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

[Linyu Yuan]

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@xxxxxxxxxxx]
> > > > > > 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@xxxxxxxxxxxx/T/#m7b907aa6da4023cb20fa00a57813d31fd84e081f
 do you think we need to read event count before checking 
DWC3_EVENT_PENDING  ?
> BR,
> Thinh