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

[Linyu Yuan]

On 1/11/2023 8:00 AM, Thinh Nguyen wrote:
> > one more question, is it legacy PCIe device still exist in real world ? and
> > any VID/PID info ?
> Currently, all dwc3 PCIe devices are affected. Some setups are more


if non PCIe device have no such issue, can we do some improvement for it ?

like new flag or static key/jump label to improve interrupt handler ?


> noticeable than others. The dwc3 driver is implemented to probe platform
> devices. So, dwc3 PCIe devices are wrapped as platform devices for the
> dwc3 driver. Since we're going through the platform device code path,
> the pci layer falls back to using legacy interrupt instead of MSI (last
> I check awhile ago).
>
> A little more detail on this problem:
> PCIe legacy interrupt will emulate interrupt line by sending an
> interrupt assert and deassert messages. After the interrupt assert
> message is sent, interrupts are continuously generated until the
> deassert message is sent. If there's a register write to unmask/mask
> interrupt or clearing events falls in between these messages, then there
> may be a race.
>
> Let's say we don't have event pending check, this can happen:
>
> Normal scenario
> ---------------
>      event_count += n # controller generates new events
>    interrupt asserts
>      write(mask irq)
>      event_count -= n # dwc3 clears events
>    interrupt deasserts
>      write(unmask irq)
>
>
> Race scenario
> -------------
>      event_count += n # new events
>    interrupt asserts
>      write(mask irq)
>      event_count -= n # clear events
>      event_count += n # more events come and hard irq handler gets called
> 		     # again as interrupt is generated, but cached
> 		     # events haven't been handled. This breaks
> 		     # serialization and causes lost events.
>      write(mask irq)
>
>      event_count -= n # clear events
>    interrupt deasserts
>      write(unmask irq) # events handled


if mask irq is not working, the race will happen like this, thanks for 
explanation.


> For MSI, this won't be a problem because it's edge-triggered and the way
> it sends interrupt is different.
>
> BR,
> Thinh