Possible bug in 8250.c

Possible bug in 8250.c

[Ivica Mikec]

 Hi!


I noticed a problem in 8250.c. 

My board has only one UART port, and is 16550 compatible, so in function serial8250_interrupt I see that serial_in function is called twice. Second time, code "else if (end == NULL)" is executed and function return IRQ_NONE. This causes an entry in /proc/irq/spurious:

count 239
unhandled 1
last_unhandled 4294700846 ms

But this is not a spurious interrupt.

Regards.

Re: Possible bug in 8250.c

[Jiri Slaby]

On 08/19/2011 07:44 PM, Ivica Mikec wrote:
> 
>  Hi!
> 
> 
> I noticed a problem in 8250.c. 
> 
> My board has only one UART port, and is 16550 compatible, so in function serial8250_interrupt I see that serial_in function is called twice. Second time, code "else if (end == NULL)" is executed and function return IRQ_NONE. This causes an entry in /proc/irq/spurious:
> 
> count 239
> unhandled 1
> last_unhandled 4294700846 ms
> 
> But this is not a spurious interrupt.

How did you find out? Have you checked that the port signals that it
raised an interrupt? I.e. does it go through the 'if (!(iir &
UART_IIR_NO_INT))' branch?

What other devices are bound to the same interrupt? Attach /proc/interrupts.

regards,
-- 
js
suse labs

Re: Possible bug in 8250.c

[Ivica Mikec]

I traced the function using jtag debugger. 

UART is not sharing interrupts:
========================================= Console ========================
Serial: 8250/16550 driver, 1 ports, IRQ sharing disabled
serial8250.0: ttyS0 at MMIO 0xae023400 (irq = 53) is a 16550A
console [ttyS0] enabled, bootconsole disabled
=========================================================================
And /proc/interrupts:

=========================================================================
cat /proc/interrupts
              CPU0
 53:       4534                XXXXX  serial
 56:         12                   XXXXX  phy_interrupt
153:   20262068          XXXXX  timer
ERR:          0
=========================================================================

So in first iteration, interrupt is cleared, and in second, function will execute:

          } else if (end == NULL)
                        end = l;

which will terminate the loop, but the return code will be IRQ_RETVAL(0).




On Fri 19/08/11 12:01 , Jiri Slaby <jslaby@suse.cz> wrote:

> On 08/19/2011 07:44 PM, Ivica Mikec wrote:
> > 
> > Hi!
> > 
> > 
> > I noticed a problem in 8250.c. 
> > 
> > My board has only one UART port, and is 16550 compatible, so in function
> serial8250_interrupt I see that serial_in function is called twice. Second
> time, code "else if (end == NULL)" is executed and function return
> IRQ_NONE. This causes an entry in /proc/irq/spurious:
> > 
> > count 239
> > unhandled 1
> > last_unhandled 4294700846 ms
> > 
> > But this is not a spurious interrupt.
> 
> How did you find out? Have you checked that the port signals that it
> raised an interrupt? I.e. does it go through the 'if (!(iir &
> UART_IIR_NO_INT))' branch?
> 
> What other devices are bound to the same interrupt? Attach
> /proc/interrupts.
> 
> regards,
> -- 
> js
> suse labs
> 
> 
> 

Re: Possible bug in 8250.c

[Alan Cox]

> So in first iteration, interrupt is cleared, and in second, function will execute:
> 
>           } else if (end == NULL)
>                         end = l;
> 
> which will terminate the loop, but the return code will be IRQ_RETVAL(0).

If you are getting an extra IRQ when then chip itself is indicating no
IRQ then the IRQ is spurious. That probably means you have a problem
elsewhere - eg the IRQ controller being misconfigured between level/edge
triggered.

Alan