netdev - Re: Race between "Generic PHY" and "bcm53xx" drivers after -EPROBE

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Date:   Mon, 20 Sep 2021 21:58:56 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Rafał Miłecki <zajec5@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <rmk+kernel@...linux.org.uk>
Subject: Re: Race between "Generic PHY" and "bcm53xx" drivers after
 -EPROBE_DEFER

On Mon, Sep 20, 2021 at 11:10:39AM -0700, Florian Fainelli wrote:
> > But even so, what's a "pseudo PHY" exactly? I think that's at the bottom
> > of this issue. In the Linux device model, a device has a single driver.
> > In this case, the same MDIO device either has a switch driver, if you
> > accept it's a switch, or a PHY driver, if you accept it's a PHY.
> > I said it's "broken" because the expectation seems to be that it's a switch,
> > but it looks like it's treated otherwise. Simply put, the same device
> > can't be both a switch and a PHY.
> 
> A pseudo-PHY is a device that can snoop and respond to MDIO bus
> requests. I understand it cannot be both, just explaining to you how the
> people at Broadcom have been seeing the world from their perspective.
> Anything that is found at MDIO address 0x1e/30 is considered a MDIO
> attached switch, that's all.

Nothing wrong with that per se, at NXP we've been thinking about RevMII
as well, and having a switch expose itself as a PHY over MDIO, in any
case something a bit richer/more interactive than a fixed-link.

One way to bypass that limitation is to access the switch registers
through a different device compared to the PHY device. At the very
least, a different MDIO address. Even better, not over MDIO at all, but
something faster, SPI, PCIe etc.