[systemd-devel] Which udev action is run on boot for my device?

[Lennart Poettering]

On Mo, 26.10.20 11:28, Pekka Paalanen (ppaalanen at gmail.com) wrote:

> > So the ultimate quesiton is: what kind of trigger was executed on my
> > device on boot time?
> >
>
> Hi,
>
> FWIW, I was totally baffled by a similar problem with a DRM device. My
> rule was doing the same thing: match to "add" and skip otherwise. My
> rule did not appear to take effect at boot.
>
> The problem was not that "add" was not emitted - it was and processed
> with my rule. The problem was that the DRM driver then shortly after
> emitted also "changed" (this is how DRM communicates monitor hotplug
> events). Because my rule did not match "change", the udev property I
> was adding was effectively *removed*. IOW, any monitor hotplug would
> cause my udev setting to disappear.
>
> Therefore I concluded that if you want your rule to apply always (e.g.
> set a property that you expect to always find on the device), do not
> filter it with ACTION at all. New actions could be added any time, and
> then your rule breaks. At most, maybe it is safe to skip if
> ACTION=="remove", but I'm not sure about even that.

That is the correct way: check for the one "negative" action type
(i.e. "remove"), and run your rules on all the other action types,
i.e. on all "positive" action types. (Where "positive" means: the
device exists and is there, and (mostly) useable, and "negative"
means: the device doesn#t exist anymore)

Lennart

--
Lennart Poettering, Berlin