Arch Linux

jwatte

Member

Registered: 2012-06-22

Posts: 58

udev sometimes doesn't create symlinks -- race condition?

udev sometimes doesn't create symlinks I specify in /etc/udev/rules.d/*.rules
Many times, it works fine, especially when cold booting.
When rebooting, it almost always misses a few rules.
Sometimes when cold booting, it will miss rules, too.

Here's a typical rule, intended to recognize my two Microsoft HD webcams and create aliases "_Lcam" and "_Rcam."

KERNELS=="2-2:1.0", SUBSYSTEM=="video4linux", SYMLINK+="_Lcam"
KERNELS=="1-2:1.0", SUBSYSTEM=="video4linux", SYMLINK+="_Lcam"

KERNELS=="3-1.4:1.0", SUBSYSTEM=="video4linux", SYMLINK+="_Rcam"
KERNELS=="4-1.4:1.0", SUBSYSTEM=="video4linux", SYMLINK+="_Rcam"

Note that, although I have not moved these devices around, the physical bus numbers have changed over time. This is also concerning to me -- I thought the idea was that the USB bus numbering is based on the physical hardware (at PCI/Ex/HyperTransport level) and thus won't change. That appears to be mistaken.

So, if the physical USB busses will be re-numbered, and I have two identical devices plugged in (there is no "serial number" for a webcam,) what can I do to tell them apart?

As it is, these devices happen to be one plugged into a USB3 adapter and one into a USB2 adapter, so I suppose I could separate on the "xhci" vs "ehci" driver way up the chain, but that seems very fragile, not to mention not working for the two ACM serial devices I both have plugged into USB2 ports...