linux-kernel - Re: USB stick as root device does not work

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Message-ID: <Pine.LNX.4.44L0.1105231332090.2020-100000@iolanthe.rowland.org>
Date:	Mon, 23 May 2011 13:33:55 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Randy Dunlap <rdunlap@...otime.net>
cc:	Jens Axboe <axboe@...nel.dk>, TejunHeo <tj@...nel.org>,
	Micha Nelissen <micha@...i.hopto.org>,
	lud <linux-usb@...r.kernel.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: USB stick as root device does not work

On Mon, 23 May 2011, Randy Dunlap wrote:

> [adding cc: of linux-usb mailing list]

Better to ask people familiar with the block layer (CC'ed).

Alan Stern

> On Sun, 22 May 2011 16:13:17 +0200 Micha Nelissen wrote:
> 
> > Hi,
> > 
> > Using USB stick as root device does not work for me. I specify
> > 'root=/dev/sdb1 rootwait' on the kernel commandline, but the kernel does
> > not wait for the USB stick to be scanned and its partitions found. I
> > have a harddisk which is /dev/sda, and this USB stick would be /dev/sdb.
> > 
> > I think blk_lookup_devt returns the block device number even if it does
> > not exist yet, and init/do_mounts.c:473 therefore skips waiting:
> > 
> >         if ((ROOT_DEV == 0) && root_wait) {
> > 
> > due to ROOT_DEV having become non-zero already.
> > 
> > If I use the commandline: 'root=/dev/sdb1 rootdelay=5' then it does
> > mount and boot properly. However, using rootwait seems safer and faster
> > to me, that it's meant for this use case.
> > 
> > Thanks for any pointers,
> > 
> > Micha
> > --
> 
> ---
> ~Randy
> *** Remember to use Documentation/SubmitChecklist when testing your code ***

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