1 files changed, 32 insertions, 6 deletions

diff --git a/Documentation/networking/netdevices.txt b/Documentation/networking/netdevices.txt
index ce1361f95243..37869295fc70 100644
--- a/Documentation/networking/netdevices.txt
+++ b/Documentation/networking/netdevices.txt
@@ -20,6 +20,30 @@ private data which gets freed when the network device is freed. If
 separately allocated data is attached to the network device
 (dev->priv) then it is up to the module exit handler to free that.
 
+MTU
+===
+Each network device has a Maximum Transfer Unit. The MTU does not
+include any link layer protocol overhead. Upper layer protocols must
+not pass a socket buffer (skb) to a device to transmit with more data
+than the mtu. The MTU does not include link layer header overhead, so
+for example on Ethernet if the standard MTU is 1500 bytes used, the
+actual skb will contain up to 1514 bytes because of the Ethernet
+header. Devices should allow for the 4 byte VLAN header as well.
+
+Segmentation Offload (GSO, TSO) is an exception to this rule.  The
+upper layer protocol may pass a large socket buffer to the device
+transmit routine, and the device will break that up into separate
+packets based on the current MTU.
+
+MTU is symmetrical and applies both to receive and transmit. A device
+must be able to receive at least the maximum size packet allowed by
+the MTU. A network device may use the MTU as mechanism to size receive
+buffers, but the device should allow packets with VLAN header. With
+standard Ethernet mtu of 1500 bytes, the device should allow up to
+1518 byte packets (1500 + 14 header + 4 tag).  The device may either:
+drop, truncate, or pass up oversize packets, but dropping oversize
+packets is preferred.
+
 
 struct net_device synchronization rules
 =======================================
@@ -43,16 +67,17 @@ dev->get_stats:
 
 dev->hard_start_xmit:
         Synchronization: netif_tx_lock spinlock.
+
         When the driver sets NETIF_F_LLTX in dev->features this will be
         called without holding netif_tx_lock. In this case the driver
         has to lock by itself when needed. It is recommended to use a try lock
-        for this and return -1 when the spin lock fails. 
+        for this and return NETDEV_TX_LOCKED when the spin lock fails.
         The locking there should also properly protect against 
-        set_multicast_list
-        Context: Process with BHs disabled or BH (timer).
-        Notes: netif_queue_stopped() is guaranteed false
-               Interrupts must be enabled when calling hard_start_xmit.
-                (Interrupts must also be enabled when enabling the BH handler.)
+        set_multicast_list.
+
+        Context: Process with BHs disabled or BH (timer),
+                 will be called with interrupts disabled by netconsole.
+
         Return codes: 
         o NETDEV_TX_OK everything ok. 
         o NETDEV_TX_BUSY Cannot transmit packet, try later 
@@ -74,4 +99,5 @@ dev->poll:
         Synchronization: __LINK_STATE_RX_SCHED bit in dev->state.  See
                 dev_close code and comments in net/core/dev.c for more info.
         Context: softirq
+                 will be called with interrupts disabled by netconsole.