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Message-ID: <Pine.LNX.4.44L0.1103231110400.2211-100000@iolanthe.rowland.org>
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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