sfc: don't insert mc_list on low-latency firmware if it's too long

If the mc_list is longer than 256 addresses, we enter mc_promisc mode.
If we're in mc_promisc mode and the firmware doesn't support cascaded
multicast, normally we also insert our mc_list, to prevent stealing by
another VI. However, if the mc_list was too long, this isn't really
helpful - the MC groups that didn't fit in the list can still get
stolen, and having only some of them stealable will probably cause
more confusing behaviour than having them all stealable. Since
inserting 256 multicast filters takes a long time and can lead to MCDI
state machine timeouts, just skip the mc_list insert in this overflow
condition.

Signed-off-by: Edward Cree <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ethernet/sfc/ef10.c

···
 	bool mc_promisc;
 /* Whether in multicast promiscuous mode when last changed */
 	bool mc_promisc_last;
+	bool mc_overflow; /* Too many MC addrs; should always imply mc_promisc */
 	bool vlan_filter;
 	struct list_head vlan_list;
 };
···
 	struct netdev_hw_addr *mc;
 	unsigned int i, addr_count;
 
+	table->mc_overflow = false;
 	table->mc_promisc = !!(net_dev->flags & (IFF_PROMISC | IFF_ALLMULTI));
 
 	addr_count = netdev_mc_count(net_dev);
···
 	netdev_for_each_mc_addr(mc, net_dev) {
 		if (i >= EFX_EF10_FILTER_DEV_MC_MAX) {
 			table->mc_promisc = true;
+			table->mc_overflow = true;
 			break;
 		}
 		ether_addr_copy(table->dev_mc_list[i].addr, mc->addr);
···
 			}
 		} else {
 			/* If we failed to insert promiscuous filters, don't
-			 * rollback.  Regardless, also insert the mc_list
+			 * rollback.  Regardless, also insert the mc_list,
+			 * unless it's incomplete due to overflow
 			 */
 			efx_ef10_filter_insert_def(efx, vlan,
 						   EFX_ENCAP_TYPE_NONE,
 						   true, false);
-			efx_ef10_filter_insert_addr_list(efx, vlan, true, false);
+			if (!table->mc_overflow)
+				efx_ef10_filter_insert_addr_list(efx, vlan,
+								 true, false);
 		}
 	} else {
 		/* If any filters failed to insert, rollback and fall back to