Udev Tidbits Picked Up at LCA (Shallow Thoughts)

Akkana's Musings on Open Source, Science, and Nature.

Wed, 07 Feb 2007

Udev Tidbits Picked Up at LCA

A couple of udev tips I picked up at LinuxConf, mostly from talking to folks in the hallways:

I'd been having trouble getting my laptop to read its built-in memory stick since upgrading to Ubuntu Edgy. It's basically the same problem I described in an earlier article: the machine boots, sees the built-in reader with no card there, and udev creates /dev/sda but not /dev/sda1. Later, I insert a memory stick, but the reader (like so many other USB-based flash card readers) does not generate an event, so no new device is created.

In that earlier article, the solution was to change the udev rule that creates the device and add something like NAME{all_partitions}="stick". The all_partitions tells it to create /dev/stick1, /dev/stick2 etc. up through the possible maximum number of partitions. (It would be nice to limit it just to /dev/stick1, but there doesn't seem to be any way to do that.)

Unfortunately, in edgy, the udev rules have been rewritten to be a lot more general, and adding {all_partitions} wasn't working. But LinuxConf gave me two solutions to this problem:

First, I was able to pester one of the hal developers about hal's annoying mandatory polling. (This is the official Ubuntu solution to the problem, by the way: if you let hald wake up twice a second to poll every device on the USB bus to see whether anything new has been added, then you'll get that /dev/sda1 device appearing. I wasn't the only one at the conference, I was happy to find, who was unhappy about this hald misbehavior. It got mentioned in at least two other talks as an example of inefficient behavior that can eat batteries and CPU, and a questioner during the hal talk echoed my opinion that the polling should be made optional for those of us who don't want it.)

Anyway, I asked him what hald does to create the /dev/sda1 device once it sees a card. It turns out that touch /dev/sda causes udev to wake up and re-check the device, and create any new device nodes which might have appeared. Hurrah! That's a much cleaner workaround than sudo mknod.

But at breakfast a few days later, I found myself sitting next to a udev expert. He took a look at the file I'd created, /etc/udev/rules.d/memstick.rules, and after a few minutes of fiddling discovered what it was missing: a crucial ACTION=="add" directive which hadn't been required under the old system. The working line now looks like this:

KERNEL=="sda", ACTION=="add", OPTIONS=="all_partitions", NAME{all_partitions}="stick"

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[ 21:14 Feb 07, 2007    More linux | permalink to this entry ]