Re: [PATCH] usb: dwc3: Clear DWC3_EVENT_PENDING when count is 0

[Linyu Yuan <quic_linyyuan@quicinc.com>]

On 1/10/2023 11:05 AM, Linyu Yuan wrote:
>
> On 1/10/2023 10:53 AM, Thinh Nguyen wrote:
>> On Tue, Jan 10, 2023, Linyu Yuan wrote:
>>> On 1/10/2023 2:28 AM, Thinh Nguyen wrote:
>>>> On Fri, Jan 06, 2023, Linyu Yuan wrote:
>>>>> On 1/5/2023 5:54 PM, 정재훈 wrote:
>>>>>>> -----Original Message-----
>>>>>>> From: Linyu Yuan [mailto:quic_linyyuan@quicinc.com]
>>>>>>> Sent: Thursday, January 5, 2023 12:35 PM
>>>>>>> To: JaeHun Jung; Felipe Balbi; Greg Kroah-Hartman; Thinh Nguyen
>>>>>>> Cc: open list:USB XHCI DRIVER; open list; Seungchull Suh; 
>>>>>>> Daehwan Jung
>>>>>>> Subject: Re: [PATCH] usb: dwc3: Clear DWC3_EVENT_PENDING when 
>>>>>>> count is 0
>>>>>>>
>>>>>>>
>>>>>>> On 1/5/2023 11:29 AM, Linyu Yuan wrote:
>>>>>>>> On 1/2/2023 1:08 PM, JaeHun Jung wrote:
>>>>>>>>> Sometimes very rarely, The count is 0 and the DWC3 flag is set 
>>>>>>>>> has
>>>>>>>>> status.
>>>>>>>>> It must not have these status. Because, It can make happen 
>>>>>>>>> interrupt
>>>>>>>>> storming status.
>>>>>>>> could you help explain without clear the flag, how interrupt 
>>>>>>>> storming
>>>>>>>> happen ?
>>>>>>>>
>>>>>>>> as your change didn't touch any hardware register, i don't know 
>>>>>>>> how it
>>>>>>>> fix storming.
>>>>>>>>
>>>>>> H/W interrupts are still occur on IP.
>>>>> I guess we should fix it from IP layer.
>>>>>
>>>> How are you certain the problem is from IP layer?
>>> I think all IRQ is from DWC3 controller IP. if it is not IP layer, 
>>> could you
>>> share how to prevent from SW layer ?
>>>
>>> seem IRQ can happen when event count is zero ,  why this can happen 
>>> ? does
>>> it mean event count register is not trust ?
>> When the interrupt is unmasked, the controller will generate interrupts
>> as events are received. Normally, the flag checking for pending event
>> should be cleared before unmasking the interrupt, but we clear it after
>> to account for possible false interrupt due to the nature of legacy pci
>> interrupt. This exposes a gap where the interrupts can come but the flag
>> isn't cleared. While it should be rare and shouldn't be too much of a
>> problem, we can avoid this scenario with some additional checks.
>>
>>>>> but when checking DWC3_EVENT_PENDING flag, it will auto clear in 
>>>>> dwc3 thread
>>>>> irq handler.
>>>>>
>>>>> there is one possible root cause is it cleared only after irq 
>>>>> enabled in
>>>>> dwc3_process_event_buf(),
>>>>>
>>>>> we should move unmask irq operation at end of this function.
>>>>>
>>>> This interrupt storm can happen because we clear the evt->flags 
>>>> _after_
>>>> we unmask the interrupt. This was done to prevent false interrupt from
>>>> delay interrupt deassertion, which can be a problem for legacy pci
>>>> interrupt.
>>> thanks for explain, i didn't know that.
>>>> see 7441b273388b ("usb: dwc3: gadget: Fix event pending check")
>>>>
>>>> The change JaeHun Jung did should be fine.
>>> agree.
>> The change may still need some additional check as suggested in my
>> response:
>> https://lore.kernel.org/linux-usb/20230109190914.3blihjfjdcszazdd@synopsys.com/T/#m7b907aa6da4023cb20fa00a57813d31fd84e081f 
>>
>  do you think we need to read event count before checking 
> DWC3_EVENT_PENDING  ?
sorry for this noise, may be i have a little understanding of the legcy 
pci issue now.
>>
>> BR,
>> Thinh