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

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Message-ID: <228f24db-0a26-4d38-8f73-a9691262cc5f@arinc9.com>
Date:   Sat, 23 Sep 2023 20:51:44 +0300
From:   Arınç ÜNAL <arinc.unal@...nc9.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     "Russell King (Oracle)" <linux@...linux.org.uk>,
        Rob Herring <robh+dt@...nel.org>,
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Subject: Re: [PATCH net-next v2 00/10] define and enforce phylink bindings

On 23.09.2023 18:12, Andrew Lunn wrote:
> On Sat, Sep 23, 2023 at 09:28:41AM +0300, Arınç ÜNAL wrote:
>> On 23.09.2023 01:36, Andrew Lunn wrote:
>>> 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.

There's no requirement for an MDIO bus or a PHY. If the MAC is connected to
a PHY, only the PHY node needs to be described. The PHY can be controlled
by any interface, I2C, MDIO, or something else. If there's no PHY, the
fixed-link property would be used to describe the link.

> 
>> 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.

The fixed-link property would be used to describe the link of the MAC here.

Arınç