void sppp_input (struct net_device * dev, struct sk_buff * skb);
This can be called directly by cards that do not have timing constraints but is normally called from the network layer after interrupt servicing to process frames queued via netif_rx.
We process the options in the card. If the frame is destined for the protocol stacks then it requeues the frame for the upper level protocol. If it is a control from it is processed and discarded here.