netdev - Re: [PATCH] net: dsa: mv88e6xxx: prevent 2500BASEX mode override

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Date:   Mon, 15 Feb 2021 16:16:27 +0100
From:   Marek Behun <marek.behun@....cz>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Nathan Rossi <nathan@...hanrossi.com>, netdev@...r.kernel.org,
        Nathan Rossi <nathan.rossi@...i.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH] net: dsa: mv88e6xxx: prevent 2500BASEX mode override

On Mon, 15 Feb 2021 14:57:57 +0000
Russell King - ARM Linux admin <linux@...linux.org.uk> wrote:

> On Mon, Feb 15, 2021 at 02:47:56PM +0100, Marek Behun wrote:
> > On Mon, 15 Feb 2021 06:15:59 +0000
> > Nathan Rossi <nathan@...hanrossi.com> wrote:
> >   
> > > diff --git a/drivers/net/dsa/mv88e6xxx/chip.c b/drivers/net/dsa/mv88e6xxx/chip.c
> > > index 54aa942eed..5c52906b29 100644
> > > --- a/drivers/net/dsa/mv88e6xxx/chip.c
> > > +++ b/drivers/net/dsa/mv88e6xxx/chip.c
> > > @@ -650,6 +650,13 @@ static void mv88e6xxx_validate(struct dsa_switch *ds, int port,
> > >  	if (chip->info->ops->phylink_validate)
> > >  		chip->info->ops->phylink_validate(chip, port, mask, state);
> > >  
> > > +	/* Advertise 2500BASEX only if 1000BASEX is not configured, this
> > > +	 * prevents phylink_helper_basex_speed from always overriding the
> > > +	 * 1000BASEX mode since auto negotiation is always enabled.
> > > +	 */
> > > +	if (state->interface == PHY_INTERFACE_MODE_1000BASEX)
> > > +		phylink_clear(mask, 2500baseX_Full);
> > > +  
> > 
> > I don't quite like this. This problem should be either solved in
> > phylink_helper_basex_speed() or somewhere in the mv88e6xxx code, but near
> > the call to phylink_helper_basex_speed().
> > 
> > Putting a solution to the behaviour of phylink_helper_basex_speed() it
> > into the validate() method when phylink_helper_basex_speed() is called
> > from a different place will complicate debugging in the future. If
> > we start solving problems in this kind of way, the driver will become
> > totally unreadable, IMO.  
> 
> If we can't switch between 1000base-X and 2500base-X, then we should
> not be calling phylink_helper_basex_speed() - and only one of those
> two capabilities should be set in the validation callback. I thought
> there were DSA switches where we could program the CMODE to switch
> between these two...

There are. At least Peridot, Topaz and Amethyst support switching
between these modes. But only on some ports.

This problem happnes on Peridot X, I think.

On Peridot X there are
- port 0: RGMII
- ports 9-10: capable of 1, 2.5 and 10G SerDes (10G via
  XAUI/RXAUI, so multiple lanes)
- ports 1-8: with copper PHYs
  - some of these can instead be set to use the unused SerDes lanes
    of ports 9-10, but only in 1000base-x mode

So the problem can happen if you set port 9 or 10 to only use one
SerDes lane, and use the spare lanes for the 1G ports.
On these ports 2500base-x is not supported, only 1000base-x (maybe
sgmii, I don't remember)

Marek