linux-kernel - Re: [PATCH v2 net-next 3/3] net: phy: aquantia: add support for aqr115c

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Date: Fri, 28 Jun 2024 02:18:45 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Daniel Golle <daniel@...rotopia.org>
Cc: Bartosz Golaszewski <brgl@...ev.pl>,
	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>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v2 net-next 3/3] net: phy: aquantia: add support for
 aqr115c

On Thu, Jun 27, 2024 at 11:42:45PM +0100, Daniel Golle wrote:
> Hi Bartosz,
> 
> On Thu, Jun 27, 2024 at 01:30:17PM +0200, Bartosz Golaszewski wrote:
> > From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> > 
> > Add support for a new model to the Aquantia driver. This PHY supports
> > Overlocked SGMII mode with 2.5G speeds.
> 
> I don't think that there is such a thing as "Overclocked SGMII mode with
> 2.5G speed".

Unfortunately, there is. A number of vendors say they do this, without
saying quite what they actually do.  As you point out, symbol
replication does not work, and in-band signalling also makes no
sense. So they throw all that away. Leaving just the higher clock
rate, single speed, and no in-band signalling.

In the end, that looks very similar to 2500BaseX with broken inband
signalling.

> Hence I assume that what you meant to say here is that the PHY uses
> 2500Base-X as interface mode and performs rate-adaptation for speeds
> less than 2500M (or half-duplex) using pause frames.

Not all systems assume rate adaptation. Some are known to use SGMII
for 10/100/1G with inband signalling, and then swap to 2500BaseX
without inband-signalling for 2.5G operation!

2.5G is a mess.

    Andrew