Nested class devices and the future of the device model [LWN.net]

Nested class devices and the future of the device model

Posted Oct 23, 2005 23:03 UTC (Sun) by zblaxell (subscriber, #26385)
In reply to: Nested class devices and the future of the device model by kleptog
Parent article: Nested class devices and the future of the device model

"they work with versions 2.6.8 to 2.6.12 but after that you need..."

...new versions of udev and hotplug to match, installed before rebooting.

Actually you don't *need* anything. I'm running 2.6.13.2 with udev 0.056 (Debian), and it works just fine in the default configuration. I have udev on hold at the moment as it seems that some or all later versions of udev are severely broken regardless of kernel version--after a few bad reboots I put everything on hold, and will revisit this problem the next time some other package forces me to upgrade udev.

AFAICT, with recent udev versions all of your old udev user-space configuration breaks, and you get new features that weren't in older udevs. It's possible that some really new devices or features in the kernel are therefore not supported at all by older udev, but that means new stuff won't ever work, not that old stuff that previously worked breaks. Of course if you had non-default configuration, it has most likely reverted to the default.

If you have a static /dev tree on your root filesystem, it will still work in 2.6.13 whether you use udev or not (assuming udev *itself* isn't broken, in which case it will be broken no matter what kernel you use). If you rely on a tmpfs-mounted /dev supplied by udev versions later than 0.056...I don't know what happens.

The biggest change IMHO between 2.6.12 and later kernels is that devfs went away. It's necessary to do some gymastics with 'mount --bind' to get the original root filesystem and make a '/dev/console' in it so that the subsequent reboot will succeed (assuming you've also installed udev)--my systems have had empty /dev on the root filesystem for some years now. This is such a huge, all-encompassing, all-breaking change, that it's pointless to discuss the various udev flavors.

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