netdev - Re: [RFC PATCH 1/1] phylink: Set speed to SPEED_UNKNOWN when there is no PHY connected

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Date:   Thu, 29 Aug 2019 12:34:48 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Arseny Solokha <asolokha@...kras.ru>,
        netdev <netdev@...r.kernel.org>,
        Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        leandro.maciel.dorileo@...el.com
Subject: Re: [RFC PATCH 1/1] phylink: Set speed to SPEED_UNKNOWN when there is
 no PHY connected

Hi Russell,

On Wed, 28 Aug 2019 at 20:14, Russell King - ARM Linux admin
<linux@...linux.org.uk> wrote:
>
> On Wed, Aug 28, 2019 at 05:58:02PM +0300, Vladimir Oltean wrote:
> > phylink_ethtool_ksettings_get can be called while the interface may not
> > even be up, which should not be a problem. But there are drivers (e.g.
> > gianfar) which connect to the PHY in .ndo_open and disconnect in
> > .ndo_close. While odd, to my knowledge this is again not illegal and
> > there may be more that do the same. But PHYLINK for example has this
> > check in phylink_ethtool_ksettings_get:
> >
> >       if (pl->phydev) {
> >               phy_ethtool_ksettings_get(pl->phydev, kset);
> >       } else {
> >               kset->base.port = pl->link_port;
> >       }
> >
> > So it will not populate kset->base.speed if there is no PHY connected.
> > The speed will be 0, by way of a previous memset. Not SPEED_UNKNOWN.
> > It is arguable whether that is legal or not. include/uapi/linux/ethtool.h
> > says:
> >
> >       All values 0 to INT_MAX are legal.
> >
> > By that measure it may be. But it sure would make users of the
> > __ethtool_get_link_ksettings API need make more complicated checks
> > (against -1, against 0, 1, etc). So far the kernel community has been ok
> > with just checking for SPEED_UNKNOWN.
> >
> > Take net/sched/sch_taprio.c for example. The check in
> > taprio_set_picos_per_byte is currently not robust enough and will
> > trigger this division by zero, due to PHYLINK not setting SPEED_UNKNOWN:
> >
> >       if (!__ethtool_get_link_ksettings(dev, &ecmd) &&
> >           ecmd.base.speed != SPEED_UNKNOWN)
> >               picos_per_byte = div64_s64(NSEC_PER_SEC * 1000LL * 8,
> >                                          ecmd.base.speed * 1000 * 1000);
>
> The ethtool API says:
>
>  * If it is enabled then they are read-only; if the link
>  * is up they represent the negotiated link mode; if the link is down,
>  * the speed is 0, %SPEED_UNKNOWN or the highest enabled speed and
>  * @duplex is %DUPLEX_UNKNOWN or the best enabled duplex mode.
>
> So, it seems that taprio is not following the API... I'd suggest either
> fixing taprio, or getting agreement to change the ethtool API.
>

How would you suggest rewriting the line above in taprio to make
correct and robust use of the ethtool API?

> --
> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
> FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
> According to speedtest.net: 11.9Mbps down 500kbps up

Regards,
-Vladimir