Re: hotplugging on an embedded system under 2.6.17(-rcX)

[Greg KH <greg@kroah.com>]

On Tue, Jun 06, 2006 at 11:56:33AM +0200, Guennadi Liakhovetski wrote:
> Hi all,
> 
> We are building an embedded system based on oe / bitbaker distro and 
> 2.6.17(-rc5) kernel. And we'd like to use hotplug on it. Hotplug in a 
> generic sense, not as _the_ /proc/sys/kernel/hotplug = /sbin/hotplug 
> thing. And this is my first embedded 2.6 project with hotplug...

Why do you want to use hotplug?  Seriously, if you don't need it I
wouldn't just add it for the fun of it.

What do you want to accomplish?  automatic loading of modules?  network
connections bring up/down?  udev dynamic and persistant device nodes?
All of this needs to be determined before you grab a desktop version of
the udev and hotplug packages.

> First I was pretty much unaware of the way udev / hotplug co-exist now, so 
> we just installed _the_ "hotplug" package in the same way we were doing 
> under 2.4 and then we noticed that the boot-time is VERY long, upon closer 
> investigation it turned out to be the hotplug script that was using up 
> most of that time being started multiple times on each modprobe.

What "hotplug package" did you use?

> Now, I've read that now the actual "hotplpug" package is deprecated and 
> superseded by the udev (from kernel 2.6.15). But one requires a pretty 
> modern version of udev to really fully replace "hotplug" and best 
> co-operate with the kernel. First question - starting from which version 
> of udev is advised for 2.6.15+ kernels?

The latest.

> Secondly, the version of udev we have from oe is 071.

That's quite old, why are you using that version?  I would not recommend
it.

> If I just install it I see the udevd is started, it even dynamically
> manages / creates device nodes, and I can see with strace that it also
> receives all modprobe / rmmod events from the kernel, but, I suspect
> it is not quite properly configured yet, e.g., when I plug in a USB
> keyboard usbhid doesn't get loaded. Ok, I can read docs, install the
> latest version of udev, configure it, but before I do all that - will
> it be _much_ faster than the hotplug?  If not - we'll have to look for
> another solution...

It is much faster than hotplug, but it all comes down to what your udev
rules are doing.  What are they doing?

> Are there any ready embedded fast hotplug solutions? Will it be faster if 
> one compiles most stuff in the kernel? Our main problem is the startup 
> time when about 10-20 kernel modules get loaded...

Do you need 20 modules?  Why?

thanks,

greg k-h


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