Each rtnet device registered with the stack is represented by a node
into /sys/devices/virtual/rtnet/.

Setting the address of the real device probed by the kernel into the
rtnet_device.sysbind field before calling rt_register_rtnetdev(),
causes the "adapter" symlink to be installed in the newly created
attribute directory, pointing at that device.

e.g.

static int probe_handler(struct pci_dev *pdev, const struct pci_device_id *ent)
{
	...
	rtdev->sysbind = &pdev->dev;
	...
	ret = rt_register_rtnetdev(rtdev);
	...
}

$ ls -l /sys/devices/virtual/rtnet/
total 0
drwxr-xr-x    2 root     root             0 Mar 30 12:50 enp1s0
drwxr-xr-x    2 root     root             0 Mar 30 12:50 rtlo

$ ls -l /sys/devices/virtual/rtnet/enp1s0/
total 0
lrwxrwxrwx    1 root     root             0 Mar 30 12:51 adapter -> ../../../pci0000:00/0000:00:13.0/0000:01:00.0
lrwxrwxrwx    1 root     root             0 Mar 30 12:51 subsystem -> ../../../../class/rtnet
-rw-r--r--    1 root     root          4096 Mar 30 12:50 uevent