I found myself with a Proxmox server that wouldn’t do anything with its network card. It took me a while to realize that at one point I had bound it to a VM. Even after removing it from the VM, the host wouldn’t do anything with it.

Discover which driver a device is using:

lspci -knn

In my case I found the culprit: the driver for the network card was still claimed by vfio-pci

08:00.0 Network controller [0280]: Mellanox Technologies MT27500 Family [ConnectX-3] [15b3:1003]
Subsystem: Mellanox Technologies MT27500 Family [ConnectX-3] [15b3:0050]
Kernel driver in use: vfio-pci
Kernel modules: mlx4_core

I finally found in this post how to tell the kernel to unbind from vfio-pci and bind to the network driver mlx4_core. Given the PCI bus location and device ID from the command, I was able to reclaim my network adapter to my host successfully:

echo -n "0000:08:00.0" > /sys/bus/pci/drivers/vfio-pci/unbind
echo -n "15b3 1003" > /sys/bus/pci/drivers/vfio-pci/remove_id
echo -n "0000:08:00.0" > /sys/bus/pci/drivers/mlx4_core/bind

Success!