Re: device properties and change events

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Mar 27, 2013 at 04:47:39AM +0000, Keith Pine wrote:
> I'm using udev-147 in CentOS 6.3.

My sympathies :)

> I have two rules that import properties into the udev db. I'll use a
> simplified example.
> 
>     IMPORT{program}="program1 %k"
>     ACTION="add", IMPORT{program}="program2 %N"
> 
> program1 exports MY_FOO, and program2 exports MY_BAR. I've noticed that
> when a change event is triggered, MY_BAR is no longer available in the
> udev database. So it appears change events clear the existing environment.

What is triggering a change event?  What type of device?  What driver?

> The work done in program2 is relatively expensive so I'd rather only run
> it when the device is first added.
> 
> It looks like I can re-import MY_BAR if I use IMPORT{db} in a separate
> rule:
> 
>     IMPORT{program}="program1 %k"
>     ACTION="add", IMPORT{program}="program2 %N"
>     ACTION="change", IMPORT{db}="MY_BAR"
> 
> 
> Is this the correct approach? Or is there another way to persist the
> properties that were imported during add, besides re-importing program2?

I think that is correct.  When a device "changes", all of it's
attributes are thrown away as they obviously changed (the kernel told us
so.)

> Also, is it possible to determine *what* causes a change event to occur?

What type of device is this?

> With 'udevadm monitor' I can see change events being triggered whenever my
> application exits (for the devices it has opened), and would like to
> determine why. I was hoping udev could give me a hint.

Your kernel driver should tell you why, what driver is it?

thanks,

greg k-h