.. SPDX-License-Identifier: GPL-2.0

=====================================================
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 main sets of features on each netdevice,
first and second are initialized by the driver:

 1. netdev->hw_features set contains features whose state may possibly
    be changed (enabled or disabled) for a particular device by user's
    request.  Drivers normally initialize this set before registration or
    in the ndo_init callback. Changes after registration should be made
    very carefully as other parts of the code may assume hw_features are
    static. At the very least changes must be made under rtnl_lock and
    the netdev instance lock, and followed by netdev_update_features().

 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->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.

On top of those three main sets, each netdev has:

 1. Sets which control features inherited by child devices (VLAN, MPLS,
    hw_enc for L3/L4 tunnels). These sets allow the driver to limit which
    netdev->features are propagated, in case HW cannot perform the offloads
    with the extra headers present.

 2. netdev->mangleid_features, TSO features which are supported only when
    IP ID field can be mangled (constant instead of incrementing) during TSO.

 3. netdev->gso_partial_features, additional TSO features which HW can
    support via NETIF_F_GSO_PARTIAL.

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. If the device uses the
netdev instance lock, that lock must be held as well. 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.

For "ops locked" drivers (see Documentation/networking/netdevices.rst),
ethtool callbacks that may end up invoking netdev_update_features() must
opt back into rtnl_lock by setting the matching ETHTOOL_OP_NEEDS_RTNL_*
bit in ``ethtool_ops::op_needs_rtnl``. The ethtool core then keeps
rtnl_lock held across those SET callbacks so the contract above still
holds.

ndo_features_check is called for each skb before that skb is passed to
ndo_start_xmit. Driver may perform any non-trivial checks (e.g. exact
header geometry / length) and withdraw features like HW_CSUM or TSO,
requesting the networking stack to fall back to the software implementation.

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, except that NETIF_F_VLAN_CHALLENGED may be changed.
Care must be taken as changes to NETIF_F_VLAN_CHALLENGED 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.

 * 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.

* hsr-tag-ins-offload

This should be set for devices which insert an HSR (High-availability Seamless
Redundancy) or PRP (Parallel Redundancy Protocol) tag automatically.

* hsr-tag-rm-offload

This should be set for devices which remove HSR (High-availability Seamless
Redundancy) or PRP (Parallel Redundancy Protocol) tags automatically.

* hsr-fwd-offload

This should be set for devices which forward HSR (High-availability Seamless
Redundancy) frames from one port to another in hardware.

* hsr-dup-offload

This should be set for devices which duplicate outgoing HSR (High-availability
Seamless Redundancy) or PRP (Parallel Redundancy Protocol) frames
automatically in hardware.

Part V: Related device flags
============================

* netdev->netmem_tx

This is not a netdev feature bit. Drivers support netmem TX by setting
netdev->netmem_tx to one of the values in enum netmem_tx_mode.
See Documentation/networking/netmem.rst.