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| -rw-r--r-- | Documentation/networking/netdev-features.txt | 181 |
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diff --git a/Documentation/networking/netdev-features.txt b/Documentation/networking/netdev-features.txt deleted file mode 100644 index 58dd1c1e3c65..000000000000 --- a/Documentation/networking/netdev-features.txt +++ /dev/null @@ -1,181 +0,0 @@ -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 UDP segmentation offload - -NETIF_F_GSO_UDP_L4 accepts a single UDP header with a payload that exceeds -gso_size. On segmentation, it segments the payload on gso_size boundaries and -replicates the network and UDP headers (fixing up the last one if less than -gso_size). - - * 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 is meant to be used by drivers that don't need locking at all, -e.g. software tunnels. - -This is also used in a few legacy drivers that implement their -own locking, don't use it for new (hardware) 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. - -* rx-gro-hw - -This requests that the NIC enables Hardware GRO (generic receive offload). -Hardware GRO is basically the exact reverse of TSO, and is generally -stricter than Hardware LRO. A packet stream merged by Hardware GRO must -be re-segmentable by GSO or TSO back to the exact original packet stream. -Hardware GRO is dependent on RXCSUM since every packet successfully merged -by hardware must also have the checksum verified by hardware. |