linux-kernel - Re: USB storage no-boot regression (bisected)

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Date:	Tue, 14 Apr 2009 18:38:07 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Jeff Garzik <jeff@...zik.org>
Cc:	Linux USB kernel mailing list <linux-usb@...r.kernel.org>,
	Greg KH <greg@...ah.com>,
	Alan Stern <stern@...land.harvard.edu>,
	LKML <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: USB storage no-boot regression (bisected)

On Tue, 14 Apr 2009 17:06:14 -0400
Jeff Garzik <jeff@...zik.org> wrote:

> 
> Once of the x86-64 machines I use for testing runs off of two 2GB USB 
> flash drives, one for Fedora 10 userland, and one for kernel
> repository 
> + builds.
> 
> It boots correctly in 2.6.27, but fails with the same symptoms in 
> 2.6.28, 2.6.29 and 2.6.30-rc1:
> 
> 	1) The kernel boots
> 	2) After time passes, kernel begins executing initramfs
> 	   userland
> 	3) the kernel prints out probe messages for the USB keyboard,
> 	   SCSI probe messages for the two USB flash drives
> 
> Or IOW, the keyboard and two SCSI drives appear after initramfs
> begins booting.  And this is for drivers built into the kernel
> (though same behavior with modules).
> 
> This no-boot regression is 100% reproducible, and neatly bisects down
> to
> 
> > commit 8520f38099ccfdac2147a0852f84ee7a8ee5e197
> > Author: Alan Stern <stern@...land.harvard.edu>
> > Date:   Mon Sep 22 14:44:26 2008 -0400
> > 
> >     USB: change hub initialization sleeps to delayed_work
> >     
> >     This patch (as1137) changes the hub_activate() routine,
> > replacing the power-power-up and debounce delays with delayed_work
> > calls.  The idea is that on systems where the USB stack is compiled
> > into the kernel rather than built as modules, these delays will no
> > longer block the boot thread.  At least 100 ms is saved for each
> > root hub, which can add up to a significant savings in total boot
> > time. 
> >     Arjan van de Ven was very pleased to see that this shaved 700
> > ms off his computer's boot time.  Since his total boot time is on
> > the order of two seconds, the improvement is considerable.
> >     
> >     Signed-off-by: Alan Stern <stern@...land.harvard.edu>
> >     Tested-by: Arjan van de Ven <arjan@...radead.org>
> >     Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>
> 
> 
> My preliminary guess is that this made things --too-- asynchronous,
> and for some reason userland begins executing before the SCSI core 
> initializes the USB storage as Linux block devices.
> 
> In any case, I cannot boot because of the above commit :)

the fundamental problem with USB is that you don'tm know when you're
done probing ... devices show up when they feel like or more or less.

This change just made it go faster enough for you to be out of luck;
fundamentally your userland needs to wait if the device it wants is not
there. 

Or you pass a kernel boot parameter to wait.. there is one (root_delay
or something, haven't used it in a while).


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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