Re: [PATCH 2/2] net: dsa: microchip: Provide Module 4 KSZ9477 errata (DS80000754C)

[Vladimir Oltean <olteanv@gmail.com>]

Hi Lukasz,

On Tue, Aug 29, 2023 at 10:35:33AM +0200, Lukasz Majewski wrote:
> Hi Vladimir,
> 
> > On Fri, Aug 25, 2023 at 06:48:41PM +0000, Tristram.Ha@microchip.com
> > wrote:
> > > > > IMHO adding functions to MMD modification would facilitate
> > > > > further development (for example LED setup).  
> > > > 
> > > > We already have some KSZ9477 specific initialization done in the
> > > > Micrel PHY driver under drivers/net/phy/micrel.c, can we converge
> > > > on the PHY driver which has a reasonable amount of infrastructure
> > > > for dealing with workarounds, indirect or direct MMD accesses
> > > > etc.?  
> > > 
> > > Actually the internal PHY used in the KSZ9897/KSZ9477/KSZ9893
> > > switches are special and only used inside those switches.  Putting
> > > all the switch related code in Micrel PHY driver does not really
> > > help.  When the switch is reset all those PHY registers need to be
> > > set again, but the PHY driver only executes those code during PHY
> > > initialization.  I do not know if there is a good way to tell the
> > > PHY to re-initialize again.  
> > 
> > Suppose there was a method to tell the PHY driver to re-initialize
> > itself. What would be the key points in which the DSA switch driver
> > would need to trigger that method? Where is the switch reset at
> > runtime?
> 
> Tristam has explained why adding the internal switch PHY errata to
> generic PHY code is not optimal.

Yes, and I didn't understand that explanation, so I asked a
clarification question.

> If adding MMD generic code is a problem - then I'm fine with just
> clearing proper bits with just two indirect writes in the
> drivers/net/dsa/microchip/ksz9477.c
> 
> I would also prefer to keep the separate ksz9477_errata() function, so
> we could add other errata code there.
> 
> Just informative - without this patch the KSZ9477-EVB board's network
> is useless when the other peer has EEE enabled by default (like almost
> all non managed ETH switches).

No, adding direct PHY MMD access code to the ksz9477 switch driver is
not even the biggest problem - even though, IIUC, the "workaround" to
disable EEE advertisement could be moved to ksz9477_get_features() in
drivers/net/phy/micrel.c, where phydev->supported_eee could be cleared.

The biggest problem that I see is that Oleksij Rempel has "just" added
EEE support to the KSZ9477 earlier this year, with an ack from Arun
Ramadoss: 69d3b36ca045 ("net: dsa: microchip: enable EEE support").
I'm not understanding why the erratum wasn't a discussion topic then.

I am currently on vacation and won't be able to look very deeply into
the problem, but IIUC, your patch undoes that work, and so, it needs an
ACK from Oleksij.