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

[Thinh Nguyen]

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.

> 
> +               }
>                 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