usb: dwc3: gadget: Fix event pending check

The DWC3_EVENT_PENDING flag is used to protect against invalid call to
top-half interrupt handler, which can occur when there's a delay in
software detection of the interrupt line deassertion.

However, the clearing of this flag was done prior to unmasking the
interrupt line, creating opportunity where the top-half handler can
come. This breaks the serialization and creates a race between the
top-half and bottom-half handler, resulting in losing synchronization
between the controller and the driver when processing events.

To fix this, make sure the clearing of the DWC3_EVENT_PENDING is done at
the end of the bottom-half handler.

Fixes: d325a1d ("usb: dwc3: gadget: Prevent losing events in event cache")
Cc: stable@vger.kernel.org
Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/8670aaf1cf52e7d1e6df2a827af2d77263b93b75.1656380429.git.Thinh.Nguyen@synopsys.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Thinh Nguyen

drivers/usb/dwc3/gadget.c

@@ -4249,7 +4249,6 @@ static irqreturn_t dwc3_process_event_buf(struct dwc3_event_buffer *evt)
 	}
 
 	evt->count = 0;
-	evt->flags &= ~DWC3_EVENT_PENDING;
 	ret = IRQ_HANDLED;
 
 	/* Unmask interrupt */
@@ -4261,6 +4260,9 @@ static irqreturn_t dwc3_process_event_buf(struct dwc3_event_buffer *evt)
 		dwc3_writel(dwc->regs, DWC3_DEV_IMOD(0), dwc->imod_interval);
 	}
 
+	/* Keep the clearing of DWC3_EVENT_PENDING at the end */
+	evt->flags &= ~DWC3_EVENT_PENDING;
+
 	return ret;
 }