Add definitions to support a local loopback link layer

Documentation

@@ -1 +1 @@
-Subproject commit ce750c0899c2138b37b52a48d020a3eceb92fd04
+Subproject commit cae6e67e0534485cd42119346a2e0121d6652965

drivers/net/Kconfig

@@ -5,12 +5,21 @@
 
 comment "General Ethernet MAC Driver Options"
 
-config NETDEV_MULTINIC
+config NETDEV_LOOPBACK
-	bool "Multiple network interface support"
+	bool "Local loopback support"
 	default n
+	---help---
+		Add support for the local network loopback device, lo.
+
+config NETDEV_MULTINIC
+	bool "Multiple network interface support"
+	default n if !NETDEV_LOOPBACK
+	default y if NETDEV_LOOPBACK
 	---help---
 		Select this option if you board and/or MCU are capable of supporting
-		multiple Ethernet MAC drivers.
+		multiple link layer drivers.  NOTE that the local loopback device
+		is considered to be a a link layer driver so if local loopback
+		support is used you probably need to select this option.
 
 config NETDEV_LATEINIT
 	bool "Late driver initialization"

include/nuttx/net/net.h

@@ -76,6 +76,7 @@
 enum net_lltype_e
 {
   NET_LL_ETHERNET = 0, /* Ethernet */
+  NET_LL_LOOPBACK,     /* Local loopback */
   NET_LL_SLIP,         /* Serial Line Internet Protocol (SLIP) */
   NET_LL_PPP,          /* Point-to-Point Protocol (PPP) */
   NET_LL_TUN,          /* TUN Virtual Network Device */

include/nuttx/net/netconfig.h

@@ -157,7 +157,7 @@
 #  define MAX_NET_DEV_MTU   CONFIG_NET_ETH_MTU
 
 #else
-  /* Perhaps only Unix domain sockets */
+  /* Perhaps only Unix domain sockets of the loopback device */
 
 #  define NET_LL_HDRLEN(d)  0
 #  define NET_DEV_MTU(d)    0
@@ -166,6 +166,10 @@
 
 #endif /* MULTILINK or SLIP or ETHERNET */
 
+/* For the loopback device, we will use the largest representable MTU */
+
+#define NET_LO_MTU          UINT16_MAX
+
 /* Layer 3/4 Configuration Options ******************************************/
 
 /* IP configuration options */
@@ -346,6 +350,7 @@
  */
 
 #define TCP_MSS(d,h)        (NET_DEV_MTU(d) - NET_LL_HDRLEN(d) - TCP_HDRLEN - (h))
+#define LO_TCP_MSS(h)       (NET_LO_MTU - (h))
 
 /* If Ethernet is supported, then it will have the smaller MSS */
 
@@ -402,15 +407,17 @@
  * See the note above regarding the TCP MSS and CONFIG_NET_MULTILINK.
  */
 
+#define NET_LO_TCP_RECVWNDO LO_TCP_MSS(0)
+
 #ifdef CONFIG_NET_SLIP
 #  ifndef CONFIG_NET_SLIP_TCP_RECVWNDO
-#    define CONFIG_NET_SLIP_TCP_RECVWNDO SLIP_TCP_MSS
+#    define CONFIG_NET_SLIP_TCP_RECVWNDO SLIP_TCP_MSS(0)
 #  endif
 #endif
 
 #ifdef CONFIG_NET_ETHERNET
 #  ifndef CONFIG_NET_ETH_TCP_RECVWNDO
-#    define CONFIG_NET_ETH_TCP_RECVWNDO ETH_TCP_MSS
+#    define CONFIG_NET_ETH_TCP_RECVWNDO ETH_TCP_MSS(0)
 #  endif
 #endif
 

net/netdev/netdev_register.c

@@ -61,8 +61,9 @@
  * Pre-processor Definitions
  ****************************************************************************/
 
-#define NETDEV_SLIP_FORMAT      "sl%d"
 #define NETDEV_ETH_FORMAT       "eth%d"
+#define NETDEV_LO_FORMAT        "lo"
+#define NETDEV_SLIP_FORMAT      "sl%d"
 #define NETDEV_TUN_FORMAT       "tun%d"
 
 #if defined(CONFIG_NET_SLIP)
@@ -190,6 +191,17 @@ int netdev_register(FAR struct net_driver_s *dev, enum net_lltype_e lltype)
 
       switch (lltype)
         {
+#ifdef CONFIG_NETDEV_LOOPBACK
+          case NET_LL_LOOPBACK:  /* Local loopback */
+            dev->d_llhdrlen = 0;
+            dev->d_mtu      = NET_LO_MTU;
+#ifdef CONFIG_NET_TCP
+            dev->d_recvwndo = NET_LO_TCP_RECVWNDO;
+#endif
+            devfmt          = NETDEV_LO_FORMAT;
+            break;
+#endif
+
 #ifdef CONFIG_NET_ETHERNET
           case NET_LL_ETHERNET:  /* Ethernet */
             dev->d_llhdrlen = ETH_HDRLEN;