invalidating udev cache, how?

[Koen Kooi <k.kooi@student.utwente.nl>]

Hi,

The udev 124 has cache (/etc/dev.tar) to avoid doing the coldplug udev 
dance that can take a few seconds to a minute depending on machine speed 
and kernel options. It is working a bit too well at the moment:

* user boots image with 2.6.26 kernel on an omap board
* user gets /dev/fb0
* user fixes the bootargs in uboot to enable the overlay
* user doesn't get /dev/fb1

so remove /etc/dev.tar and reboot: /dev/fb1 appears.

I also encountered a case where the permissions on /dev/null where wrong 
during first boot (it's a mystery why that happened) and udev cached 
those making ssh daemons fail on boot.

My current ideas:

1) remove /etc/dev.tar if its > x weeks old
2) recreate it on shutdown
3) remove it after x times

option 1) breaks on systems without an RTC and/or no /etc/timestamp
option 2) moves the slowness to shutdown
option 3) requires extra logic and filesystem access

and all options don't fix the first case I mentioned, they all take a 
while to make /dev/fb1 appear.

The key is that it should be transparent to users, so adding a check for 
.e.g 'ignore_dev.tar=1' in bootargs wouldn't work, since that implies 
that users are aware of the problem and know how to 'fix' it.

Does anyone have other ways to invalidate the cache, and if not, which 
option would get your vote?

regards,

Koen