netdev - Re: [PATCH net] net: sched: taprio: Avoid division by zero on invalid link speed

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Date:   Tue, 01 Oct 2019 09:51:13 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     olteanv@...il.com
Cc:     vinicius.gomes@...el.com, jhs@...atatu.com,
        xiyou.wangcong@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH net] net: sched: taprio: Avoid division by zero on
 invalid link speed

From: Vladimir Oltean <olteanv@...il.com>
Date: Sun, 29 Sep 2019 02:37:22 +0300

> The check in taprio_set_picos_per_byte is currently not robust enough
> and will trigger this division by zero, due to e.g. PHYLINK not setting
> kset->base.speed when there is no PHY connected:
 ...
> Russell King points out that the ethtool API says zero is a valid return
> value of __ethtool_get_link_ksettings:
> 
>    * 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.
> 
> The chosen path was to fix taprio.
> 
> Fixes: 7b9eba7ba0c1 ("net/sched: taprio: fix picos_per_byte miscalculation")
> Signed-off-by: Vladimir Oltean <olteanv@...il.com>

Applied and queued up for -stable.