net: Add documentation for netdev features handling

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.]