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-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 mdoe.