linux-kernel - Re: [PATCH 2/2] net: mdio: support c45 peripherals on c22 busses

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Message-ID: <YGtdv++nv3H5K43E@arch-linux>
Date:   Mon, 5 Apr 2021 20:58:07 +0200
From:   Danilo Krummrich <danilokrummrich@...develop.de>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        davem@...emloft.net, hkallweit1@...il.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, jeremy.linton@....com
Subject: Re: [PATCH 2/2] net: mdio: support c45 peripherals on c22 busses

On Mon, Apr 05, 2021 at 03:33:55PM +0200, Andrew Lunn wrote:
> On Sun, Apr 04, 2021 at 09:23:55PM +0200, Danilo Krummrich wrote:
> > So currently every driver should check for the flag MII_ADDR_C45 and report an
> > error in case it's unsupported.
> > 
> > What do you think about checking the bus' capabilities instead in
> > mdiobus_c45_*()? This way the check if C45 is supported can even happen before
> > calling the driver at all. I think that would be a little cleaner than having
> > two places where information of the bus' capabilities are stored (return value
> > of read/write functions and the capabilities field).
> > 
> > I think there are not too many drivers setting their capabilities though, but
> > it should be easy to derive this information from how and if they handle the
> > MII_ADDR_C45 flag.
> 
> I actually don't think anything needs to change. The Marvell PHY
> probably probes due to its C22 IDs. The driver will then requests C45
> access which automagically get converted into C45 over C22 for your
> hardware, but remain C45 access for bus drivers which support C45.
> 
Thanks Andrew - I agree, for the Marvell PHY to work I likly don't need any
change, since I also expect that it will probe with the C22 IDs. I'll try
this soon.

However, this was about something else - Russell wrote:
> > > We have established that MDIO drivers need to reject accesses for
> > > reads/writes that they do not support [..]
The MDIO drivers do this by checking the MII_ADDR_C45 flag if it's a C45 bus
request. In case they don't support it they return -EOPNOTSUPP. So basically,
the bus drivers read/write functions (should) encode the capability of doing
C45 transfers.

I just noted that this is redundant to the bus' capabilities field of
struct mii_bus which also encodes the bus' capabilities of doing C22 and/or C45
transfers.

Now, instead of encoding this information of the bus' capabilities at both
places, I'd propose just checking the mii_bus->capabilities field in the
mdiobus_c45_*() functions. IMHO this would be a little cleaner, than having two
places where this information is stored. What do you think about that?
> 	  Andrew