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

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

On Mon, Apr 05, 2021 at 08:58:07PM +0200, Danilo Krummrich wrote:
> On Mon, Apr 05, 2021 at 03:33:55PM +0200, Andrew Lunn wrote:
> 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?

It would be good to clean that up, but that's a lot of work given the
number of drivers - not only in terms of making the changes but also
making sure that the changes are as correct as would be sensibly
achievable... then there's the problem of causing regressions by doing
so.

The two ways were introduced at different times to do two different
things: the checking in the read/write methods in each driver was the
first method, which was being added to newer drivers. Then more
recently came the ->capabilities field.

So now we have some drivers that:
- do no checks and don't fill in ->capabilities either (some of which
  are likely C22-only.)
- check in their read/write methods for access types they don't support
  (e.g. drivers/net/ethernet/marvell/mvmdio.c) and don't fill in
  ->capabilities. Note, mvmdio supports both C22-only and C45-only
  interfaces with no C22-and-C45 interfaces.
- do fill in ->capabilities with MDIOBUS_C22_C45 (and hence have no
  checks in their read/write functions).

Sometimes, its best to leave stuff alone... if it ain't broke, don't
make regressions.

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