netdev - Re: [PATCH net-next v2 00/10] define and enforce phylink bindings

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Date: Sat, 23 Sep 2023 17:12:40 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Arınç ÜNAL <arinc.unal@...nc9.com>
Cc: "Russell King (Oracle)" <linux@...linux.org.uk>,
	Rob Herring <robh+dt@...nel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
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Subject: Re: [PATCH net-next v2 00/10] define and enforce phylink bindings

On Sat, Sep 23, 2023 at 09:28:41AM +0300, Arınç ÜNAL wrote:
> On 23.09.2023 01:36, Andrew Lunn wrote:
> > > I agree. My patch description here failed to explain the actual issue,
> > > which is missing hardware descriptions. Here's what I understand. An
> > > ethernet-controller is a MAC. For the MAC to work properly with its link
> > > partner, at least one of these must be described:
> > > - pointer to a PHY to retrieve link information from the PHY
> > > - pointer to a PCS to retrieve link information from the PCS
> > > - pointer to an SFP to retrieve link information from the SFP
> > > - static link information
> > 
> > You are missing:
> > 
> > - The MAC has firmware driving the PHY, nothing for linux to do.
> > 
> > There are properties in ethernet-controller.yaml the MAC driver would
> > however like to use such as local-mac-address, max-frame-size,
> > nvmem-cell-names etc.
> 
> This is interesting. This is clearly a hardware difference of the ethernet
> controller.
> 
> I believe this fits case 1. There's still an MDIO bus the ethernet
> controller uses, there's still a PHY on the MDIO bus which the ethernet
> controller uses.

Why must there be an MDIO bus? All the bus provides is a communication
channel to the PHY. There are PHYs which are memory mapped, or use
I2C. SFP are a good example of I2C, which Linux maps to MDIO just to
make things simple, but the hardware is I2C. Why must there be a PHY?
Maybe it is a Base-K link, i.e. a baseboard link to a switch, or a BMC
or something.

> The only difference is the firmware of the ethernet
> controller controls... What exactly does the firmware control that a Linux
> driver would have controlled instead? Just configuring the link settings of
> the MAC?

A MAC driver implements struct ethtool_ops:::get_link_settings and
set_link_settings. For a MAC driver using phylib or phylink they
typically then call into phylib or phylink to do the actual work,
maybe with a bit of pre-processing in the MAC driver.

A MAC driver using firmware would typically make an RPC into the
firmware to implement these calls.

There is a MAC driver currently under review which does not have a PHY
at all. The MAC is directly connected to a switch, all within one
IC. The link is always running at 5Gbps, the link is always up. It is
physically impossible to connect a PHY, so get_link_settings just
returns hard coded values.

	Andrew