Re: Is there any docs or design guides for adding hotplugablity to

[David Brownell <david-b@pacbell.net>]

Mark Atwood wrote:
> Other than just reading the source, of course.

Start there, and then ask questions.  The docs that exist
are on the http://linux-hotplug.sf.net website, and in the
source code for the various subsystems that do hotplugging.

Inside the kernel, one basically invokes the program identified
by /proc/sys/kernel/hotplug, using a simple convention (only
argv[1] is standard) with call_usermodehelper() to report those
events.  Normally one wants the events to be reported by some
subsystem/framework code, rather than individual drivers, since
it's a lose to expect every driver be changed.


> I'm working on a Linux appliance, and I feel that using hotplug events
> to signal things like moving in range of an 802.11 access point,
> getting carrier on an ethernet port, and someone plugging a cable into
> the serial port, would make a lot of stuff much much easier.
> 
> I suspect that a lot of this would be useful out in the "regular
> linux" world as well. Especially the ethernet carrier detect. (Windows
> can do it, and will invoke DHCP when you plug in an ethernet cable, it
> would be nice if Linux can do the same.)

I agree that carrier detect would be good to report through hotplug,
since that's actually the relevant event.  Since the network drivers
don't actually agree on an initialization model (some register
before going "up", others don't -- see the special casing in
the /etc/hotplug/net.agent script for a start), some of the basic
issues there relate to the networking stack.

Last I looked, the network stack didn't actually treat carrier
detect as an interesting event, which makes such changes be unduly
complex.  If you're interested in updating the 2.5 stack to do
such things, I'd suggest discussing this on the netdev list and
shipping code before the planned feature freeze on Hallowe'en... :)

- Dave







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