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

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Date:   Mon, 5 Mar 2018 11:45:58 +0200
From:   Roger Quadros <rogerq@...com>
To:     Felipe Balbi <balbi@...nel.org>
CC:     <linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: dwc3: Prevent indefinite sleep in _dwc3_set_mode
 during suspend/resume

Felipe,

On 05/03/18 10:49, Felipe Balbi wrote:
> 
> Hi,
> 
> 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.

And there is no point in clearing the DWC3_EP_END_TRANSFER_PENDING flag
in dwc3_gadget_free_endpoints() since we're freeing the dwc3_ep memory there.

dwc3_gadget_init_endpoints() will start with a clean slate.

-- 
cheers,
-roger

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