ethernet: Retire M_HASFCS

The mbuf flag M_HASFCS was introduced for drivers to indicate the net
stack that packets include FCS (Frame Check Sequence). In principle, to
be efficient, FCS should always be processed by hardware, firmware, or
at last sort the driver. Well, Ethernet specifies that damaged frames
should be discarded, thus only good ones will be passed up to the net
stack, then it makes no senses for the net stack to see FCS just to trim
it.

The last consumer of the flag M_HASFCS has been removed since change [1].
It is time to retire it.

1. 105a4f7 ng_atmllc: remove

Reviewed by:	kp
MFC after:	never
Differential Revision:	https://reviews.freebsd.org/D42391

sys/net/debugnet.c

@@ -570,10 +570,6 @@ debugnet_input_one(struct ifnet *ifp, struct mbuf *m)
 		    m->m_len, m->m_pkthdr.len);
 		goto done;
 	}
-	if ((m->m_flags & M_HASFCS) != 0) {
-		m_adj(m, -ETHER_CRC_LEN);
-		m->m_flags &= ~M_HASFCS;
-	}
 	eh = mtod(m, struct ether_header *);
 	etype = ntohs(eh->ether_type);
 	if ((m->m_flags & M_VLANTAG) != 0 || etype == ETHERTYPE_VLAN) {

sys/net/ethernet.h

@@ -40,7 +40,6 @@
 /*
  * Ethernet-specific mbuf flags.
  */
-#define	M_HASFCS	M_PROTO5	/* FCS included at end of frame */
 #define	M_BRIDGE_INJECT	M_PROTO6	/* if_bridge-injected frame */
 
 /*

sys/net/if_ethersubr.c

@@ -588,16 +588,6 @@ ether_input_internal(struct ifnet *ifp, struct mbuf *m)
 	 */
 	ETHER_BPF_MTAP(ifp, m);
 
-	/*
-	 * If the CRC is still on the packet, trim it off. We do this once
-	 * and once only in case we are re-entered. Nothing else on the
-	 * Ethernet receive path expects to see the FCS.
-	 */
-	if (m->m_flags & M_HASFCS) {
-		m_adj(m, -ETHER_CRC_LEN);
-		m->m_flags &= ~M_HASFCS;
-	}
-
 	if (!(ifp->if_capenable & IFCAP_HWSTATS))
 		if_inc_counter(ifp, IFCOUNTER_IBYTES, m->m_pkthdr.len);