Re: [PATCH 4/6] serial: 8250_dw: Rework IIR_NO_INT handling to stop interrupt storm

[Ilpo Järvinen]

On Sat, 24 Jan 2026, Andy Shevchenko wrote:

> On Fri, Jan 23, 2026 at 07:27:37PM +0200, Ilpo Järvinen wrote:
> > INTC10EE UART can end up into an interrupt storm where it reports
> > IIR_NO_INT (0x1). If the storm happens during active UART operation, it
> > is promptly stopped by IIR value change due to Rx or Tx events.
> > However, when there is no activity, either due to idle serial line or
> > due to specific circumstances such as during shutdown that writes
> > IER=0, there is nothing to stop the storm.
> > 
> > During shutdown the storm is particularly problematic because
> > serial8250_do_shutdown() calls synchronize_irq() that will hang in
> > waiting for the storm to finish which never happens.
> > 
> > This problem can also result in triggering a warning:
> > 
> >   irq 45: nobody cared (try booting with the "irqpoll" option)
> >   [...snip...]
> >   handlers:
> >     serial8250_interrupt
> >   Disabling IRQ #45
> > 
> > Normal means to reset interrupt status by reading LSR, MSR, USR, or RX
> > register do not result in the UART deasserting the IRQ.
> > 
> > Add a quirk to INTC10EE UARTs to enable Tx interrupts if UART's Tx is
> > currently empty and inactive. Rework IIR_NO_INT to keep track of the
> > number of consecutive IIR_NO_INT, and on third one perform the quirk.
> > Enabling Tx interrupts should change IIR value from IIR_NO_INT to
> > IIR_THRI which has been observed to stop the storm.
> 
> ...
> 
> > +	u64			no_int_count;
> 
> Why so big?
> 
> ...

No particular reason, it's a leftover from debugging this issue.

> > +		d->no_int_count++;
> > +		if (d->no_int_count > 2 && quirks & DW_UART_QUIRK_IER_KICK)
> > +			dw8250_quirk_ier_kick(p);
> 
> Usual way is to use modulo. And perhaps use 4 for the sake of avoiding
> division:
> 
> 		if (d->no_int_count == 3 && quirks & DW_UART_QUIRK_IER_KICK)
> 			dw8250_quirk_ier_kick(p);
> 
> 		d->no_int_count = (d->no_int_count + 1) % 4;

This doesn't look equivalent code as it only fires on 4th NO_INT, but I 
guess the difference doesn't matter that much so changing to your 
suggestion so that the kick will only occurs on fourth NO_INT interrupt.

-- 
 i.

> where 4 may be defined with meaningful name. With that u8 is more than enough.
>
> >  		return 0;
> 
>