Date:   Fri, 02 Nov 2018 16:36:19 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Felipe Balbi <balbi@...nel.org>
Cc:     Paul Elder <paul.elder@...asonboard.com>,
        Alan Stern <stern@...land.harvard.edu>, Bin Liu <b-liu@...com>,
        kieran.bingham@...asonboard.com, gregkh@...uxfoundation.org,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        rogerq@...com
Subject: Re: [PATCH 4/6] usb: gadget: add functions to signal udc driver to delay status stage

Hi Felipe,

On Friday, 2 November 2018 15:17:20 EET Felipe Balbi wrote:
> Laurent Pinchart writes:
> >>>>> +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 :/
> 
> add a flag to gadget structure. Something like
> "supports_explicit_status_stage" and add a new return value
> USB_EXPLICIT_STATUS_STAGE.
> 
> Then, take uvc for example, implement the new setup:
> 
> if (supports_explicit_status_stage)
> 	return USB_EXPLICIT_STATUS_STAGE;
> 
> then on dwc3 you would:
> 
> switch (ret) {
> case USB_EXPLICIT_STATUS_STAGE:
> case USB_GADGET_DELAYED_STATUS:
> 	wait_for_status_request_queue();
>         break;
> default:
> 	start_status_stage();
> }
> 
> If this works with dwc3 + uvc, then we have a good recipe on how to
> implement for the other drivers.

Given that we need to delay the status stage and not the data stage, we can't 
explicitly request the status stage through a usb request queue. Would a new 
explicit function call work for you for that purpose ?

> > Alan, Felipe, how do we move forward with this ? There are several issues
> > with the existing control request handling mechanism in the USB gadget
> > stack, and while Paul could work on improving the mechanism, we need to
> > provide clear guidance regarding the direction we want to take.
> > 
> > For reference, the issues I know about for the USB_GADGET_DELAYED_STATUS
> > mechanism are
> > 
> > - The mechanism is inherently racy. It relies on signaling the delay at
> > the very end of the processing in the setup handler, which by definition
> > occurs after the work to process the control request is queued (in the
> > generic sense, regardless of whether this involves a kernel workqueue or
> > passing the work to userspace). There is thus a race window after queuing
> > the work and before signaling the delay during which the work handler
> > could signal completion.
> 
> We won't fix this until all functions and UDCs are converted over, but
> it's doable.

It could be fixed by signaling the delay through an explicit function call 
before queueing the work instead of through a return value though, but I agree 
that long term requesting the status stage explicitly would likely be cleaner.

> > - The mechanism is poorly documented. As Paul mentioned, comments in the
> > code state that USB_GADGET_DELAYED_STATUS delay the "data/status stages".
> > This is very unclear, and the only three UDCs that implement the
> > mechanism seem to do so in different ways:
> > 
> >   - The mtu driver states in a comment that it will "handle the delay
> >   STATUS phase till receive ep_queue on ep0".
> > 
> >   - The bdc driver states in a comment that "The ep0 state will remain
> >   WAIT_FOR_DATA_START till we received ep_queue on ep0".
> > 
> >   - The dwc3 driver seems to handle USB_GADGET_DELAYED_STATUS for the
> >   SET_CONFIG request only.
> 
> that's the only one that has needed it so far. I'm all for making status
> stage ALWAYS explicit, that will, in the long run, simplify UDC
> drivers and make the API easier to understand.
> 
> > - The mechanism relies on queueing a request to the UDC to signal that it
> > should continue with the status stage. That request can be queued either
> > by the USB gadget function driver directly, or by the composite layer in
> > usb_composite_setup_continue() (the latter is restricted to requests that
> > carry no data as it sets the request length to 0). This is problematic if
> > we want to delay the status phase after completing the data phase, in
> > order to validate the setup phase data and the data phase data (for a
> > control OUT request) together.
> 
> It shouldn't cause problems, actually. Most of the issues come from the
> fact that sometimes gadget driver just returns a success and expects UDC
> to initiate status stage and sometimes gadget driver wants to handle
> status stage explicitly.

Requesting the status stage explicitly requires an API to do so (quite 
obviously). Your proposal is to use the usb request queue as a signal to 
continue to the next stage. My point is that this can't work for control OUT 
requests, where we may want to delay the status stage after the data is 
received by the UDC, and thus after the request is queued. We need a different 
API for that.

For control IN, we may want to delay the data stage if we can't respond 
directly, or proceed with the data stage immediately. In both cases this can 
be signaled by a request being queued. There is no need to delay the status 
stage as it's initiated by the host.

For control OUT, we may want to delay the data stage if we need to validate 
the setup stage data asynchronously. Proceeding to the data stage can be 
signaled by queueing a request. We may also want to delay the status stage if 
we need to validate the data stage data asynchronously. This can't be signaled 
by queueing a request.

I wonder if there's really a use case for delaying the data stage of control 
OUT requests, as it seems to me that we can perform the asynchronous 
validation of the setup and data stages together, in which case we would 
always proceed to the data stage, and only potentially delay the status stage. 
However, if we switch to an explicit API where the transition from the setup 
to the data stage is triggered by queueing a request, and given that such a 
transition may need to be delayed for the control IN case, delaying the data 
stage for control OUT would essentially come for free.

In any case we need an API to delay the status stage of a control OUT request. 
There are two options here. We can consider that the status stage shouldn't be 
delayed by default and add a new function to be called from the data stage 
completion handler to request a status stage delay (using the return value of 
the completion handler isn't a good idea as it would be racy as explained 
above). A second function would be needed to request the status stage (which, 
as explained above too, can't be done by queueing a request). A second option 
is to consider that the status stage is delayed by default until explcitly 
required. In both cases the same new function is needed to request the status 
stage.

Note that delaying the data stage and delaying the status stage are two 
different problems, and don't necessarily need to be solved together. However, 
if I understand things correctly, we currently delay the data stage of a few 
control OUT requests (they all have a 0 bytes data stage) as a mean to 
completion of the request. I believe that this use case could be implemented 
by delaying the status stage instead, so the two are still related in a way.

If we end up moving to explicit state handling, with the data stage being 
entered by queueing a request, and the status stage being entered by calling a 
new function, control OUT requests with 0 bytes of data that can be handled 
synchronously in the setup handler would require function drivers to both 
queue a zero-length request and call the status function. This would make the 
function code more complex, and I wonder whether a shortcut would be a good 
idea, perhaps in the form of a flag in the request that tells the UDC to 
automatically proceed to the status stage immediately after the data stage. Or 
we could make that behaviour the default when the request doesn't have a 
completion handler (as moving explicitly to the status stage should be done at 
the earliest from the data stage completion handler).

> > For those reasons I think a new mechanism is needed. It should either
> > signal the status phase delay through an explicit function call instead
> > of a return value (to solve the race mentioned above), or by requiring
> > all requests to be explicitly completed (but that will require changing
> > all USB function drivers). Furthermore, the mechanism need to support
> > delaying the status phase after queuing the request for the data phase,
> > so we need an explicit way to signal that the UDC should proceed with the
> > status phase, other than queueing the request.
> > 
> > Thoughts ? Preferences ?
> 
> how about making status stage always explicit?

If we implement a proof of concept, could you help us converting drivers over 
to the new API ? I'll assume we'll have to address all UDCs first, and then 
the function drivers.

-- 
Regards,

Laurent Pinchart

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