On Wed, Apr 22, 2009 at 05:21:51PM -0400, Alan Stern wrote: > [CC: list trimmed, since this may no longer be of general interest] > > On Wed, 22 Apr 2009, David VomLehn wrote: > > > I'm going to see if I can come up with some sort of concrete proposal for > > waiting for hotpluggable boot devices. I'd appreciate it if you could think > > about how to implement the USB-specific part of this. Once we have something > > that makes sense, and works, we can go back and pick up the details for > > the console, IP autoconfig, and anything else that someone might need done. > > Whatever you come up with, it should operate on a per-subsystem basis, > perhaps together with an additional "all subsystems are done with their > boot probing" mechanism. Absolutely. > (USB disk drives will still be a problem... I'm going to punt on them > for now.) I'm quite willing to punt on them for now, too, since we do have rootdelay and rootwait. If we do a good job of things, though, we might be able to improve on those mechanisms. > Would it be sufficient to have a "number of pending probes" counter > together with a waitqueue? Your console routine could first wait for > all the async stuff to end, so that all the USB host controllers had > been registered, and then wait (on the new waitqueue) for the counter > to go to 0. The hub driver would maintain the counter and signal the > waitqueue whenever the counter was decremented to 0. I think that something like this is reasonable. It requires that all concerned bus subsystems increment the number of pending probes by the time the async stuff ends, but I think that's reasonable. And I hope that enough other people think about this enough to let us know whether it is. > Is it safe to say that we don't have to worry about udev or other > userspace hotplug mechanisms? After all, we are talking about things > that happen before the first userspace process is set up. And if we > had to wait for drivers to be modprobed into the kernel, we'd again be > facing unpredictable delays. One data point--the kernel doesn't even export the console interfaces to modules, so you can't have a hotplugged console. The concern is really only those devices you need to finish your boot with enough functionality to do everything else--that functionality will include udev, which can handle anything else. > Alan Stern David VomLehn -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
