From: Paul Elder <paul.elder@ideasonboard.com>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
Bin Liu <b-liu@ti.com>,
kieran.bingham@ideasonboard.com, gregkh@linuxfoundation.org,
linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
balbi@kernel.org, rogerq@ti.com
Subject: Re: [PATCH 4/6] usb: gadget: add functions to signal udc driver to delay status stage
Date: Thu, 1 Nov 2018 19:40:59 -0400 [thread overview]
Message-ID: <20181101234059.GA28068@garnet.amanokami.net> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1810180956590.1521-100000@iolanthe.rowland.org>
Hi Alan,
On Thu, Oct 18, 2018 at 10:07:36AM -0400, Alan Stern wrote:
> On Thu, 18 Oct 2018, Laurent Pinchart wrote:
>
> > Hi Bin,
> >
> > On Thursday, 11 October 2018 19:10:21 EEST Bin Liu wrote:
> > > On Tue, Oct 09, 2018 at 10:49:01PM -0400, Paul Elder wrote:
> > > > A usb gadget function driver may or may not want to delay the status
> > > > stage of a control OUT request. An instance it might want to is to
> > > > asynchronously validate the data of a class-specific request.
> > > >
> > > > Add a function usb_ep_delay_status to allow function drivers to choose
> > > > to delay the status stage in the request completion handler. The UDC
> > > > should then check the usb_ep->delayed_status flag and act accordingly to
> > > > delay the status stage.
> > > >
> > > > Also add a function usb_ep_send_response as a wrapper for
> > > > usb_ep->ops->send_response, whose prototype is added as well. This
> > > > function should be called by function drivers to tell the UDC what to
> > > > reply in the status stage that it has requested to be delayed.
> > > >
> > > > Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
> > > > Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> > > > ---
> > > >
> > > > drivers/usb/gadget/udc/core.c | 35 +++++++++++++++++++++++++++++++++++
> > > > include/linux/usb/gadget.h | 11 +++++++++++
> > > > 2 files changed, 46 insertions(+)
> > > >
> > > > diff --git a/drivers/usb/gadget/udc/core.c b/drivers/usb/gadget/udc/core.c
> > > > index af88b48c1cea..1ec5ce6b43cd 100644
> > > > --- a/drivers/usb/gadget/udc/core.c
> > > > +++ b/drivers/usb/gadget/udc/core.c
> > > > @@ -443,6 +443,41 @@ void usb_ep_fifo_flush(struct usb_ep *ep)
> > > > }
> > > > EXPORT_SYMBOL_GPL(usb_ep_fifo_flush);
> > > >
> > > > +/**
> > > > + * usb_ep_ep_delay_status - set delay_status flag
> > > > + * @ep: the endpoint whose delay_status flag is being set
> > > > + *
> > > > + * This function instructs the UDC to delay the status stage of a control
> > > > + * request. It can only be called from the request completion handler of
> > > > a
> > > > + * control request.
> > > > + */
> > > > +void usb_ep_delay_status(struct usb_ep *ep)
> > > > +{
> > > > + ep->delayed_status = true;
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(usb_ep_delay_status);
> > >
> > > Is usb_ep_set_delay_status() better? I thought it implies get/return
> > > action if a verb is missing in the function name.
> >
> > For what it's worth, I understand the function name as "delay the status
> > stage", with "delay" being a verb. Maybe the short description could be
> > updated accordingly.
>
> Is there a reason for adding a new function for this? This is exactly
> what the USB_GADGET_DELAYED_STATUS return value from the setup callback
> is meant for (and it is already used by some gadget drivers).
In theory, we might be able to use USB_GADGET_DELAYED_STATUS for this.
However, there are a few ambiguities that prevent us from doing so.
First of all, we want to delay only the status stage for control OUT
requests; according to composite.h, USB_GADGET_DELAYED_STATUS is for
delaying the "data/status stages". Does this mean that it delays the
status stage only or does it delay both stages? If the slash means
"and", then we cannot use USB_GADGET_DELAYED_STATUS.
Furthermore, we have found that USB_GADGET_DELAYED_STATUS is racey,
which has already been observed in the UVC gadget driver previously [0].
The raceiness stems from the fact that things can happen in between
returning USB_GADGET_DELAYED_STATUS and the composite layer reacting to
it - especially if usb_composite_setup_continue is called within that
window it causes a WARN. In any case, the fact that the mechanism itself
is racey suggests that it needs improvement, and using it wouldn't be a
good solution in this case.
> Is it a question of when the gadget driver learns that it will need to
> delay the status stage? If that's the case,
Not really.
> why not always return
> USB_GADGET_DELAYED_STATUS from the setup callback? Then instead of
> calling usb_ep_delay_status() when a delay is needed, you could queue
> the status request when a delay isn't needed.
Theoretically this might work, but see the problems mentioned above.
> As a more general solution, Felipe has said that a UDC driver should
> _never_ carry out the status stage transaction until the gadget driver
> has told it to do so. Then there would be no need for any sort of
> delay indicator.
Yeah, but,
> (But implementing this would require significant
> changes to a bunch of different drivers...)
exactly :/
[0] https://www.spinics.net/lists/linux-usb/msg169208.html
Paul
next prev parent reply other threads:[~2018-11-01 23:41 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-10 2:48 [PATCH 0/6] usb: gadget: add mechanism to asynchronously validate data stage of ctrl out request Paul Elder
2018-10-10 2:48 ` [PATCH 1/6] usb: uvc: include videodev2.h in g_uvc.h Paul Elder
2018-10-10 13:42 ` Laurent Pinchart
2018-10-10 2:48 ` [PATCH 2/6] usb: gadget: uvc: enqueue usb request in setup handler for control OUT Paul Elder
2018-10-10 2:49 ` [PATCH 3/6] usb: gadget: uvc: package setup and data for control OUT requests Paul Elder
2018-10-10 2:49 ` [PATCH 4/6] usb: gadget: add functions to signal udc driver to delay status stage Paul Elder
2018-10-11 16:10 ` Bin Liu
2018-10-17 23:45 ` Laurent Pinchart
2018-10-18 12:46 ` Bin Liu
2018-10-18 14:07 ` Alan Stern
2018-11-01 23:40 ` Paul Elder [this message]
2018-11-02 12:44 ` Laurent Pinchart
[not found] ` <87h8gzy5y7.fsf@linux.intel.com>
2018-11-02 14:36 ` Laurent Pinchart
2018-11-02 16:18 ` Alan Stern
2018-11-02 17:10 ` Laurent Pinchart
2018-11-02 19:46 ` Alan Stern
2018-11-06 11:24 ` Felipe Balbi
2018-11-06 15:01 ` Alan Stern
2018-11-07 6:53 ` Felipe Balbi
2018-11-06 11:17 ` Felipe Balbi
2018-11-06 14:51 ` Alan Stern
2018-11-07 7:00 ` Felipe Balbi
2018-11-07 16:23 ` Alan Stern
2018-12-14 3:47 ` Paul Elder
2018-12-14 15:35 ` Alan Stern
2018-10-10 2:49 ` [PATCH 5/6] usb: musb: gadget: implement send_response Paul Elder
2018-10-11 16:07 ` Bin Liu
2018-10-31 23:26 ` Paul Elder
2018-10-10 2:49 ` [PATCH 6/6] usb: gadget: uvc: allow ioctl to send response in status stage Paul Elder
2018-10-10 12:57 ` [PATCH 0/6] usb: gadget: add mechanism to asynchronously validate data stage of ctrl out request Laurent Pinchart
2018-10-11 19:31 ` Bin Liu
2018-10-17 23:42 ` Laurent Pinchart
2018-10-18 12:40 ` Bin Liu
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