linux-kernel - Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting root

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Message-ID: <485BDCE4.2010303@s5r6.in-berlin.de>
Date:	Fri, 20 Jun 2008 18:37:56 +0200
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	Vegard Nossum <vegard.nossum@...il.com>,
	Matthew Wilcox <matthew@....cx>,
	Pekka J Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org, dsd@...too.org,
	mdharm-usb@...-eyed-alien.net, linux-usb@...r.kernel.org,
	vegardno@....uio.no, James.Bottomley@...senpartnership.com,
	linux-scsi@...r.kernel.org, Greg KH <greg@...ah.com>
Subject: Re: [RFC/PATCH] usb-storage: wait for device scanning before mounting
 root

Alan Stern wrote:
> New USB devices can appear at any time.  None are present when the 
> system is started; they are all detected dynamically by the khubd 
> thread.
> 
> While I'm not familiar with the details of any other hotpluggable 
> buses (like Firewire), I imagine much the same is true for them.

It's very similar with FireWire.  FireWire devices announce SBP-2 target
capability (if they have it) after they became ready.  The ieee1394 or
firewire core then scans or re-scans them in a process context.  Then
upper layer drivers like the SBP-2 driver are matched, bound, probed.

If SBP-2 targets went through a power state change, they typically first
power everything up (spin up media etc.) before they announce their
protocol capabilities, and this may of course take a while.  Before that
they just are inaccessible, and it is unknown whether they could become
candidates for access by upper layer drivers.
-- 
Stefan Richter
-=====-==--- -==- =-=--
http://arcgraph.de/sr/
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