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

From: Linyu Yuan <quic_linyyuan@quicinc.com>
To: "Thinh Nguyen" <Thinh.Nguyen@synopsys.com>,
	정재훈 <jh0801.jung@samsung.com>
Cc: '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>
Subject: Re: [PATCH] usb: dwc3: Clear DWC3_EVENT_PENDING when count is 0
Date: Fri, 3 Feb 2023 10:18:57 +0800	[thread overview]
Message-ID: <1ffed097-af47-a179-3854-f7b238d8b6de@quicinc.com> (raw)
In-Reply-To: <20230202200650.cqntlk7m2krsafgd@synopsys.com>

On 2/3/2023 4:06 AM, Thinh Nguyen wrote:
> On Thu, Feb 02, 2023, Linyu Yuan wrote:
>> On 2/2/2023 2:57 AM, Thinh Nguyen wrote:
>>> On Tue, Jan 31, 2023, Linyu Yuan wrote:
>>>> hi Thinh,
>>>>
>>>>
>>>> regarding your suggestion, assume it is not PCIe type,  still have one
>>>> question,
>>>>
>>>>
>>>> -       if (evt->flags & DWC3_EVENT_PENDING)
>>>> +       if (evt->flags & DWC3_EVENT_PENDING) {
>>>> +               if (!evt->count) {
>>>> +                       u32 reg = dwc3_readl(dwc->regs, DWC3_GEVNTSIZ(0));
>>>> +
>>>> +                       if (!(reg & DWC3_GEVNTSIZ_INTMASK))
>>>> +                               evt->flags &= ~DWC3_EVENT_PENDING;
>>>>
>>>> do we need to return IRQ_WAKE_THREAD  ?
>>> No, if evt->count is 0, but GEVNTCOUNT is > 0, the controller will
>>> generate interrupt. The evt->count will be updated and the events will
>>> be handled on the next interrupt.
>>
>> when will next interrupt happen ?
> Immediately after. You can test this by just return IRQ_HANDLED and not
> clear the GEVNTCOUNT to see its behavior.

if it immediately, it will be good.

정재훈  could you update a new patch which Thinh suggest.

maybe we didn't find the root cause of irq strom, but the change have no side effect.

>
>> as when enter here, i guess GEVENTCOUNT is already > 0, but we didn't read
>> it.
> GEVNTCOUNT is always updating as new events are generated. We only clear
> however many events we process, but that doesn't stop it from
> incrementing.

just consider if there is a case that next GEVNETCOUNT increase which 
happen long time later,

maybe think too much.

>
> BR,
> Thinh
>
>>
>>>> +               }
>>>>                   return IRQ_HANDLED;
>>>>
>>>> as here return IRQ HANDLED, how can we make sure a new IRQ will be handled
>>>> after previous IRQ thread clean PENDING flag ?
>>> If evt->count > 0, that means the bottom half is still running. So,
>>> leave it be. If evt->count == 0, then the cached events are processed,
>>> we're safe to clear the PENDING flag. New interrupt will be generated if
>>> GEVNTCOUNT is > 0.
>>>
>>>> +       }
>>>>
>>>>
>>>> also for non-PCIe controller, consider IRQ mask register working correctly,
>>>>
>>>> consider a case IRQ happen before IRQ thread exit,  here just return
>>>> IRQ_HANDLED.
>>>>
>>>> once IRQ thread exit, it will clean PENDING flag, so next IRQ event will run
>>>> normally.
>>>>
>>>> if 정재훈 saw PENDING flag is not cleared, does it mean IRQ thread have no
>>>> chance to exit ?
>>> The PENDING flag should be cleared eventually when the bottom half
>>> completes. I don't expect the interrupt storm to block the IRQ thread
>>> forever, but I can't guarantee the device behavor. 정재훈 can confirm.
>>> This change should resolve the interrupt storm.
>>>
>>> BR,
>>> Thinh