[systemd-devel] Resolving systemd naming problems on multi-port PCI cards

[Jordan Hargrave]

The current systemd naming scheme for Network cards has a problem
correctly naming multi-port NIC devices in a PCI slot.

Systemd currently generates names of the form:

enpAsBfCdD

pA = PCI bus number
sB = PCI device number (confusingly called 'SLOT')
fC = PCI function number
[dD = NIC device port (sysfs dev_port)]

eg. enp5s0f0 for a NIC at 05:00.0, dev_port = 0

These names already aren't necessarily persistent if PCI bus topology
changes (Bus number changes due to adding cards across reboot, etc).

--or--
ensBfCdD

sB = _SUN slot
fC = PCI function number
[dD = NIC device port (sysfs dev_port)]

eg. ens2f0d1 for a single-port NIC at 0?:00.0 in PCI slot 2, dev_port = 1

The problem is the 2nd naming scheme cannot handle multi-port NICs.
Multi-port NICs often have one or more bridges before the PCI slot
number itself.

eg. for my quad-port Intel NIC in PCI slot 2 the devices are actually:
44:00.0
44:00.1
45:00.0
45:00.1

Using the 2nd naming scheme, the names generated are:
ens2f0
ens2f1
ens2f0
ens2f1

Oops. Problem. There is a name collision.  So depending on who gets
initialized first I'll see either:

ens2f0
ens2f1
enp69s0f0
enp69s1f0

or
enp68s0f0
enp68s1f0
ens2f0
ens2f1

There is a way to fix this by combining the two naming schemes, with a
bit of a hack.

enpAsBfCdD

pA = PCI bus # (no change)
sB = _SUN slot # (no change)
fC = This is what changes. Instead of C = function number (0..7) it is
Device:Function (0..31)
dD = Device port (no change)

On my system this generates new names:
enp4s0 at /sys/devices/pci0000:00/0000:00:03.0 SLOT 3      => enp3s4f0
enp4s0d1 at /sys/devices/pci0000:00/0000:00:03.0 1 SLOT 3  => enp3s4f0d1
enp4s0f1 at /sys/devices/pci0000:00/0000:00:03.0 SLOT 3    => enp3s4f1
enp4s0f1d1 at /sys/devices/pci0000:00/0000:00:03.0 SLOT 3  => enp3s4f1d1
enp4s0f2 at /sys/devices/pci0000:00/0000:00:03.0 SLOT 3    => enp3s4f2
enp4s0f2d1 at /sys/devices/pci0000:00/0000:00:03.0 SLOT 3  => enp3s4f2d1
enp4s0f3 at /sys/devices/pci0000:00/0000:00:03.0 SLOT 3    => enp3s4f3
enp4s0f3d1 at /sys/devices/pci0000:00/0000:00:03.0 SLOT 3  => enp3s4f3d1
enp4s0f4 at /sys/devices/pci0000:00/0000:00:03.0 SLOT 3    => enp3s4f4
enp4s0f4d1 at /sys/devices/pci0000:00/0000:00:03.0 SLOT 3  => enp3s4f4d1
enp4s0f5 at /sys/devices/pci0000:00/0000:00:03.0 SLOT      => enp3s4f5
enp4s0f5d1 at /sys/devices/pci0000:00/0000:00:03.0 SLOT 3  => enp3s4f5d1
enp4s0f6 at /sys/devices/pci0000:00/0000:00:03.0 SLOT 3    => enp3s4f6
enp4s0f6d1 at /sys/devices/pci0000:00/0000:00:03.0 SLOT 3  => enp3s4f6d1
enp4s0f7 at /sys/devices/pci0000:00/0000:00:03.0 SLOT 3    => enp3s4f7
enp4s0f7d1 at /sys/devices/pci0000:00/0000:00:03.0 SLOT 3  => enp3s4f7d1
enp4s1 at /sys/devices/pci0000:00/0000:00:03.0 SLOT 3      => enp3s4f8
      (Device 1:0 => Function 8)
enp4s1d1 at /sys/devices/pci0000:00/0000:00:03.0 SLOT 3    =>
enp3s4f8d1   (Device 1:0 => Function 8)

enp68s0f0 at /sys/devices/pci0000:40/0000:40:03.0 SLOT 2   => enp68s2f0
enp68s0f1 at /sys/devices/pci0000:40/0000:40:03.0 SLOT 2   => enp68s2f1
enp69s0f0 at /sys/devices/pci0000:40/0000:40:03.0 SLOT 2   => enp69s2f0
enp69s0f1 at /sys/devices/pci0000:40/0000:40:03.0 SLOT 2   => enp69s2f1

This way it is always able to determine the physical PCI slot the device is in.

This scheme still does have a limitation... the names may not be
persistent if PCI topology changes due to the PCI bus number still
being part of the name.