sc->sc_ethif can't be NULL, it is set definitively in pppoe_create.
PPPoE can't by anything else than PADI sent in pppoe_send_padi, it
is only called when this is true.
PPPoE state can't be anything else than initial state in
pppoe_connect, this function is called from PPP core only when PPP
is in the dead phase, if PPP is in the dead phase it means the link
protocol is dead as well.
PPPoE can't be anything else than data phase in pppoe_disconnect
this function is only called by PPP core only when PPP session is up,
if PPP session is UP it means the link protocol is UP as well.
PPPoE can't by anything else than PADR sent in pppoe_send_padr, it
is only called when this is true.
PPPoE can't by anything else than PADO sent in pppoe_send_pado, it
is only called when this is true.
PPPoE can't by anything else than PADO sent in pppoe_send_pads, it
is only called when this is true.
PPPoE can't be anything else than session phase in pppoe_xmit,
function is only called by pppoe_write and pppoe_netif_output
which are both called by PPP core only when PPP session is up, if
PPP session is UP it means the link protocol is UP as well.
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