netdev - Re: [PATCH net-next 0/5] net: phy: C45-over-C22 access

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Message-ID: <dcea8c36e626dc31ee1ddd8c867eb999@walle.cc>
Date:   Tue, 24 Jan 2023 15:41:56 +0100
From:   Michael Walle <michael@...le.cc>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Yisen Zhuang <yisen.zhuang@...wei.com>,
        Salil Mehta <salil.mehta@...wei.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Broadcom internal kernel review list 
        <bcm-kernel-feedback-list@...adcom.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Marek Behún <kabel@...nel.org>,
        Xu Liang <lxu@...linear.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 0/5] net: phy: C45-over-C22 access

>> The problem is in the middle.  get_phy_c45_devs_in_pkg() uses
>> mdiobus_c45_read(). Does mdiobus_c45_read() mean perform a C45 bus
>> transaction, or access the C45 address space? I would say it means
>> perform a C45 bus transaction. It does not take a phydev, so we are
>> below the concept of PHYs, and so C45 over C22 does not exist at this
>> level.
> 
> C45-over-C22 is a PHY thing, it isn't generic. We shouldn't go poking
> at the PHY C45-over-C22 registers unless we know for certain that the
> C22 device we are accessing is a PHY, otherwise we could be writing
> into e.g. a switch register or something else.
> 
> So, the mdiobus_* API should be the raw bus API. If we want C45 bus
> cycles then mdiobus_c45_*() is the API that gives us that, vs C22 bus
> cycles through the non-C45 API.
> 
> C45-over-C22 being a PHY thing is something that should be handled by
> phylib, and currently is. The phylib accessors there will use C45 or
> C45-over-C22 as appropriate.

I think the crux is get_phy_device(). It is used for two different
cases:
  (1) to scan the mdio bus
  (2) to add a c45 phy, i.e. in the DT/fwnode case

For (1) we must not use indirect access. And for (2) we know for
a fact that it must be a PHY and thus we can (and have to) fall back
to c45-over-c22.

Btw. for the DT case, it seems we need yet another property
to indicate broken MDIO busses.

-michael