linux-kernel - Re: Question on the removable flag of USB devices

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Message-ID: <20090526173432.GU28077@one-eyed-alien.net>
Date:	Tue, 26 May 2009 10:34:32 -0700
From:	Matthew Dharm <mdharm-kernel@...-eyed-alien.net>
To:	Stefan Richter <stefanr@...6.in-berlin.de>
Cc:	Rogério Brito <rbrito@...rs.sf.net>,
	Alan Stern <stern@...land.harvard.edu>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: Question on the removable flag of USB devices

On Tue, May 26, 2009 at 06:36:09PM +0200, Stefan Richter wrote:
> Matthew Dharm wrote:
> > On Mon, May 25, 2009 at 01:45:54PM -0300, Rogério Brito wrote:
> >> The question is: how are devices determined to be removable or not?
> >> Sorry if this question is too naive.
> > 
> > It's a bit that the device reports.  The usb-storage driver passes this bit
> > to the SCSI layer.
> 
> Wasn't this bit actually about removable _media_ rather than
> hotpluggable transports?  (I.e. hardware where you can pull the medium
> out of the drive mechanism.)  So in this sense, an USB attached HDD
> should actually not present itself as "removable" unless it's a SyQuest
> or Zip or something like that.

In fact, yes, that's the meaning of that bit.  The device indicates if the
storage media may be removed from the device.

That said, a great many devices simply lie about this.  Probably developers
doing a cut-and-paste from templates which have the bit set one way or the
other.  The "popular" OSes don't seem to really care much about this bit...

Matt

-- 
Matthew Dharm                              Home: mdharm-usb@...-eyed-alien.net 
Maintainer, Linux USB Mass Storage Driver

C:  Like the Furby?
DP: He gives me the creeps.  Think the SPCA will take him?
					-- Cobb and Dust Puppy
User Friendly, 1/2/1999

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