[systemd-devel] Trying to come up with udev rule for USB geographic naming

[Paul D. DeRocco]

That is, I want a rule for creating a symlink that describes the USB port
something is plugged into, rather than the specific device plugged into
it. Linux USB has a way of representing the location of each physical
jack, even when hubs are involved. For devices like USB serial and USB
MIDI, I'd like apps to be able to refer to devices by where they're
plugged in.

I used udevadm to generate the info for a USB MIDI converter. The first
three levels look like this:

looking at device
'/devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0/sound/card1/m
idiC1D0':
  KERNEL=="midiC1D0"
  SUBSYSTEM=="sound"
  DRIVER==""

looking at parent device
'/devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0/sound/card1':
  KERNELS=="card1"
  SUBSYSTEMS=="sound"
  DRIVERS==""
  ATTRS{id}=="XMidi1X1"
  ATTRS{number}=="1"

looking at parent device
'/devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0':
  KERNELS=="1-1.2:1.0"
  SUBSYSTEMS=="usb"
  DRIVERS=="snd-usb-audio"
  ATTRS{authorized}=="1"
  ATTRS{bAlternateSetting}==" 0"
  ATTRS{bInterfaceClass}=="01"
  ATTRS{bInterfaceNumber}=="00"
  ATTRS{bInterfaceProtocol}=="00"
  ATTRS{bInterfaceSubClass}=="01"
  ATTRS{bNumEndpoints}=="00"
  ATTRS{supports_autosuspend}=="1"

(There may be spurious line breaks in there, due to email.)

I'd like an alias for this device to be "/dev/midi1-1.2:1.0". I tried the
following:

KERNEL=="midiC*", DRIVERS=="snd-usb-audio", SYMLINK+="midi%k"

It did bupkis. The man page seems to imply that, while a single rule can't
match items from different parents, it's okay to match something in the
device and then something else in a parent. So I would expect the KERNEL
key would match the device itself, DRIVERS would match a parent, but what
then does %k refer to, the kernel name in the device or the kernel name in
the parent?

But it not only didn't create a "midi1-1.2:1.0", it didn't create a
"midimidiC1D0" either, so it doesn't look like the rule matched at all.

There is already a symlink
"/dev/snd/by-path/platform-3f980000.usb-usb-0:1.2:1.0", but it refers to
the useless "controlC1" interface. It also doesn't appear to be generated
by an explicit rule anywhere that might show me what to do.

What am I doing wrong? Do I need a rule that just matches the MIDI device,
and then runs a script that searches for the parent name?

-- 

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco at ix.netcom.com