Re: about split the udev

[David Zeuthen <david@fubar.dk>]

On Mon, 2004-01-12 at 17:28, Robert Love wrote:
> On Mon, 2004-01-12 at 08:54, Kay Sievers wrote:
> 
> > Huhh, what a magic :)
> > 
> > udev is a part of hotplug.
> > udev _sends_ DBUS-messages.
> > HAL receives these messages with the node name _from_ udev.
> 
> HAL _also_ installs a hotplug script to listen for events and emit a
> D-BUS signal, because HAL wants to know about all hotplug events, not
> just ones that udev handles.  So I think Martin's idea is actually
> pretty good.
> 
> It would mean we end up with a really clean progression for hardware
> integration:
> 
> 	kernel -> hotplug (via /sbin/hotplug) -> hal.hotplug ->
> 	udev daemon (via D-BUS) -> HAL daemon (via D-BUS) ->
> 	upper levels of the desktop (via D-BUS)
> 
> Which I like.

Yeah, but udev and hal.hotplug works in different ways with respect to
d-bus integration.

hal.hotplug explicitly sends the hotplug events as d-bus messages
addressed to the org.freedesktop.Hal service - no one can piggy back on
these.

In contrast udev, when invoked, claims the org.kernel.udev service and
emits a signal on the org.kernel.udev.NodeMonitor interface - any
application can pick up this signal if they subscribe to it on the
system messagebus.

Due to the nice security infrastructure of D-BUS one can actually trust
that the event originated from udev.

(BUT... note that we do in udev today is fundamentally racey as multiple
copies of udev run at the same time. So in reality a copy of udev
doesn't always claim the org.kernel.udev service; it becomes a
"secondary owner" of the service instead. 
This means that signals stemming from such a copy doesn't seem to
originate from the org.kernel.udev service. This is an issue I think
need to be solved in d-bus; I've already raised it on message-bus-list
but I haven't got a reply yet.)

What we want to do with hotplug is probably similar to what we do with
udev rather than how hal.hotplug works. If people are interested, I'll
be happy to submit code for this. Of course we'll run into the same
issues as we have with udev d-bus integration today.

Cheers,
David



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