netdev - Re: [PATCH v14 2/4] phy: Add media type and speed serdes configuration interfaces

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Date:   Tue, 16 Feb 2021 09:37:56 +0100
From:   Steen Hegelund <steen.hegelund@...rochip.com>
To:     Andrew Lunn <andrew@...n.ch>,
        Kishon Vijay Abraham I <kishon@...com>
CC:     Vinod Koul <vkoul@...nel.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Lars Povlsen <lars.povlsen@...rochip.com>,
        Bjarni Jonasson <bjarni.jonasson@...rochip.com>,
        Microchip UNG Driver List <UNGLinuxDriver@...rochip.com>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v14 2/4] phy: Add media type and speed serdes
 configuration interfaces

Hi Andrew and Kishon,

On Mon, 2021-02-15 at 15:07 +0100, Andrew Lunn wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you
> know the content is safe
> 
> On Mon, Feb 15, 2021 at 05:25:10PM +0530, Kishon Vijay Abraham I
> wrote:
> > Okay. Is it going to be some sort of manual negotiation where the
> > Ethernet controller invokes set_speed with different speeds? Or the
> > Ethernet controller will get the speed using some out of band
> > mechanism
> > and invokes set_speed once with the actual speed?
> 
> Hi Kishon
> 
> There are a few different mechanism possible.
> 
> The SFP has an EEPROM which contains lots of parameters. One is the
> maximum baud rate the module supports. PHYLINK will combine this
> information with the MAC capabilities to determine the default speed.
> 
> The users can select the mode the MAC works in, e.g. 1000BaseX vs
> 2500BaseX, via ethtool -s. Different modes needs different speeds.
> 
> Some copper PHYs will change there host side interface baud rate when
> the media side interface changes mode. 10GBASE-X for 10G copper,
> 5GBase-X for 5G COPPER, 2500Base-X for 2.5G copper, and SGMII for
> old school 10/100/1G Ethernet.
> 
> Mainline Linux has no support for it, but some 'vendor crap' will do
> a
> manual negotiation, simply trying different speeds and see if the
> SERDES establishes link. There is nothing standardised for this, as
> far as i know.
> 
>     Andrew

Yes, in case I mention the only way to ensure communication is human
intervention to set the speed to the highest common denominator.

BR
Steen