Re: [ANNOUNCE] hotplug-ng 002 release

Per Svennerbrandt
2005-05-18T23:01:36Z

* Per Liden (per@fukt.bth.se) wrote:
> On Mon, 9 May 2005, Per Svennerbrandt wrote:
>
> > * Per Liden (per@fukt.bth.se) wrote:
> > > On Fri, 6 May 2005, Greg KH wrote:
> > >
> > > [...]
> > > > Now, with the 2.6.12-rc3 kernel, and a patch for module-init-tools, the
> > > > USB hotplug program can be written with a simple one line shell script:
> > > > modprobe $MODALIAS
> > >
> > > Nice, but why not just convert all this to a call to
> > > request_module($MODALIAS)? Seems to me like the natural thing to do.
> >
> > I actually have a pretty hackish proof-of-consept patch that does
> > basicly that, and have been running it on my systems for the past five
> > months or so, if anybody's interested.
>
> Ah! Please post the patches.

Ok, so I finally decided that it's probably silly of me to hold of a
working patch just because I wanted my first submission to the kernel
to somehow be a marvellous example of ellegancy and not just some
quick hack that I threw toghether as a personal proof-of-consept.

> > Along with it I also have a patch witch exports the module aliases for
> > PCI and USB devices through sysfs. With it the "coldplugging" of a
> > system (module wise) can be reduced to pretty much:
> >
> > #!/bin/sh
> >
> > for DEV in /sys/bus/{pci,usb}/devices/*; do
> > modprobe `cat $DEV/modalias`
> > done
>
> Nice! This is really what coldplugging _should_ look like. Hmm, maybe
> even coldplug the system by request_module()'ing those as well at some
> stage?

Well, the code already generates all the requests necessary. :) The only
problem now of course beeing that it generates those at a "stage" ;) when
userspace usually isn't ready to fullfill those.

I'm currently thinking about maybe making all the requests sleep on a
waitqueue untill the root filesystem becomes availible, shoudn't be too
hard if I remember the code correctly... Could potentialy end up using
a lot of resourses though.

And, yeah, I know this could all be done quite easily with scripts in a
initrd or similar, but that is in fact *exactly* what I'm trying to
avoid here, to reduce complexity and keep thinks as simple as they
possibly can be. I'm not proposing this as a generic solution for
everyone, rather the opposite in fact. I do however beleve there to be
enough demand out there for this particular kind of "special case" to
warrant (optional) support for it in the mainline kernel.

> > (And I actually run exactly that on my laptop, and it works surpricingly
> > well. (Largly due to the fact that the usb-controller is always attached
> > below the pci-bus of course, but it really wouldn't take that much work
> > to make it do the right thing even without relying on any specific
> > ordering/topology))
> >
> > With the above in place my system does all the module-loading that I
> > care about automaticly, and most importantly does so without relying
> > on an /etc/hotplug/ dir with everything and it's grandma in it (or at
> > least thousands of lines of shellscripting).
>
> This is exactly what I'm looking for as well.

For example, I see absolutely no need for a fullblown hotplug system
on my minimalst-userspace router/firewall. I just want the kernel to
be able to load the right modules, on demand, as it boots (and perhaps
when I plug in a usb flashdrive, every third, or so, year).

> > But since the request_modalias() thing seemed as such an obvious thing
> > to do I have been reluctant to submit it fearing that I must have missed
> > some fundamental flaw in it or you guys would have implemented it that
> > way a long time ago? (at least since Rusty rewrote the module
> > loader). Was I wrong*?
> >
> > Greg, Rusty, what do you think?
>
> I'd like to get a better understanding of that as well. Why invent a
> second on demand module loader when we have kmod? The current approach
> feels like a step back to something very similar to the old kerneld.
>
> /Per L

Sorry for the delay, patches to follow.

/ Per Svennerbrandt