Re: Does Linux support more than 2TB on a USB attached device?

On Wed, Oct 03, 2007 at 04:59:48PM -0400, Alan Stern wrote:
> On Wed, 3 Oct 2007, Alan Cox wrote:
> 
> > > scsi_host_alloc creates a host structure with max_cmd_len set to 12.
> > > 
> > > To allow USB Storage to accept 16-byte CDBs, the following will need to
> > > be added:
> > > 
> > >          host->max_cmd_len = 16;  /* or some symbol */
> > > 
> > > That should allow for USB storage devices larger than 2TiB.
> > 
> > The question is whether USB storage as a specification permits this, and
> > what other hardware does if this happens.
> 
> The specification does allow it.  I don't see any reason why that can't 
> be added.  Richard, would you like to submit a patch?

Strictly speaking, the specification is generally silent on this point.  If
they are using the "transparent SCSI" protocol, they could legally fill the
command bytes with beer and it would be legal.

That said, I see no reason setting max_cmd_len to 16 would cause a problem.
The protocols which do implement a command-size limit enforce that limit
later in the protocol handler.

Matt

-- 
Matthew Dharm                              Home: [email protected] 
Maintainer, Linux USB Mass Storage Driver

Your lips are twitching.  You're playing Quake aren't you.
					-- Stef to Greg
User Friendly, 8/11/1998

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