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

[Linyu Yuan <quic_linyyuan@quicinc.com>]

On 1/10/2023 11:13 AM, Linyu Yuan wrote:
>
> 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.
one more question, is it legacy PCIe device still exist in real world ? 
and any VID/PID info ?
>>>
>>> BR,
>>> Thinh