Date:   Wed, 5 May 2021 01:50:37 +0000
From:   Thinh Nguyen <Thinh.Nguyen@...opsys.com>
To:     Wesley Cheng <wcheng@...eaurora.org>,
        Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        "balbi@...nel.org" <balbi@...nel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
CC:     "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jackp@...eaurora.org" <jackp@...eaurora.org>
Subject: Re: [PATCH v2] usb: dwc3: gadget: Avoid canceling current request for
 queuing error

Wesley Cheng wrote:
> 
> 
> On 5/3/2021 10:22 PM, Thinh Nguyen wrote:
>> Wesley Cheng wrote:
>>>
>>>
>>> On 5/3/2021 8:12 PM, Thinh Nguyen wrote:
>>>> Hi Wesley,
>>>>
>>>> Wesley Cheng wrote:
>>>>>
>>>>>
>>>>> On 5/3/2021 7:20 PM, Thinh Nguyen wrote:
>>>>>> Hi,
>>>>>>
>>>>>> Wesley Cheng wrote:
>>>>>>> If an error is received when issuing a start or update transfer
>>>>>>> command, the error handler will stop all active requests (including
>>>>>>> the current USB request), and call dwc3_gadget_giveback() to notify
>>>>>>> function drivers of the requests which have been stopped.  Avoid
>>>>>>> having to cancel the current request which is trying to be queued, as
>>>>>>> the function driver will handle the EP queue error accordingly.
>>>>>>> Simply unmap the request as it was done before, and allow previously
>>>>>>> started transfers to be cleaned up.
>>>>>>>
>>>>>
>>>>> Hi Thinh,
>>>>>
>>>>>>
>>>>>> It looks like you're still letting dwc3 stopping and cancelling all the
>>>>>> active requests instead letting the function driver doing the dequeue.
>>>>>>
>>>>>
>>>>> Yeah, main issue isn't due to the function driver doing dequeue, but
>>>>> having cleanup (ie USB request free) if there is an error during
>>>>> usb_ep_queue().
>>>>>
>>>>> The function driver in question at the moment is the f_fs driver in AIO
>>>>> mode.  When async IO is enabled in the FFS driver, every time it queues
>>>>> a packet, it will allocate a io_data struct beforehand.  If the
>>>>> usb_ep_queue() fails it will free this io_data memory.  Problem is that,
>>>>> since the DWC3 gadget calls the completion with -ECONNRESET, the FFS
>>>>> driver will also schedule a work item (within io_data struct) to handle
>>>>> the completion.  So you end up with a flow like below
>>>>>
>>>>> allocate io_data (ffs)
>>>>>  --> usb_ep_queue()
>>>>>    --> __dwc3_gadget_kick_transfer()
>>>>>    --> dwc3_send_gadget_ep_cmd(EINVAL)
>>>>>    --> dwc3_gadget_ep_cleanup_cancelled_requests()
>>>>>    --> dwc3_gadget_giveback(ECONNRESET)
>>>>> ffs completion callback
>>>>> queue work item within io_data
>>>>>  --> usb_ep_queue returns EINVAL
>>>>> ffs frees io_data
>>>>> ...
>>>>>
>>>>> work scheduled
>>>>>  --> NULL pointer/memory fault as io_data is freed
>>>
>>> Hi Thinh,
>>>
>>>>
>>>> sounds like a race issue.
>>>>
>>>
>>> It'll always happen if usb_ep_queue() fails with an error. Sorry for not
>>> clarifying, but the "..." represents executing in a different context
>>> :). Anything above the "..." is in the same context.
>>>>>
>>>>>> BTW, what kinds of command and error do you see in your setup and for
>>>>>> what type endpoint? I'm thinking of letting the function driver to
>>>>>> dequeue the requests instead of letting dwc3 automatically
>>>>>> ending/cancelling the queued requests. However, it's a bit tricky to do
>>>>>> that if the error is -ETIMEDOUT since we're not sure if the controller
>>>>>> had already cached the TRBs.
>>>>>>
>>>>>
>>>>> Happens on bulk EPs so far, but I think it wouldn't matter as long as
>>>>> its over the FFS interface. (and using async IO transfers)
>>>>
>>>> Do you know which command and error code? It's strange if
>>>> UPDATE_TRANSFER command failed.
>>>>
>>>
>>> Sorry for missing that part of the question.  It is a no xfer resource
>>> error on a start transfer command.  So far this happens on low system
>>> memory test cases, so there may be some sequences that were missed,
>>> which led to this particular command error.
>>>
>>> Thanks
>>> Wesley Cheng
> 
> Hi Thinh,
> 
>>
>> No xfer resource usually means that the driver attempted to send
>> START_TRANSFER without waiting for END_TRANSFER command to complete.
>> This may be a dwc3 driver issue. Did you check this?
>>
>> Thanks,
>> Thinh
>>
>>
> 
> Yes, we know the reason why this happens, and its due to one of the
> downstream changes we had that led to the scenario above.  Although,
> that has been fixed, I still believe the error path is a potential
> scenario we'd still want to address.
> 
> I think the returning success always on dwc3_gadget_ep_queue(), and
> allowing the error in the completion handler/giveback at the function
> driver level to do the cleanup is a feasible solution.  Doesn't change
> the flow of the DWC3 gadget, and so far all function drivers we've used
> handle this in the correct manner.
> 
> Thanks
> Wesley Cheng

Right. I think for now we should do that (return success always except
for cases of disconnect or already in-flight etc). This helps keeping it
simple and avoid some pitfalls dealing with giving back the request.
Currently we return the error status to dwc3_gadget_ep_queue if we
failed to send a command that may not even related to the same request
being queued.

This way, I think it matches with how we handle it in the driver. We
always put the request in the pending list (queued) first and possibly
start/update the controller with new data.

Thanks,
Thinh


> 
>>>
>>>>>
>>>>>> This seems to add more complexity and I don't have a good solution to
>>>>>> it. Since you're already cancelling all the active request anyway, what
>>>>>> do you think of always letting dwc3_gadget_ep_queue() to go through with
>>>>>> success, but report failure through request completion?
>>>>>>
>>>>>
>>>>> We do have something similar as well downstream (returning success
>>>>> always on dwc3_gadget_ep_queue()) and its been working for us also.
>>>>> Problem is we don't test the ISOC path much, so this is the only type of
>>>>> EP that might come into question...
>>>>>
>>>>
>>>> It should be similiar with isoc. I can't think of a potential issue yet.
>>>>
>>>>> Coming up with a way to address the concerns you brought up was a bit
>>>>> difficult as there were scenarios we needed to consider.  next_request()
>>>>> doesn't always have to be the request being queued (even if ep queue
>>>>> triggered it).  There was no easy way to determine if kick transfer was
>>>>> due to ep queue, but even if there was, we'd need to remember the
>>>>> previous point as well.
>>>>>
>>>>
>>>> Yeah, there are a few pitfalls. I don't have a good solution to it if we
>>>> want to return failure immediately and let the function driver handle
>>>> the dequeue (if it wants to).
>>>>
>>>> Thanks,
>>>> Thinh
>>>>
>>>
>>
> 

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