.org>
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 <8fdffc76-4b2f-44ea-9800-1e5d3624d94e@lunn.ch>
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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.