usb: gadget: f_fs: Only return delayed status when len is 0

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Jul 19, 2018 at 08:48:32AM +0300, Felipe Balbi wrote:
> 
> Hi,
> 
> Jerry Zhang <zhangjerry@google.com> writes:
> > Commit 1b9ba000 ("Allow function drivers to pause control
> > transfers") states that USB_GADGET_DELAYED_STATUS is only
> > supported if data phase is 0 bytes.
> >
> > It seems that when the length is not 0 bytes, there is no
> > need to explicitly delay the data stage since the transfer
> > is not completed until the user responds. However, when the
> > length is 0, there is no data stage and the transfer is
> > finished once setup() returns, hence there is a need to
> > explicitly delay completion.
> >
> > This manifests as the following bugs:
> >
> > Prior to 946ef68ad4e4 ('Let setup() return
> > USB_GADGET_DELAYED_STATUS'), when setup is 0 bytes, ffs
> > would require user to queue a 0 byte request in order to
> > clear setup state. However, that 0 byte request was actually
> > not needed and would hang and cause errors in other setup
> > requests.
> >
> > After the above commit, 0 byte setups work since the gadget
> > now accepts empty queues to ep0 to clear the delay, but all
> > other setups hang.
> >
> > Fixes: 946ef68ad4e4 ("Let setup() return USB_GADGET_DELAYED_STATUS")
> > Signed-off-by: Jerry Zhang <zhangjerry@google.com>
> 
> Greg, can you pick this one manually?
> 
> Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>

Yes, I'll pick it up, thanks.

greg k-h
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