netdev - Re: Aquantia PHY in OCSGMII mode?

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Message-ID: <aK7CmzwQoINd1eYA@FUE-ALEWI-WINX>
Date: Wed, 27 Aug 2025 10:32:27 +0200
From: Alexander Wilhelm <alexander.wilhelm@...termo.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
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?

Am Wed, Aug 27, 2025 at 09:08:24AM +0100 schrieb Russell King (Oracle):
> On Wed, Aug 27, 2025 at 07:57:28AM +0200, Alexander Wilhelm wrote:
> > Hi Vladimir,
> > 
> > One of our hardware engineers has looked into the issue with the 100M link and
> > found the following: the Aquantia AQR115 always uses 2500BASE-X (GMII) on the
> > host side. For both 1G and 100M operation, it enables pause rate adaptation.
> > However, our MAC only applies rate adaptation for 1G links. For 100M, it uses a
> > 10x symbol replication instead.
> 
> This sounds like a misunderstanding, specifically:
> 
> "our MAC only applies rate adaptation for 1G links. For 100M, it uses
> 10x symbol replication instead."
> 
> It is the PHY that does rate adaption, so the MAC doesn't need to
> support other speeds. Therefore, if the PHY is using a 2.5Gbps link
> to the MAC with rate adaption for 100M, then the MAC needs to operate
> at that 2.5Gbps speed.
> 
> You don't program the MAC differently depending on the media side
> speed, unlike when rate adaption is not being used.

You're right. The flow control with rate adaptation is controlled by PHY. The
MAC should remain on the 2.5Gbps speed. Therefore I wonder why it uses 10x
symbol repetition.


Best regards
Alexander Wilhelm