netdev - Re: Aquantia PHY in OCSGMII mode?

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Message-ID: <aK7MV2TrkVKwOEpr@shell.armlinux.org.uk>
Date: Wed, 27 Aug 2025 10:13:59 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Alexander Wilhelm <alexander.wilhelm@...termo.com>
Cc: Vladimir Oltean <vladimir.oltean@....com>, Andrew Lunn <andrew@...n.ch>,
	Heiner Kallweit <hkallweit1@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Aquantia PHY in OCSGMII mode?

On Wed, Aug 27, 2025 at 11:03:42AM +0200, Alexander Wilhelm wrote:
> Am Wed, Aug 27, 2025 at 09:47:49AM +0100 schrieb Russell King (Oracle):
> > On Wed, Aug 27, 2025 at 10:41:11AM +0200, Alexander Wilhelm wrote:
> > > Set to 100M:
> > > 
> > >     fsl_dpaa_mac: [DEBUG] <memac_link_down> called
> > >     fsl_dpaa_mac: [DEBUG] <memac_link_up> called
> > >     fsl_dpaa_mac: [DEBUG] * mode: 0
> > >     fsl_dpaa_mac: [DEBUG] * phy_mode(interface): 2500base-x
> > >     fsl_dpaa_mac: [DEBUG] * memac_if_mode: 00000002 (IF_MODE_GMII)
> > >     fsl_dpaa_mac: [DEBUG] * speed: 2500
> > >     fsl_dpaa_mac: [DEBUG] * duplex: 1
> > >     fsl_dpaa_mac: [DEBUG] * tx_pause: 1
> > >     fsl_dpaa_mac: [DEBUG] * rx_pause: 1
> > 
> > So the PHY reported that it's using 2500base-X ("OCSGMII") for 100M,
> > which means 0x31b 3 LSBs are 4. Your hardware engineer appears to be
> > incorrect in his statement.
> 
> I asked the hardware engineer again. The point is that the MAC does not set
> SGMII for 100M. It still uses 2500base-x but with 10x paket repetition.

No one uses symbol repetition when in 2500base-x mode. Nothing supports
it. Every device datasheet I've read states clearly that symbol
repetition is unsupported when operating at 2.5Gbps.

Also think about what this means. If the link is operating at 2.5Gbps
with a 10x symbol repetition, that means the link would be passing
250Mbps. That's not compatible with _anything_.

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