netdev - Re: [RFC] mdio demux multiplexer driver

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Message-ID: <aKXtcw1G3-TQTf2r@xhacker>
Date: Wed, 20 Aug 2025 23:44:51 +0800
From: Jisheng Zhang <jszhang@...nel.org>
To: Andrew Lunn <andrew@...n.ch>, Russell King <linux@...linux.org.uk>
Cc: Russell King <linux@...linux.org.uk>,
	Florian Fainelli <florian.fainelli@...adcom.com>,
	Jakub Kicinski <kuba@...nel.org>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [RFC] mdio demux multiplexer driver

On Mon, Aug 18, 2025 at 06:27:33PM +0200, Andrew Lunn wrote:
> > > Both Ethernet and MDIO hardware exists, even if it is not used. I
> > > would separate the drivers. Have a MAC driver and an MDIO driver. List
> > > them as separate entities in DT. Always probe the MDIO0 driver. It can
> > > set the MUX registers. Don't probe MDIO1 driver. Even if it does
> > > probe, you know MDIO0 is one to be used, so MDIO1 can still set the
> > > MUX to point to MDIO0.
> > > 
> > > How messy is the address space? Are the MDIO registers in the middle
> > > of the MAC registers? Is this a standard, off the shelf MAC/MDIO IP?
> > > stmmac? Or something currently without a driver? If you are dealing
> > 
> > stmmac :(  And the MMIO reg doesn't sit together with MAC IP's.
> > As can be seen, the stmmac mdio registers sit in the middle of the
> > MAC regs. And current stmmac still tries to register a mdio driver for
> > the MDIO bus master. And to be honest, it's not the stmmac make things
> > messy, but the two MDIO masters sharing the single clk and data lines
> > makes the mess. Modeling the mmio as a demux seems a just so so but
> > not perfect solution.
> 
> So you only really have problems when MAC0 is not used. Are there any
> boards actually designed that way? If there are no such boards at the
> moment, you can delay handling that until later.
> 
> When later arrives, i would probably look at refactoring the stmmac
> MDIO code into a library. The stmmac driver can use the library, and
> you can add a new MDIO only driver around the library for when MAC0 is
> not used, but you need the MDIO bus.
> 
> For the moment, you can add setting the MUX register in the stmmac
> glue driver.

Hi Andrew, Russell

Thanks a lot for your useful inputs. I will check again and try the
demux and compare it with different solutions.

Thanks a lot!