Re: [RFC net-next PATCH 01/16] dt-bindings: net: Add pcs property

[Rob Herring]

On Tue, Oct 12, 2021 at 11:18 AM Sean Anderson <sean.anderson@xxxxxxxx> wrote:
>
> Hi Rob,
>
> On 10/12/21 9:16 AM, Rob Herring wrote:
> > On Tue, Oct 05, 2021 at 10:39:36AM +0100, Russell King (Oracle) wrote:
> >> On Mon, Oct 04, 2021 at 03:15:12PM -0400, Sean Anderson wrote:
> >> > Add a property for associating PCS devices with ethernet controllers.
> >> > Because PCS has no generic analogue like PHY, I have left off the
> >> > -handle suffix.
> >>
> >> For PHYs, we used to have phy and phy-device as property names, but the
> >> modern name is "phy-handle". I think we should do the same here, so I
> >> would suggest using "pcs-handle".
> >
> > On 1G and up ethernet, we have 2 PHYs. There's the external (typically)
> > ethernet PHY which is what the above properties are for. Then there's
> > the on-chip serdes PHY similar to SATA, PCIe, etc. which includes the
> > PCS part. For this part, we should use the generic PHY binding. I think
> > we already have bindings doing that.
>
> In the 802.3 models, there are several components which convert between
> the MII (from the MAC) and the MDI (the physical protocol on the wire).
> These are the Physical Coding Sublayer (PCS), Physical Medium Attachment
> (PMA) sublayer, and Physical Medium Dependent (PMD) sublayer. The PMD
> converts between the physical layer signaling and the on-chip (or
> on-board) signalling. The PMA performs clock recovery and converts the
> serial data from the PMD into parallel data for the PCS. The PCS handles
> autonegotiation, CSMA/CD, and conversion to the apripriate MII for
> communicating with the MAC.
>
> In the above model, generic serdes devices generally correspond to the
> PMA/PMD sublayers. The PCS is generally a separate device, both
> on the hardware and software level. It provides an ethernet-specific
> layer on top of the more generic underlying encoding. For this reason,
> the PCS should be modeled as its own device, which may then contain a
> reference to the appropriate serdes.

On the h/w I've worked on, PCS was an additional block instantiated
within the PHY, so it looked like one block to s/w. But that's been
almost 10 years ago now.

If you do have 2 h/w blocks, one option is doing something like this:

phys = <&pcs_phy>, <&sgmii_phy>;

I'm okay with 'pcs-handle', but just want to make sure we're not using
it where 'phys' would work.

> The above model describes physical layers such as 1000BASE-X or
> 10GBASE-X where the PCS/PMA/PMD is the last layer before the physical
> medium. In that case, the PCS could be modeled as a traditional PHY.
> However, when using (e.g.) SGMII, it is common for the "MDI" to be
> SGMII, and for another PHY to convert to 1000BASE-T. To model this
> correctly, the PCS/PMA/PMD layer must be considered independently from
> the PHY which will ultimately convert the MII to the MDI.