Re: hotplug (was devfs)

[Oliver.Neukum@lrz.uni-muenchen.de]

On Wed, 13 Nov 2002, Nick Craig-Wood wrote:

> On Tue, Nov 12, 2002 at 01:51:08PM +0100, Oliver Neukum wrote:
> > > Actually, here's a question: are /sbin/hotplug upcalls serialized in
> > > some fashion?  I'd hate to online a thousand devices in my disk array
> > > and have the machine forkbomb itself.
> >
> > Nope, no serialisation. You don't have any guarantee even that
> > addition will be delivered before removal.
>
> And that is why (we finally discovered) we were getting
> non-deterministic device numbering of USB nodes.
>
> We have machines with 6 x 4 port USB <-> serial converters attached.
> These would get randomly assigned usb device ids and hence random
> /dev/ttyUSB nodes.  Not very useful when there is a load of different
> things attached to the 24 serial ports!

Please clarify. Did this happen if you connected a hub these gadgets
sit on? Do you use the same type of device six times ?

> Sometimes we also found that one of the devices wouldn't get
> initialised properly.
>
> We fixed these problems by removing hotplug and loading the relevant
> kernel modules in the correct order and voila a perfectly

Modules ? Plural?

> deterministic order for the /dev/ttyUSBs with all devices initialised.
> Plugging in our USB bus with 24 devices on it does indeed produce a
> mini-forkbomb effect ;-) (Especially since these Keyspan devices are
> initialised twice - once without firmware and once with firmware.)

There's a further problem.

> So - perhaps hotplug ought to be serialised?

Definitely, but how far?

	Regards
		Oliver




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