Subject: Re: [PATCH V1 net-next 0/3] net: stmmac: implement clocks

From: Andrew Lunn <[email-masked]>
Date: Thu, 25 Feb 2021 03:33:38 +0100

> Hi Andrew,

> I don't have experience with Ethernet switch, according to your
> points, you mean we can connect STMMAC to an Ethernet switch, and
> then Ethernet switch managed STMMAC by the MDIO bus but without
> checking whether STMMAC interface is opened or not, so STMMAC needs
> clocks for MDIO even interface is closed, right?

Correct. The MDIO bus has a different life cycle to the MAC. If any of
stmmac_xgmac2_mdio_read(), stmmac_xgmac2_mdio_write(),
stmmac_mdio_read(), and stmmac_mdio_write() need clocks ticking, you
need to ensure the clock is ticking, because these functions can be
called while the interface is not opened.

> > You said you copied the FEC driver. Take a look at that, it was initially broken in
> > this way, and i needed to extend it when i got a board with an Ethernet switch
> > attached to the FEC.
> 

> Could you point me how to implement clocks management to cover above
> Ethernet switch case? Or can we upstream this first and then fix it
> later for such case?

I actually got is wrong on the first attempt. So you need to look at:

42ea4457ae net: fec: normalize return value of pm_runtime_get_sync() in MDIO write
14d2b7c1a9 net: fec: fix initial runtime PM refcount
8fff755e9f net: fec: Ensure clocks are enabled while using mdio bus

And no, you cannot fix it later, because your patches potentially
break existing systems using an Ethernet switch. See:

ommit da29f2d84bd10234df570b7f07cbd0166e738230
Author: Jose Abreu <[email-masked]>
Date:   Tue Jan 7 13:35:42 2020 +0100

    net: stmmac: Fixed link does not need MDIO Bus
    
    When using fixed link we don't need the MDIO bus support.

...
    Tested-by: Florian Fainelli <f.fainelli@gmail> # Lamobo R1 (fixed-link + MDIO sub node for roboswitch).

So there are boards which make use of a switch and MDIO. Florian might
however be able to run tests for you, if you ask him.

   Andrew