Udev - Trapping "Add" - Subsystem "module"

Udev - Trapping "Add" - Subsystem "module"

[Kilau, Scott]

Hi everyone,
 
I have a quick question that I am hoping has a quick answer.  =)

 
I want to hook my driver when its added to the module subsystem.
 
After spending quite a bit of time trying to figure out why I
couldn't get the right hook, I finally found that it can't
be trapped, currently on purpose.
 

My rule (currently I put in "10-dgap.rules"):

ACTION="add", KERNEL="dgap", SUBSYSTEM="module", \
     RUN+="/usr/sbin/dgapdl", OPTIONS+="last_rule"
 
But it won't hook, because of the rule in 05-udev-early.rules:
 
# ignore these events until someone needs them
SUBSYSTEM="module",            OPTIONS="ignore_device"
 
To verify that I did actually have the right hook okay,
I made a 01-dgap.rules and put my rule in there,
and it did hook like I wanted it to.
 
I really don't want to have a 01-dgap.rules, mainly because
I suspect its probably frowned upon...
(ie, I don't see anyone else doing it...)
 
Is there any way I can "override" the 05-udev-early.rules for
just this entry in my 10-dgap.rules file?

Thanks!
Scott Kilau

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Re: Udev - Trapping "Add" - Subsystem "module"

[Kris van Rens]

Hi,

Kilau, Scott wrote:
> Is there any way I can "override" the 05-udev-early.rules for
> just this entry in my 10-dgap.rules file?

How about taking a lower number, say, 04-dgap.rules? The rules are handled in lexical order I believe so it wouldn't even matter if you'd take the same number as another rule.

Just be sure not to overrule other rules in 05-udev-early.rules.

-Kris

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RE: Udev - Trapping "Add" - Subsystem "module"

[Kilau, Scott]

Hi Kris, 

> How about taking a lower number, say, 04-dgap.rules? The 
> rules are handled in lexical order I believe so it wouldn't 
> even matter if you'd take the same number as another rule.

I was thinking about doing it that way, but I just want to
make sure its "proper udev etiquette".

A google search doesn't seem to show anyone making any
scripts before the "05"...

And of course, all the regular udev docs suggest using 10-*whatever
for "custom" udev rules...

I just want to be sure that doing an 04- script doesn't violate
anything in udev that might not be happy about it...

Scott Kilau





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Re: Udev - Trapping "Add" - Subsystem "module"

[Kris van Rens]

Kilau, Scott wrote:
> I just want to be sure that doing an 04- script doesn't violate
> anything in udev that might not be happy about it...

Yeah, I have been searching for that too, but indeed; (probably because of the example in the howto) everybody makes rules 10-* or higher.

But, I'd say if there's nowhere to be read that it's an evil thing to make rules below 10-* then why not? As long as you're careful.

That's all the certainty I can give you I'm afraid :)

-Kris

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Re: Udev - Trapping "Add" - Subsystem "module"

[Sergey Vlasov]

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On Thu, 9 Nov 2006 12:44:52 -0600 Kilau, Scott wrote:

> My rule (currently I put in "10-dgap.rules"):
> 
> ACTION=="add", KERNEL=="dgap", SUBSYSTEM=="module", \
>      RUN+="/usr/sbin/dgapdl", OPTIONS+="last_rule"

This does not seem to be a good idea.  The module add event is
generated early - even before the module init routine is invoked; so
the program you run may even find that the module was not loaded
because the module initialization failed.  And even if the module
loads successfully, there will be a race - when your program will try
to access your module, it might break because the module init is not
yet complete.  This race might be hard to notice on an uniprocessor
machine, because the udevd process which handles uevents will most
likely be scheduled only after the module init code completes (but you
should be able to reproduce it easily if you insert some sleeps in the
module init code).

The proper way to handle initialization is to have your module
register some devices and then catch addition of these devices in udev
rules.  At this time you can be sure that the corresponding device
file has been created by udevd.

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RE: Udev - Trapping "Add" - Subsystem "module"

[Kilau, Scott]

 
> The proper way to handle initialization is to have your module
> register some devices and then catch addition of these devices in udev
> rules.  At this time you can be sure that the corresponding device
> file has been created by udevd.

Ah, thanks for the answer.
Yes, this sure does seem like a better solution.

Thanks for the suggestion, I will give it a whirl!
Scott Kilau

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