> > However, the /sbin/hotplug interface can be used... But that's an
> > argument for a later time, after some more design work is done :)
>
> No, it cannot. It's event based. We need a simple and robust way
> to get that information.
I was going to make a similar point.
The "original" /sbin/hotplug framework was just publishing events
that the kernel knew about ... "here's a new USB device". While it
was a design goal that it work without (what we now call) "usbfs",
clearly a number of things work better when usbfs is available since
then the hotplug agent has more information available to it.
Two points that I believe sold that original /sbin/hotplug approach
vs. alternatives, beyond the obvious "it works and solves the problem":
- Cost in the kernel was less than 512 bytes; it used
existing mechanisms, which were always present.
(It takes a bit more space now, mostly because more
subsystems have learned about it.)
- It solved the problem without needing two contentious
capabilities that added to overall system costs (memory,
maintainability, administerability, etc): a filesystem (like
usbfs or devfs); and a user mode daemon (like devfsd
or the various early usbds).
Now in general, there are two ways to deliver the information
userspace needs to handle an event: (a) in the event itself,
or (b) through external state. The "poll a filesystem" approach
relies purely on (b), hotplug uses (a) for minimal functionality
but (!!) relies on (b) to deliver full functionality.
In short, I would disagree that a pure event based approach
is the way to go. In general it needs to be supported by good
system facilities that let user mode tools "introspect" about
the system device configuration (without needing to grot
around in /dev/kmem to discover such stuff) or examine
(and ideally, modify) bindings of drivers to devices as part
of device configuration.
Event delivery is only one of the mechanisms Linux needs,
and it shouldn't be overloaded.
- Dave
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