netdev - Re: [PATCH net-next 0/2] dt-bindings: define property describing supported ethernet PHY modes

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Message-ID: <20210325014328.6ce00864@thinkpad>
Date:   Thu, 25 Mar 2021 01:43:28 +0100
From:   Marek Behún <kabel@...nel.org>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     netdev@...r.kernel.org, Andrew Lunn <andrew@...n.ch>,
        "David S . Miller" <davem@...emloft.net>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Rob Herring <robh@...nel.org>, devicetree@...r.kernel.org,
        pali@...nel.org
Subject: Re: [PATCH net-next 0/2] dt-bindings: define property describing
 supported ethernet PHY modes

On Wed, 24 Mar 2021 17:11:25 -0700
Florian Fainelli <f.fainelli@...il.com> wrote:

> On 3/24/2021 4:45 PM, Marek Behún wrote:
> > On Wed, 24 Mar 2021 16:16:41 -0700
> > Florian Fainelli <f.fainelli@...il.com> wrote:
> >   
> >> On 3/24/2021 4:00 PM, Marek Behún wrote:  
> >>> On Wed, 24 Mar 2021 14:19:28 -0700
> >>> Florian Fainelli <f.fainelli@...il.com> wrote:
> >>>     
> >>>>> Another problem is that if lower modes are supported, we should
> >>>>> maybe use them in order to save power.      
> >>>>
> >>>> That is an interesting proposal but if you want it to be truly valuable,
> >>>> does not that mean that an user ought to be able to switch between any
> >>>> of the supported PHY <=> MAC interfaces at runtime, and then within
> >>>> those interfaces to the speeds that yield the best power savings?    
> >>>
> >>> If the code determines that there are multiple working configurations,
> >>> it theoretically could allow the user to switch between them.
> >>>
> >>> My idea was that this should be done by kernel, though.
> >>>
> >>> But power saving is not the main problem I am trying to solve.
> >>> What I am trying to solve is that if a board does not support all modes
> >>> supported by the MAC and PHY, because they are not wired or something,
> >>> we need to know about that so that we can select the correct mode for
> >>> PHYs that change this mode at runtime.    
> >>
> >> OK so the runtime part comes from plugging in various SFP modules into a
> >> cage but other than that, for a "fixed" link such as a SFF or a soldered
> >> down PHY, do we agree that there would be no runtime changing of the
> >> 'phy-mode'?  
> > 
> > No, we do not. The PHY can be configured (by strapping pins or by
> > sw) to change phy-mode depending on the autonegotiated copper speed.
> > 
> > So if you plug in an ethernet cable where on the otherside is only 1g
> > capable device, the PHY will change mode to sgmii. But if you then plug
> > a 5g capable device, the PHY will change mode to 5gbase-r.
> > 
> > This happens if the PHY is configured into one of these changing
> > configurations. It can also be configured to USXGMII, or 10GBASER with
> > rate matching.
> > 
> > Not many MACs in kernel support USXGMII currently.
> > 
> > And if you use rate matching mode, and the copper side is
> > linked in lower speed (2.5g for example), and the MAC will start
> > sending too many packets, the internal buffer in the PHY is only 16 KB,
> > so it will fill up quickly. So you need pause frames support. But this
> > is broken for speeds <= 1g, according to erratum.
> > 
> > So you really want to change modes. The rate matching mode is
> > basically useless.  
> 
> OK, so whenever there is a link change you are presumably reading the
> mode in which the PHY has been reconfigured to, asking the MAC to
> configured itself appropriately based on that, and if there is no
> intersection, error out?

No. At initialization I tell the PHY to change between
  10gbase-r / 5gbase-r / 2500base-x / sgmii
according to the copper side. The PHY will do this alone on change on
copper side. I don't need to do this.

(This already works with current version of marvell10g driver - but
 kernel is not configuring this, it has to be configure via strapping
 pins.)

But I can tell the PHY at initialization to change instead between
  xaui / 5gbase-r / 2500base-x / sgmii
Again the PHY will do this on its own whenever speed on the copper side
changes.

But I need to know which of this settings I should use.

Marek