netdev - RE: [PATCH v1 net-next 3/3] net: dsa: microchip: add KSZ9893 switch support

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Date:   Tue, 5 Mar 2019 01:24:44 +0000
From:   <Tristram.Ha@...rochip.com>
To:     <f.fainelli@...il.com>
CC:     <UNGLinuxDriver@...rochip.com>, <netdev@...r.kernel.org>,
        <andrew@...n.ch>, <pavel@....cz>
Subject: RE: [PATCH v1 net-next 3/3] net: dsa: microchip: add KSZ9893 switch
 support

> >  static int ksz9477_phy_read16(struct dsa_switch *ds, int addr, int reg)
> > @@ -389,6 +399,10 @@ static int ksz9477_phy_write16(struct dsa_switch
> *ds, int addr, int reg,
> >  	/* No real PHY after this. */
> >  	if (addr >= dev->phy_port_cnt)
> >  		return 0;
> > +
> > +	/* No gigabit support.  Do not write to this register. */
> > +	if (!(dev->features & GBIT_SUPPORT) && reg == MII_CTRL1000)
> > +		return 0;
> 
> There should not be any reasons to write to this register if the
> linkmode capabilities are correctly reflected which you do take care of,
> did you find PHYLIB writing to that register?

I found the register is being written, although not trying to advertise gigabit speed.

Problem is the switch wants to have a 100Mbit variant but does not properly prepare the PHY for that.  From the outside the PHY looks like it is gigabit capable.  Internally gigabit support is completely removed.

Just to be safe the driver does not want to touch that register.