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Message-ID: <bfcfac21-3df7-7f47-576b-0717f2bab393@quicinc.com>
Date:   Tue, 10 Jan 2023 11:13:39 +0800
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Subject: Re: [PATCH] usb: dwc3: Clear DWC3_EVENT_PENDING when count is 0
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From:   Linyu Yuan <quic_linyyuan@quicinc.com>
To:     Thinh Nguyen <Thinh.Nguyen@synopsys.com>
CC:     =?UTF-8?B?7KCV7J6s7ZuI?= <jh0801.jung@samsung.com>,
        'Felipe Balbi' <balbi@kernel.org>,
        'Greg Kroah-Hartman' <gregkh@linuxfoundation.org>,
        "'open list:USB XHCI DRIVER'" <linux-usb@vger.kernel.org>,
        'open list' <linux-kernel@vger.kernel.org>,
        'Seungchull Suh' <sc.suh@samsung.com>,
        'Daehwan Jung' <dh10.jung@samsung.com>
References: <CGME20230102050839epcas2p4b9d09d926f9a14c3b8e8df2574d334c3@epcas2p4.samsung.com>
 <20230102050831.105499-1-jh0801.jung@samsung.com>
 <af32e2f5-de45-38af-2b4f-47c1ac21ea9e@quicinc.com>
 <c4e01a0a-1c98-3103-2b91-2fe0ba8c3118@quicinc.com>
 <000201d920eb$c3715c50$4a5414f0$@samsung.com>
 <0bbd2355-2290-17c7-6860-d8b25930aed6@quicinc.com>
 <20230109182813.sle5h34wdgglnlph@synopsys.com>
 <e7217471-5b08-eee6-fba8-06e879118d77@quicinc.com>
 <20230110025310.nowjnrmo3oag76xd@synopsys.com>
 <4ced9c3e-c7b5-e0a0-88ec-1ac383d893a2@quicinc.com>
In-Reply-To: <4ced9c3e-c7b5-e0a0-88ec-1ac383d893a2@quicinc.com>
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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