netdev - Re: [PATCH v1] net: sfp: add quirks for ODI DFP-34X-2C2

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Message-ID: <ZdyiigYNGf8WVZNu@shell.armlinux.org.uk>
Date: Mon, 26 Feb 2024 14:39:06 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Shengyu Qu <wiagn233@...look.com>
Cc: andrew@...n.ch, hkallweit1@...il.com, davem@...emloft.net,
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH v1] net: sfp: add quirks for ODI DFP-34X-2C2

On Mon, Feb 26, 2024 at 10:16:46PM +0800, Shengyu Qu wrote:
> Hi Russell,
> 
> > On Mon, Feb 26, 2024 at 09:23:46PM +0800, Shengyu Qu wrote:
> > > ODI DFP-34X-2C2 is capable of 2500base-X, but incorrectly report its
> > > capabilities in the EEPROM.
> > > So use sfp_quirk_2500basex for this module to allow 2500Base-X mode.
> > This was previously submitted by Sergio Palumbo, and comes in two
> > different forms - an OEM version and non-OEM. There was extensive
> > discussion about this, and the result is that I'm not accepting this
> > quirk for this module.
> > 
> > The reason is that the module _defaults_ to 1000base-X and requires
> > manual reconfiguration by the user to operate at 2500base-X.
> > Unfortunately, there is no way for the kernel to know whether that
> > reconfiguration has occurred.
> No, In the firmware of this stick, the speed rate is configured to auto
> negotiation rather than fixed 1000base-X.

How does this "auto negotiation" work?

I mean *exactly* how does it work? How does it know whether the host is
operating at 1000base-X or 2500base-X?

There is *no* inband protocol to allow this to be negotiated.

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