[netdevice] Allow MTU to be changed at runtime

Provide a settings applicator to modify netdev->max_pkt_len in
response to changes to the "mtu" setting (DHCP option 26).

Note that as with MAC address changes, drivers are permitted to
completely ignore any changes in the MTU value.  The net result will
be that iPXE effectively uses the smaller of either the hardware
default MTU or the software configured MTU.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/dhcp.h

 /** Root path */
 #define DHCP_ROOT_PATH 17
 
+/** Maximum transmission unit */
+#define DHCP_MTU 26
+
 /** Vendor encapsulated options */
 #define DHCP_VENDOR_ENCAP 43
 

src/net/netdev_settings.c

 	.description = "Interface name",
 	.type = &setting_type_string,
 };
+const struct setting mtu_setting __setting ( SETTING_NETDEV, mtu ) = {
+	.name = "mtu",
+	.description = "MTU",
+	.type = &setting_type_int16,
+	.tag = DHCP_MTU,
+};
 
 /**
  * Store MAC address setting
 struct init_fn netdev_redirect_settings_init_fn __init_fn ( INIT_LATE ) = {
 	.initialise = netdev_redirect_settings_init,
 };
+
+/**
+ * Apply network device settings
+ *
+ * @ret rc		Return status code
+ */
+static int apply_netdev_settings ( void ) {
+	struct net_device *netdev;
+	struct settings *settings;
+	struct ll_protocol *ll_protocol;
+	size_t old_max_pkt_len;
+	size_t mtu;
+	int rc;
+
+	/* Process settings for each network device */
+	for_each_netdev ( netdev ) {
+
+		/* Get network device settings */
+		settings = netdev_settings ( netdev );
+
+		/* Get MTU */
+		mtu = fetch_uintz_setting ( settings, &mtu_setting );
+
+		/* Do nothing unless MTU is specified */
+		if ( ! mtu )
+			continue;
+
+		/* Update maximum packet length */
+		ll_protocol = netdev->ll_protocol;
+		old_max_pkt_len = netdev->max_pkt_len;
+		netdev->max_pkt_len = ( mtu + ll_protocol->ll_header_len );
+		if ( netdev->max_pkt_len != old_max_pkt_len ) {
+			DBGC ( netdev, "NETDEV %s MTU is %zd\n",
+			       netdev->name, mtu );
+		}
+
+		/* Close and reopen network device if MTU has increased */
+		if ( netdev_is_open ( netdev ) &&
+		     ( netdev->max_pkt_len > old_max_pkt_len ) ) {
+			netdev_close ( netdev );
+			if ( ( rc = netdev_open ( netdev ) ) != 0 ) {
+				DBGC ( netdev, "NETDEV %s could not reopen: "
+				       "%s\n", netdev->name, strerror ( rc ) );
+				return rc;
+			}
+		}
+	}
+
+	return 0;
+}
+
+/** Network device settings applicator */
+struct settings_applicator netdev_applicator __settings_applicator = {
+	.apply = apply_netdev_settings,
+};

src/net/udp/dhcp.c

 	DHCP_PARAMETER_REQUEST_LIST,
 	DHCP_OPTION ( DHCP_SUBNET_MASK, DHCP_ROUTERS, DHCP_DNS_SERVERS,
 		      DHCP_LOG_SERVERS, DHCP_HOST_NAME, DHCP_DOMAIN_NAME,
-		      DHCP_ROOT_PATH, DHCP_VENDOR_ENCAP, DHCP_VENDOR_CLASS_ID,
-		      DHCP_TFTP_SERVER_NAME, DHCP_BOOTFILE_NAME,
-		      DHCP_DOMAIN_SEARCH,
+		      DHCP_ROOT_PATH, DHCP_MTU, DHCP_VENDOR_ENCAP,
+		      DHCP_VENDOR_CLASS_ID, DHCP_TFTP_SERVER_NAME,
+		      DHCP_BOOTFILE_NAME, DHCP_DOMAIN_SEARCH,
 		      128, 129, 130, 131, 132, 133, 134, 135, /* for PXE */
 		      DHCP_EB_ENCAP, DHCP_ISCSI_INITIATOR_IQN ),
 	DHCP_END