L1 guide: NIC drivers

The surface

drivers/nic/driver_nic.h (34 declarations; the numbered 13):

driver_nic_register(name, ops, ctx)
driver_nic_send(dev, frame, len)
driver_nic_recv(dev, buf, maxlen)        -> length / 0 / <0
driver_nic_link_status(dev)              -> link up/down
driver_nic_get_mac(dev, out[6])

Family inits: driver_nic_e1000e_init, _igb_init, _ixgbe_init, _rtl8139_init, rtl8168_init, rtl8125_init, rtl810x_init, bcm57xx_init.

What the stack expects

Frames are raw Ethernet; the stack (net/net_core.c) owns ARP and above. RX can be polled or interrupt-driven per the ops you register. ifconfig, nicstat, the *_test commands observe.

The reference implementations

Read one Intel (e1000e) and one Realtek (rtl8169-family) driver before writing yours: descriptor rings, link-state handling, and the error paths (bus-master disable, reset on timeout) are laid out differently but the contract is identical.