[PATCH] serial: 8250: Avoid "too much work" from bogus rx timeout interrupt

Andy Shevchenko andriy.shevchenko at linux.intel.com
Mon Dec 19 12:18:08 PST 2016

On Mon, 2016-12-19 at 09:54 -0800, Doug Anderson wrote:
> Hi,
> Yes. Almost all Intel HW is using DesignWare IP for HS UARTs.
> OK, so possibly we could add this workaround in just the DesignWare
> code and then we could be more sure we're not breaking other UARTs?
> That would work for me.  It seems like it would be easier to validate
> that there are no unintended side effects if we put this just in the
> DesignWare driver.

Yes, don't need to touch others.

> Yes, I could believe that in the DMA case that my patch might not be
> the right thing to do.  I can easily just add a check for "!up->dma"
> if it makes the patch better.

At least, yes.

> > > 1. We'll get the interrupt
> > > 2. We won't do _anything_ to service the interrupt.
> > > 3. We'll return back to serial8250_interrupt(), where we'll keep
> > > looping until we get "too much work"
> > > 4. We'll break out, but the interrupt will still be active.
> > > 5. Go back to #1
> > > 
> > > ...and since this interrupt will keep firing and firing and firing
> > > with no delay in-between, we'll effectively lock the CPU up.
> > 
> > And the root cause of that is... ?
> I don't understand your question.  Basically what I'm saying is that
> we got an interrupt and did absolutely nothing to handle it or clear
> it.  Then we returned "handled".  Is it a mystery that the interrupt
> will fire again and again and again?

> Specifically:
> * reading the LSR doesn't clear the interrupt
> * The DR / BI bits aren't set.
> * serial8250_modem_status() won't clear the interrupt (reads the MSR)
> * nothing to transmit
> * we'll return "handled" since the only time serial8250_handle_irq()
> returns 0 is if we have UART_IIR_NO_INT.

My question here a bit rhetorical, we better understand root cause,
better fix would be.

> > What I think is that the root cause of this is still unknown and
> > either
> > above looks like a hack.
> I postulated a root cause of receiving a partial character, but I'd
> need to figure out how to twiddle bits in just the right way to
> somehow try to do this in a programmatic way.  I can certainly
> reproduce this in a black-box sort of way by just doing suspend/resume
> testing long enough.

Have you tried to disable C-states or set PM QoS?
Do you have same issue with and without DMA?

> Even if the root cause isn't know, though, it seems like the current
> behavior of locking up a CPU is non-ideal.  It seems like there ought
> to be some sort of way to detect and handle this case.

Have you read links I sent? In one mail I mentioned Intel's
documentation that suggests not to use RDI interrupt when DMA. Which
sounds weird.

Andy Shevchenko <andriy.shevchenko at linux.intel.com>
Intel Finland Oy

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