linux-kernel - Re: [PATCH v12 2/4] phy: Add ethernet serdes configuration option

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Date:   Wed, 27 Jan 2021 15:45:45 +0100
From:   Steen Hegelund <steen.hegelund@...rochip.com>
To:     Kishon Vijay Abraham I <kishon@...com>,
        Vinod Koul <vkoul@...nel.org>
CC:     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>,
        Andrew Lunn <andrew@...n.ch>
Subject: Re: [PATCH v12 2/4] phy: Add ethernet serdes configuration option

Hi Kishon,

On Wed, 2021-01-27 at 18:04 +0530, Kishon Vijay Abraham I wrote:
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> 

...

> > > 
> > > I'm not familiar with Ethernet. Are these generic media types? what
> > > does
> > > SR or DAC refer to?
> > 
> > The SR stands for Short Reach and is a fiber type connection used by
> > SFPs.  There also other "reach" variants.
> > 
> > DAC stands for Direct Attach Copper and is a type of cable that plugs
> > into an SFP cage and provides information back to the user via its
> > EEPROM regarding supported speed and capabilities in general.  These
> > typically supports speed of 5G or more.
> > 
> > The SFP/Phylink is the "out-of-band" method that provides the type of
> > connection: speed and media type that allows the client to adapt the
> > SerDes configuration to the type of media selected by the user.
> > 
> > > Are there other media types? What is the out-of-band
> > > mechanism by which the controller gets the media type? Why was this
> > > not
> > > required for other existing Ethernet SERDES?
> > 
> > This is probably a matter of the interface speed are now getting higher
> > and the amount of configuration needed for the SerDes have increased,
> > at the same time as this is not being a static setup, because the user
> > an plug and unplug media to the SFP cage.
> > 
> > > Are you aware of any other
> > > vendors who might require this?
> > 
> > I suspect that going forward it will become more widespread, at least
> > we have more chips in the pipeline that need this SerDes for high speed
> > connectivity.
> 
> For this case I would recommend to add new API, something like
> phy_set_media(). Configure() and Validate() is more for probing
> something that is supported by SERDES and changing the parameters. But
> in this case, I'd think the media type is determined by the cable that
> is connected and cannot be changed.
> 
> Thanks
> Kishon

I assume that you would like a separate interface for the speed information as well?

Thanks for your comments.

BR
Steen