1 Netdev features mess and how to get out from it alive 2 ===================================================== 3 4 Author: 5 Michał Mirosław <email@example.com> 6 7 8 9 Part I: Feature sets 10 ====================== 11 12 Long gone are the days when a network card would just take and give packets 13 verbatim. Today's devices add multiple features and bugs (read: offloads) 14 that relieve an OS of various tasks like generating and checking checksums, 15 splitting packets, classifying them. Those capabilities and their state 16 are commonly referred to as netdev features in Linux kernel world. 17 18 There are currently three sets of features relevant to the driver, and 19 one used internally by network core: 20 21 1. netdev->hw_features set contains features whose state may possibly 22 be changed (enabled or disabled) for a particular device by user's 23 request. This set should be initialized in ndo_init callback and not 24 changed later. 25 26 2. netdev->features set contains features which are currently enabled 27 for a device. This should be changed only by network core or in 28 error paths of ndo_set_features callback. 29 30 3. netdev->vlan_features set contains features whose state is inherited 31 by child VLAN devices (limits netdev->features set). This is currently 32 used for all VLAN devices whether tags are stripped or inserted in 33 hardware or software. 34 35 4. netdev->wanted_features set contains feature set requested by user. 36 This set is filtered by ndo_fix_features callback whenever it or 37 some device-specific conditions change. This set is internal to 38 networking core and should not be referenced in drivers. 39 40 41 42 Part II: Controlling enabled features 43 ======================================= 44 45 When current feature set (netdev->features) is to be changed, new set 46 is calculated and filtered by calling ndo_fix_features callback 47 and netdev_fix_features(). If the resulting set differs from current 48 set, it is passed to ndo_set_features callback and (if the callback 49 returns success) replaces value stored in netdev->features. 50 NETDEV_FEAT_CHANGE notification is issued after that whenever current 51 set might have changed. 52 53 The following events trigger recalculation: 54 1. device's registration, after ndo_init returned success 55 2. user requested changes in features state 56 3. netdev_update_features() is called 57 58 ndo_*_features callbacks are called with rtnl_lock held. Missing callbacks 59 are treated as always returning success. 60 61 A driver that wants to trigger recalculation must do so by calling 62 netdev_update_features() while holding rtnl_lock. This should not be done 63 from ndo_*_features callbacks. netdev->features should not be modified by 64 driver except by means of ndo_fix_features callback. 65 66 67 68 Part III: Implementation hints 69 ================================ 70 71 * ndo_fix_features: 72 73 All dependencies between features should be resolved here. The resulting 74 set can be reduced further by networking core imposed limitations (as coded 75 in netdev_fix_features()). For this reason it is safer to disable a feature 76 when its dependencies are not met instead of forcing the dependency on. 77 78 This callback should not modify hardware nor driver state (should be 79 stateless). It can be called multiple times between successive 80 ndo_set_features calls. 81 82 Callback must not alter features contained in NETIF_F_SOFT_FEATURES or 83 NETIF_F_NEVER_CHANGE sets. The exception is NETIF_F_VLAN_CHALLENGED but 84 care must be taken as the change won't affect already configured VLANs. 85 86 * ndo_set_features: 87 88 Hardware should be reconfigured to match passed feature set. The set 89 should not be altered unless some error condition happens that can't 90 be reliably detected in ndo_fix_features. In this case, the callback 91 should update netdev->features to match resulting hardware state. 92 Errors returned are not (and cannot be) propagated anywhere except dmesg. 93 (Note: successful return is zero, >0 means silent error.) 94 95 96 97 Part IV: Features 98 =================== 99 100 For current list of features, see include/linux/netdev_features.h. 101 This section describes semantics of some of them. 102 103 * Transmit checksumming 104 105 For complete description, see comments near the top of include/linux/skbuff.h. 106 107 Note: NETIF_F_HW_CSUM is a superset of NETIF_F_IP_CSUM + NETIF_F_IPV6_CSUM. 108 It means that device can fill TCP/UDP-like checksum anywhere in the packets 109 whatever headers there might be. 110 111 * Transmit TCP segmentation offload 112 113 NETIF_F_TSO_ECN means that hardware can properly split packets with CWR bit 114 set, be it TCPv4 (when NETIF_F_TSO is enabled) or TCPv6 (NETIF_F_TSO6). 115 116 * Transmit DMA from high memory 117 118 On platforms where this is relevant, NETIF_F_HIGHDMA signals that 119 ndo_start_xmit can handle skbs with frags in high memory. 120 121 * Transmit scatter-gather 122 123 Those features say that ndo_start_xmit can handle fragmented skbs: 124 NETIF_F_SG --- paged skbs (skb_shinfo()->frags), NETIF_F_FRAGLIST --- 125 chained skbs (skb->next/prev list). 126 127 * Software features 128 129 Features contained in NETIF_F_SOFT_FEATURES are features of networking 130 stack. Driver should not change behaviour based on them. 131 132 * LLTX driver (deprecated for hardware drivers) 133 134 NETIF_F_LLTX is meant to be used by drivers that don't need locking at all, 135 e.g. software tunnels. 136 137 This is also used in a few legacy drivers that implement their 138 own locking, don't use it for new (hardware) drivers. 139 140 * netns-local device 141 142 NETIF_F_NETNS_LOCAL is set for devices that are not allowed to move between 143 network namespaces (e.g. loopback). 144 145 Don't use it in drivers. 146 147 * VLAN challenged 148 149 NETIF_F_VLAN_CHALLENGED should be set for devices which can't cope with VLAN 150 headers. Some drivers set this because the cards can't handle the bigger MTU. 151 [FIXME: Those cases could be fixed in VLAN code by allowing only reduced-MTU 152 VLANs. This may be not useful, though.] 153 154 * rx-fcs 155 156 This requests that the NIC append the Ethernet Frame Checksum (FCS) 157 to the end of the skb data. This allows sniffers and other tools to 158 read the CRC recorded by the NIC on receipt of the packet. 159 160 * rx-all 161 162 This requests that the NIC receive all possible frames, including errored 163 frames (such as bad FCS, etc). This can be helpful when sniffing a link with 164 bad packets on it. Some NICs may receive more packets if also put into normal 165 PROMISC mode.