From: Vladimir Oltean <olteanv@gmail.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@gmail.com>

Applied and queued up for -stable.