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

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

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.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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