net: Add documentation for netdev features handling

v2: incorporated suggestions from Randy Dunlap
Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/netdev-features.txt

+Netdev features mess and how to get out from it alive
+=====================================================
+
+Author:
+	Michał Mirosław <mirq-linux@rere.qmqm.pl>
+
+
+
+ Part I: Feature sets
+======================
+
+Long gone are the days when a network card would just take and give packets
+verbatim.  Today's devices add multiple features and bugs (read: offloads)
+that relieve an OS of various tasks like generating and checking checksums,
+splitting packets, classifying them.  Those capabilities and their state
+are commonly referred to as netdev features in Linux kernel world.
+
+There are currently three sets of features relevant to the driver, and
+one used internally by network core:
+
+ 1. netdev->hw_features set contains features whose state may possibly
+    be changed (enabled or disabled) for a particular device by user's
+    request.  This set should be initialized in ndo_init callback and not
+    changed later.
+
+ 2. netdev->features set contains features which are currently enabled
+    for a device.  This should be changed only by network core or in
+    error paths of ndo_set_features callback.
+
+ 3. netdev->vlan_features set contains features whose state is inherited
+    by child VLAN devices (limits netdev->features set).  This is currently
+    used for all VLAN devices whether tags are stripped or inserted in
+    hardware or software.
+
+ 4. netdev->wanted_features set contains feature set requested by user.
+    This set is filtered by ndo_fix_features callback whenever it or
+    some device-specific conditions change. This set is internal to
+    networking core and should not be referenced in drivers.
+
+
+
+ Part II: Controlling enabled features
+=======================================
+
+When current feature set (netdev->features) is to be changed, new set
+is calculated and filtered by calling ndo_fix_features callback
+and netdev_fix_features(). If the resulting set differs from current
+set, it is passed to ndo_set_features callback and (if the callback
+returns success) replaces value stored in netdev->features.
+NETDEV_FEAT_CHANGE notification is issued after that whenever current
+set might have changed.
+
+The following events trigger recalculation:
+ 1. device's registration, after ndo_init returned success
+ 2. user requested changes in features state
+ 3. netdev_update_features() is called
+
+ndo_*_features callbacks are called with rtnl_lock held. Missing callbacks
+are treated as always returning success.
+
+A driver that wants to trigger recalculation must do so by calling
+netdev_update_features() while holding rtnl_lock. This should not be done
+from ndo_*_features callbacks. netdev->features should not be modified by
+driver except by means of ndo_fix_features callback.
+
+
+
+ Part III: Implementation hints
+================================
+
+ * ndo_fix_features:
+
+All dependencies between features should be resolved here. The resulting
+set can be reduced further by networking core imposed limitations (as coded
+in netdev_fix_features()). For this reason it is safer to disable a feature
+when its dependencies are not met instead of forcing the dependency on.
+
+This callback should not modify hardware nor driver state (should be
+stateless).  It can be called multiple times between successive
+ndo_set_features calls.
+
+Callback must not alter features contained in NETIF_F_SOFT_FEATURES or
+NETIF_F_NEVER_CHANGE sets. The exception is NETIF_F_VLAN_CHALLENGED but
+care must be taken as the change won't affect already configured VLANs.
+
+ * ndo_set_features:
+
+Hardware should be reconfigured to match passed feature set. The set
+should not be altered unless some error condition happens that can't
+be reliably detected in ndo_fix_features. In this case, the callback
+should update netdev->features to match resulting hardware state.
+Errors returned are not (and cannot be) propagated anywhere except dmesg.
+(Note: successful return is zero, >0 means silent error.)
+
+
+
+ Part IV: Features
+===================
+
+For current list of features, see include/linux/netdev_features.h.
+This section describes semantics of some of them.
+
+ * Transmit checksumming
+
+For complete description, see comments near the top of include/linux/skbuff.h.
+
+Note: NETIF_F_HW_CSUM is a superset of NETIF_F_IP_CSUM + NETIF_F_IPV6_CSUM.
+It means that device can fill TCP/UDP-like checksum anywhere in the packets
+whatever headers there might be.
+
+ * Transmit TCP segmentation offload
+
+NETIF_F_TSO_ECN means that hardware can properly split packets with CWR bit
+set, be it TCPv4 (when NETIF_F_TSO is enabled) or TCPv6 (NETIF_F_TSO6).
+
+ * Transmit DMA from high memory
+
+On platforms where this is relevant, NETIF_F_HIGHDMA signals that
+ndo_start_xmit can handle skbs with frags in high memory.
+
+ * Transmit scatter-gather
+
+Those features say that ndo_start_xmit can handle fragmented skbs:
+NETIF_F_SG --- paged skbs (skb_shinfo()->frags), NETIF_F_FRAGLIST ---
+chained skbs (skb->next/prev list).
+
+ * Software features
+
+Features contained in NETIF_F_SOFT_FEATURES are features of networking
+stack. Driver should not change behaviour based on them.
+
+ * LLTX driver (deprecated for hardware drivers)
+
+NETIF_F_LLTX should be set in drivers that implement their own locking in
+transmit path or don't need locking at all (e.g. software tunnels).
+In ndo_start_xmit, it is recommended to use a try_lock and return
+NETDEV_TX_LOCKED when the spin lock fails.  The locking should also properly
+protect against other callbacks (the rules you need to find out).
+
+Don't use it for new drivers.
+
+ * netns-local device
+
+NETIF_F_NETNS_LOCAL is set for devices that are not allowed to move between
+network namespaces (e.g. loopback).
+
+Don't use it in drivers.
+
+ * VLAN challenged
+
+NETIF_F_VLAN_CHALLENGED should be set for devices which can't cope with VLAN
+headers. Some drivers set this because the cards can't handle the bigger MTU.
+[FIXME: Those cases could be fixed in VLAN code by allowing only reduced-MTU
+VLANs. This may be not useful, though.]