netdev - Re: [v3 2/2] dsa: mv88e6xxx: make serdes SGMII/Fiber output amplitude configurable

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Date:   Wed, 8 Dec 2021 18:00:57 +0100
From:   Marek Behún <kabel@...nel.org>
To:     Holger Brunck <holger.brunck@...achienergy.com>
Cc:     Vladimir Oltean <olteanv@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>, Andrew Lunn <andrew@...n.ch>
Subject: Re: [v3 2/2] dsa: mv88e6xxx: make serdes SGMII/Fiber output
 amplitude configurable

On Wed, 8 Dec 2021 18:49:32 +0200
Vladimir Oltean <olteanv@...il.com> wrote:

> On Wed, Dec 08, 2021 at 06:41:31PM +0200, Vladimir Oltean wrote:
> > On Wed, Dec 08, 2021 at 05:21:04PM +0100, Marek Behún wrote:  
> > > Hello Vladimir,  
> > > > > > but the mv88e6xxx driver also drives switches that allow changing serdes
> > > > > > modes. There does not need be dedicated TX amplitude register for each serdes
> > > > > > mode, the point is that we may want to declare different amplitudes for
> > > > > > different modes.
> > > > > >
> > > > > > So the question is: if we go with your binding proposal for the whole mv88e6xxx
> > > > > > driver, and in the future someone will want to declare different amplitudes for
> > > > > > different modes on another model, would he need to deprecate your binding or
> > > > > > would it be easy to extend?
> > > > > >  
> > > > >
> > > > > ok I see. So if I follow your proposal in my case it would be something like:
> > > > > serdes-sgmii-tx-amplitude-millivolt to start with ?
> > > > >
> > > > > I can do that. Andrew what do you think?  
> > > >
> > > > Or maybe two properties:
> > > >   serdes-tx-amplitude-millivolt = <700 1000 1100>;
> > > >   serdes-tx-amplitude-modes = "sgmii", "2500base-x", "10gbase-r";
> > > > ?
> > > >
> > > > If
> > > >   serdes-tx-amplitude-modes
> > > > is omitted, then
> > > >   serdes-tx-amplitude-millivolt
> > > > should only contain one value, and this is used for all serdes modes.
> > > >
> > > > This would be compatible with your change. You only need to define the
> > > > bidning for now, your code can stay the same - you don't need to add
> > > > support for multiple values or for the second property now, it can be
> > > > done later when needed. But the binding should be defined to support
> > > > those different modes.  
> > >
> > > Vladimir, can you send your thoughts about this proposal? We are trying
> > > to propose binding for defining serdes TX amplitude.  
> > 
> > I don't have any specific concern here. It sounds reasonable for
> > different data rates to require different transmitter configurations.
> > Having separate "serdes-tx-amplitude-millivolt" and "serdes-tx-amplitude-modes"
> > properties sounds okay, although I think a prefix with "-names" at the
> > end is more canonical ("pinctrl-names", "clock-names", "reg-names" etc),
> > so maybe "serdes-tx-amplitude-millivolt-names"?
> > Maybe we could name the first element "default", and just the others
> > would be named after a phy-mode. This way, if a specific TX amplitude is
> > found in the device tree for the currently operating PHY mode, it can be
> > used, otherwise the default (first) amplitude can be used.  
> 
> Also, maybe drop the "serdes-" prefix? The property will sit under a
> SERDES lane node, so it would be a bit redundant?

Hmm. Holger's proposal adds the property into the port node, not SerDes
lane node. mv88e6xxx does not define bindings for SerDes lane nodes
(yet).