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

On Thu, Mar 25, 2021 at 12:45:25AM +0100, Marek Behún wrote:

On Wed, 24 Mar 2021 16:16:41 -0700
Florian Fainelli [off-list ref] wrote:

quoted

On 3/24/2021 4:00 PM, Marek Behún wrote:

quoted

On Wed, 24 Mar 2021 14:19:28 -0700
Florian Fainelli [off-list ref] wrote:
  

quoted

quoted

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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