system-linux: handle ethtool offload, channels, ring, coalesce and priv flags parameters

[nvandamme]

This a preliminary work to be able to pass ethtool flags via config file and apply those flags directly on configured devices.
It mainly follows the base implementations of existing ethtool/ioctl functions inside system-linux.c.

This work is a personal test to apply specific flags (priv/ring/offload) handled by intel i40e/iavf drivers (X710 variants on my side)

As i am very newly introduced to openwrt build system and SDK, i am not sure that my branch is compiling at all (i've tried via custom feed, but i doesn't seem to stick...).

Following is an ongoing documentation:

Example: Applying ethtool tunables via /etc/config/network

You can specify ethtool flags for supported family (offload, ring, coalesce, channels, priv) in a device section of /etc/config/network.

This doesn't replace existing flags already handled by netifd via ethtool (like rxpause, txpause, autoneg), it only adds support for additional ethtool tunables.

Example device config

config device
    option name 'eth1'
    list offload 'sg on'
    list offload 'gso on'
    list offload 'gro on'
    list offload 'tso off'
    list offload 'rx_gro_list off'
    list offload 'rx_udp_gro_forwarding on'
    list ring 'rx 4096'
    list ring 'tx 4096'
    list coalesce 'rx-usecs 50'
    list coalesce 'tx-frames 128'
    list channels 'rx 4'
    list channels 'tx 4'
    list priv 'my_custom_flag on'
    option txqueuelen '10000'

Supported ethtool tunable families

• list offload (ethtool -K)
• list ring (ethtool -G)
• list coalesce (ethtool -C)
• list channels (ethtool -L)
• list priv (ethtool --set-priv-flags)

Each list item is a key-value pair, e.g. list offload 'tso off'.

How it works

• When netifd brings up the device, it parses these lists and applies them using the corresponding system_set_ethtool* handler (see system-linux.c).

Notes

• You can specify multiple flags per family.
• If a flag is not supported by the driver, it will be ignored or logged as a warning.
• For advanced usage, refer to the system_set_ethtool* handlers in system-linux.c for details on what is supported.