Subject: Re: [PATCH net v1 1/1] net: dsa: microchip: don't try do read Gbit registers on non Gbit chips

From: Andrew Lunn <[email-masked]>
Date: Fri, 29 Jul 2022 15:12:10 +0200

On Fri, Jul 29, 2022 at 11:05:13AM +0200, Oleksij Rempel wrote:
> On Thu, Jul 28, 2022 at 03:25:35PM +0200, Andrew Lunn wrote:
> > On Thu, Jul 28, 2022 at 03:17:25PM +0200, Oleksij Rempel wrote:
> > > Do not try to read not existing or wrong register on chips without
> > > GBIT_SUPPORT.
> > > 
> > > Fixes: c2e866911e25 ("net: dsa: microchip: break KSZ9477 DSA driver into two files")
> > > Signed-off-by: Oleksij Rempel <[email-masked]>
> > > ---
> > >  drivers/net/dsa/microchip/ksz9477.c | 8 +++++++-
> > >  1 file changed, 7 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/net/dsa/microchip/ksz9477.c b/drivers/net/dsa/microchip/ksz9477.c
> > > index c73bb6d383ad..f6bbd9646c85 100644
> > > --- a/drivers/net/dsa/microchip/ksz9477.c
> > > +++ b/drivers/net/dsa/microchip/ksz9477.c
> > > @@ -316,7 +316,13 @@ void ksz9477_r_phy(struct ksz_device *dev, u16 addr, u16 reg, u16 *data)
> > >  			break;
> > >  		}
> > >  	} else {
> > > -		ksz_pread16(dev, addr, 0x100 + (reg << 1), &val);
> > > +		/* No gigabit support.  Do not read wrong registers. */
> > > +		if (!(dev->features & GBIT_SUPPORT) &&
> > > +		    (reg == MII_CTRL1000 || reg == MII_ESTATUS ||
> > > +		     reg == MII_STAT1000))
> > 
> > Does this actually happen?
> > 
> > If i remember this code correctly, it tries to make the oddly looking
> > PHY look like a normal PHY. phylib is then used to drive the PHY?
> > 
> > If i have that correct, why is phylib trying to read these registers?
> > It should know there is no 1G support, and should skip them.
> 
> It looks like currently undocumented silicon errata. According to the
> data sheet, the BMSR_ESTATEN should not be set BMSR_ERCAP, but this bits
> are set.
> 
> The question is what is the proper place to implement it. There is same
> PHYid for most KSZ switch PHYs, it is no possible to detect it by PHYid.
> I have following options:
> - add chips specific quirk in the ksz9477_r_phy(), just remove
>   BMSR_ESTATEN and BMSR_ERCAP.
> - notify about errata over get_phy_flags and implement get_caps quirk in
>   the PHY driver.

I would do the first. The DSA driver is already doing some emulation
of a normal PHY, so it seems odd to push a workaround into the PHY
driver when it can be part of the emulation.

    Andrew