From: Oliver Neukum <oliver@neukum.org>
To: linux-hotplug@vger.kernel.org
Subject: Re: Naming persistence of USB devices
Date: Wed, 08 Dec 2004 08:40:51 +0000 [thread overview]
Message-ID: <200412080940.51703.oliver@neukum.org> (raw)
In-Reply-To: <loom.20041208T040332-835@post.gmane.org>
> I assume there is no danger of the devices exchanging names through reboots as
> long as they stay plugged into the same ports on the same hub on the same root
Probing order on boot is deterministic. But nevertheless, device numbers
may still change if a device fails to enumerate.
> hub. What if I change any of this? What if I move the tree (I don't know the
> official USB term) of devices (hubs and peripherals) from one root hub to
Should be safe.
> another? Upon reenumeration will the /dev names change? What if I have a
> device plugged into a hub, power down and for whatever reason when I reattach
> the device I plug it into a different port, what if that port is on a different
> hub or root hub?
>
> If anyone can point me to a document explaining the fundamentals of USB and 1394
> enumeration I would appreciate it. My single biggest question is how does a
The kernel hands out device numbers strictly in order of discovery.
> device get it's /dev name, is it based off some world unique device number (like
> Ethernet MAC addresses) or maybe the Linux scans the first USB adapter/bridge it
> comes across on the PCI bus then follows some convention for the devices
> connected to hubs downstream.
The latter. USB devices in most cases lack any unique identifier. You have
a choice between naming based on topology or naming in discovery order.
You can use udev to implement either policy.
HTH
Oliver
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prev parent reply other threads:[~2004-12-08 8:40 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-12-08 3:21 Naming persistence of USB devices Adam Hunt
2004-12-08 8:40 ` Oliver Neukum [this message]
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