On 02/10/2015 07:04 AM, Nicolas Schichan wrote:
> On 02/10/2015 12:34 AM, Peter Hurley wrote:
>> Hi Nicolas,
>>
>> Thanks for the report.
>>
> [...]
>>> When a caracter is received on the UART while the kernel is printing
>>> the boot messages, as soon as the kernel configures the UART for
>>> receiving (after root filesystem mount), it gets stuck printing the
>>> following message repeatedly:
>>>
>>> serial8250: too much work for irq29
>>>
>>> Once stuck, the reception of another character allows the boot process
>>> to finish.
>>>
>>> From what I can gather, when we hit that, the UART_IIR_NO_INT is 0 (so the
>>> interrupt is raised), but the UART_LSR_DR bit is 0 as well so the UART_RX
>>> register is never read to clear the interrupt.
>>
>> The "too much work" message means serial8250_handle_irq() is returning 0,
>> ie., not handled. Which in turn means IIR indicates no interrupt is pending
>> (UART_IIR_NO_INT == 1).
>>
>> Can you log the register values for LSR and IIR at both patch locations
>> in serial8250_do_startup()?
>>
>> (I can get you a debug patch, if necessary. Let me know)
>
> Hi Peter,
>
> Thanks for your reply.
>
> Here is what I have when the issue is triggered:
>
> [ 12.154877] lsr 0x60 / iir 0x01
> [ 12.158071] lsr 0x60 / iir 0x01
> [ 12.161438] serial8250: too much work for irq29
> [ 12.165982] lsr 0x60 / iir 0x0c
> [ 12.169354] serial8250: too much work for irq29
> [ 12.173900] lsr 0x60 / iir 0x0c
> (previous two messages are repeated and printk_ratelimited())
Thanks for this information; I see I was wrong about the cause of message.
I think what happens during startup is that on this silicon clearing
the rx fifo (by serial8250_clear_fifos()) clears data ready but not
the rx timeout condition which causes a spurious rx interrupt when
interrupts are enabled.
So caught between two broken UARTs: one that underflows its rx fifo because
of unsolicited rx reads and the other that generates spurious interrupt
without unsolicited rx reads.
> When the issue is not triggered:
>
> [ 10.784871] lsr 0x60 / iir 0x01
> [ 10.788066] lsr 0x60 / iir 0x01
> [ 10.794734] VFS: Mounted root (nfs filesystem) readonly on device 0:13.
> [ 10.801654] devtmpfs: mounted
> [ 10.805169] Freeing unused kernel memory: 184K (807be000 - 807ec000)
> (userland takes over after that)
>
> I have also displayed the IIR and LSR registers when the "too much fork for
> IRQ" condition is triggered.
>
> In the serial8250_do_startup(), before the interrupt are unmasked at the end,
> the IIR looks sane and UART_IIR_NO_INT bit is set. When stuck
> serial8250_interrupt(), UART_IIR_NO_INT is cleared and the interrupt ID is set
> to 0xc which is not handled by the kernel at this time (the Kirkwood datasheet
> indicates that it is some kind of timeout condition from what I can gather).
Yes, IIR == UART_IIR_RX_TIMEOUT is to used indicate that data is in the rx fifo
but has not reached the rx trigger level yet.
ATM, I'm not exactly sure if there is a safe way to clear the spurious interrupt
from the interrupt handler.
I'm fairly certain the only way to clear the rx timeout interrupt is to read
the rx fifo, but I think this would race with actual data arrival. IOW, there
might not be a way to determine if the data read is spurious or not.
Regards,
Peter Hurley
> Here is the corresponding debug patch, for reference (against a 3.19 kernel
> this time):
>
> diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_
> index 11c6685..ed93741 100644
> --- a/drivers/tty/serial/8250/8250_core.c
> +++ b/drivers/tty/serial/8250/8250_core.c
> @@ -1661,6 +1661,12 @@ static int exar_handle_irq(struct uart_port *port)
> return ret;
> }
>
> +static inline void show_lsr_iir(struct uart_port *port)
> +{
> + printk("lsr 0x%02x / iir 0x%02x\n", serial_port_in(port, UART_LSR),
> + serial_port_in(port, UART_IIR));
> +}
> +
> /*
> * This is the serial driver's interrupt routine.
> *
> @@ -1705,6 +1711,8 @@ static irqreturn_t serial8250_interrupt(int irq, void *dev
> /* If we hit this, we're dead. */
> printk_ratelimited(KERN_ERR
> "serial8250: too much work for irq%d\n", irq);
> + if (printk_ratelimit())
> + show_lsr_iir(port);
> break;
> }
> } while (l != end);
> @@ -2107,6 +2115,7 @@ int serial8250_do_startup(struct uart_port *port)
> /*
> * Clear the interrupt registers.
> */
> + show_lsr_iir(port);
> if (serial_port_in(port, UART_LSR) & UART_LSR_DR)
> serial_port_in(port, UART_RX);
> serial_port_in(port, UART_IIR);
> @@ -2269,6 +2278,7 @@ dont_test_tx_en:
> * saved flags to avoid getting false values from polling
> * routines or the previous session.
> */
> + show_lsr_iir(port);
> if (serial_port_in(port, UART_LSR) & UART_LSR_DR)
> serial_port_in(port, UART_RX);
> serial_port_in(port, UART_IIR);
>
> Regards,
>
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