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

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

On Thu, Mar 25, 2021 at 12:45:25AM +0100, Marek Behún wrote:
> On Wed, 24 Mar 2021 16:16:41 -0700
> Florian Fainelli <f.fainelli@gmail.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@gmail.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.

Also, the sending of pause frames is only supported for 88x3310P
devices, not the 88x3310. The plain 88x3310 requires the MAC to
rate-limit in this mode.

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