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.]
  
  *  rx-fcs
  
  This requests that the NIC append the Ethernet Frame Checksum (FCS)
  to the end of the skb data.  This allows sniffers and other tools to
  read the CRC recorded by the NIC on receipt of the packet.
  
  *  rx-all
  
  This requests that the NIC receive all possible frames, including errored
  frames (such as bad FCS, etc).  This can be helpful when sniffing a link with
  bad packets on it.  Some NICs may receive more packets if also put into normal
  PROMISC mode.