linux-kernel - Re: [PATCH v2 2/3] usb: gadget: file_storage: Make CD-ROM emulation work with Mac OS-X

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Date:	Wed, 23 Mar 2011 11:17:32 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Roger Quadros <roger.quadros@...ia.com>,
	Michal Nazarewicz <mina86@...a86.com>
cc:	gregkh@...e.de, <sshtylyov@...sta.com>,
	USB list <linux-usb@...r.kernel.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] usb: gadget: file_storage: Make CD-ROM emulation
 work with Mac OS-X

On Wed, 23 Mar 2011, Roger Quadros wrote:

> > On the other hand, I don't think any implementations would get upset if
> > we simply ended the transfer with a short packet instead of adhering
> > strictly to the spec.
> >
> > The patch below should do what you want.  I haven't tested it.
> 
> I tried your patch with the CD-ROM implementation and it works perfectly. I do 
> not see the unnecessary zero padded transfers any more.
> 
> Do you think we should have this patch in? with the risk of not strictly 
> adhering to spec for cases where controller cannot stall?

There already is another place where not stalling forces the driver to 
violate the spec.  I don't think this makes things much worse... but it 
is a significant change in behavior.

This affects Michal's driver too; we should ask his opinion.  Michal, 
in case you didn't see it, the proposed patch is here:

	http://marc.info/?l=linux-usb&m=130080683528607&w=2

> Maybe the term "controller cannot stall" itself does not adhere to bulk-only 
> transport spec :).

True enough.  However I believe USB flash drives behave this way: They 
don't stall and they don't pad their data.

> > @@ -1710,24 +1683,19 @@ static int finish_reply(struct fsg_commo
> >   			common->next_buffhd_to_fill = bh->next;
> >
> >   		/*
> > -		 * For Bulk-only, if we're allowed to stall then send the
> > -		 * short packet and halt the bulk-in endpoint.  If we can't
> > -		 * stall, pad out the remaining data with 0's.
> > +		 * For Bulk-only, mark the end of the data with a short
> > +		 * packet.  If we are allowed to stall, halt the bulk-in
> > +		 * endpoint.  (Note: This violates the Bulk-Only Transport
> > +		 * specification, which requires us to pad the data if we
> 
> violates the spec only if we are not allowed to stall (i.e. stall=n) right?

Right.

> > +		 * don't halt the endpoint.  Presumably nobody will mind.)
> >   		 */

Alan Stern

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