linux-kernel - Re: [PATCH] usb: dwc3: Prevent indefinite sleep in _dwc3_set

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Date:   Mon, 05 Mar 2018 13:06:49 +0200
From:   Felipe Balbi <balbi@...nel.org>
To:     Baolin Wang <baolin.wang@...aro.org>, Roger Quadros <rogerq@...com>
Cc:     USB <linux-usb@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: dwc3: Prevent indefinite sleep in _dwc3_set_mode during suspend/resume


Hi,

Baolin Wang <baolin.wang@...aro.org> writes:
>>> Roger Quadros <rogerq@...com> writes:
>>>>> Roger Quadros <rogerq@...com> writes:
>>>>>> In the following test we get stuck by sleeping forever in _dwc3_set_mode()
>>>>>> after which dual-role switching doesn't work.
>>>>>>
>>>>>> On dra7-evm's dual-role port,
>>>>>> - Load g_zero gadget driver and enumerate to host
>>>>>> - suspend to mem
>>>>>> - disconnect USB cable to host and connect otg cable with Pen drive in it.
>>>>>> - resume system
>>>>>> - we sleep indefinitely in _dwc3_set_mode due to.
>>>>>>   dwc3_gadget_exit()->usb_del_gadget_udc()->udc_stop()->
>>>>>>    dwc3_gadget_stop()->wait_event_lock_irq()
>>>>>>
>>>>>> Let's clear the DWC3_EP_END_TRANSFER_PENDING flag on all endpoints
>>>>>> so we don't wait in dwc3_gadget_stop().
>>>>>>
>>>>>> Signed-off-by: Roger Quadros <rogerq@...com>
>>>>>> ---
>>>>>>  drivers/usb/dwc3/gadget.c | 14 ++++++++++++++
>>>>>>  1 file changed, 14 insertions(+)
>>>>>>
>>>>>> diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
>>>>>> index 2bda4eb..0a360da 100644
>>>>>> --- a/drivers/usb/dwc3/gadget.c
>>>>>> +++ b/drivers/usb/dwc3/gadget.c
>>>>>> @@ -3273,6 +3273,20 @@ int dwc3_gadget_init(struct dwc3 *dwc)
>>>>>>
>>>>>>  void dwc3_gadget_exit(struct dwc3 *dwc)
>>>>>>  {
>>>>>> +  int epnum;
>>>>>> +  unsigned long flags;
>>>>>> +
>>>>>> +  spin_lock_irqsave(&dwc->lock, flags);
>>>>>> +  for (epnum = 2; epnum < DWC3_ENDPOINTS_NUM; epnum++) {
>>>>>> +          struct dwc3_ep  *dep = dwc->eps[epnum];
>>>>>> +
>>>>>> +          if (!dep)
>>>>>> +                  continue;
>>>>>> +
>>>>>> +          dep->flags &= ~DWC3_EP_END_TRANSFER_PENDING;
>>>>>> +  }
>>>>>> +  spin_unlock_irqrestore(&dwc->lock, flags);
>>>>>> +
>>>>>>    usb_del_gadget_udc(&dwc->gadget);
>>>>>>    dwc3_gadget_free_endpoints(dwc);
>>>>>
>>>>> free endpoints is a better place for this. It's already going to free
>>>>> the memory anyway. Might as well clear all flags to 0 there.
>>>>>
>>>>
>>>> But it won't solve the deadlock issue. Since dwc3_gadget_free_endpoints()
>>>> is called after usb_del_gadget_udc() and the deadlock happens when
>>>>
>>>> usb_del_gadget_udc()->udc_stop()->dwc3_gadget_stop()->wait_event_lock_irq()
>>>>
>>>> and DWC3_EP_END_TRANSFER_PENDING flag is set.
>>>
>>> indeed. Iterating twice over the entire endpoint list seems
>>> wasteful. Perhaps we just shouldn't wait when removing the UDC since
>>> that's essentially what this patch will do, right? If you clear the flag
>>> before calling ->udc_stop(), this means the loop in dwc3_gadget_stop()
>>> will do nothing. Might as well remove it.
>>>
>>
>> This means that we will never wait for DWC3_EP_END_TRANSFER_PENDING to clear
>> in dwc3_gadget_stop() like we used to. This is perfectly fine, right?
>>
>> It makes sense to me as dwc3_gadget_stop() calls __dwc3_gadget_stop() which
>> masks all interrupts and nobody will ever clear that flag if it was set.
>
> I don't think so. It can not mask the endpoint events, please check
> the events which will be masked in DEVTEN register. The reason why we
> should wait for DWC3_EP_END_TRANSFER_PENDING to clear is that,
> sometimes the DWC3_DEPEVT_EPCMDCMPLT event will be triggered later
> than 100us, but now we may have freed the gadget irq which will cause
> crash.

We could mask command complete events as soon as ->udc_stop() is called,
right? Hmm, actually, __dwc3_gadget_stop() already clears DEVTEN
completely.

/me goes check databook

At least on revision 2.60a of the databook, bit 10 is reserved. I wonder
if that's the start of all the problems. Anybody has access to older and
newer databook revisions so we can cross-check?

best

-- 
balbi

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