[systemd-devel] Need help with a systemd/mdadm interaction.

[Greg KH]

On Tue, Nov 12, 2013 at 06:16:24PM +0900, Greg KH wrote:
> On Tue, Nov 12, 2013 at 07:54:42PM +1100, NeilBrown wrote:
> > On Tue, 12 Nov 2013 00:10:28 -0800 Greg KH <gregkh at linuxfoundation.org> wrote:
> > 
> > > On Tue, Nov 12, 2013 at 11:31:45AM +1100, NeilBrown wrote:
> > > > Alternately, is there some "all devices have been probed, nothing new will
> > > > appear unless it is hot-plugged" event.  That would be equally useful (and
> > > > probably mirrors what hardware-RAID cards do).
> > > 
> > > No, there's no way to ever know this in a hotplug world, sorry.
> > > Especially with USB devices, they show up when they show up, there's no
> > > "oh look, the bus is all scanned now and all devices currently plugged
> > > in are found" type knowledge at all.
> > > 
> > > Then there are hotplug PCI systems where people slam in PCI cards
> > > whenever they feel like it (remember, thunderbolt is PCI express...)
> > > 
> > > Sorry,
> > > 
> > > greg k-h
> > 
> > Surely something must be possible.
> 
> For USB, nope, there isn't, sorry.
> 
> > Clearly a physical hot-plug event will cause more devices to appear, but
> > there must come a point at which no more (non-virtual) devices will appear
> > unless a physical event happens?
> 
> Not for USB, sorry.
> 
> The USB bus just announces devices when it finds them, there is no "all
> is quiet" type signal or detection.
> 
> Same for PCI hotplug, devices can show up at any point in time, you
> never know when, and you don't know when all devices are "found".

Oh, and even if there was such a "flag", it wouldn't really matter,
because a device could be plugged in right after you think everything is
'quiet'.  Which is probably the reason why this isn't possible, even if
you had it, you couldn't rely on it.

greg k-h