[dhcp] Limit maximum number of DHCP discovery deferrals

For switches which remain permanently in the non-forwarding state (or
which erroneously report a non-forwarding state), ensure that iPXE
will eventually give up waiting for the link to become unblocked.

Originally-fixed-by: Wissam Shoukair <wissams@mellanox.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/config/dhcp.h

 //#define DHCP_DISC_START_TIMEOUT_SEC	4	/* as per PXE spec */
 //#define DHCP_DISC_END_TIMEOUT_SEC	32	/* as per PXE spec */
 
+/*
+ * Maximum number of discovery deferrals due to blocked links
+ * (e.g. from non-forwarding STP ports)
+ */
+#define DHCP_DISC_MAX_DEFERRALS		60
+
 /*
  * ProxyDHCP offers are given precedence by continue to wait for them
  * after a valid DHCPOFFER is received.  We'll wait through this

src/net/udp/dhcp.c

 	unsigned long elapsed = ( currticks() - dhcp->start );
 
 	/* If link is blocked, defer DHCP discovery (and reset timeout) */
-	if ( netdev_link_blocked ( dhcp->netdev ) ) {
+	if ( netdev_link_blocked ( dhcp->netdev ) &&
+	     ( dhcp->count <= DHCP_DISC_MAX_DEFERRALS ) ) {
 		DBGC ( dhcp, "DHCP %p deferring discovery\n", dhcp );
 		dhcp->start = currticks();
 		start_timer_fixed ( &dhcp->timer,
 	 * session state into packet traces.  Useful for extracting
 	 * debug information from non-debug builds.
 	 */
-	dhcppkt.dhcphdr->secs = htons ( ( ++(dhcp->count) << 2 ) |
+	dhcppkt.dhcphdr->secs = htons ( ( dhcp->count << 2 ) |
 					( dhcp->offer.s_addr ? 0x02 : 0 ) |
 					( dhcp->proxy_offer ? 0x01 : 0 ) );
 
 		return;
 	}
 
+	/* Increment transmission counter */
+	dhcp->count++;
+
 	/* Handle timer expiry based on current state */
 	dhcp->state->expired ( dhcp );
 }