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SubjectRe: [PATCH 03/16] tty: serial: 8250_core: read only RX if there is something in the FIFO
On 02/10/2015 12:46 PM, Peter Hurley wrote:
> 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.

Yep, I see no safe way to clear the spurious interrupt [1] and no idea how to
keep it from happening (other than via the unsolicited RX reads in

Unfortunately, I think this means we'll have to revert Sebastian's commit:

commit 0aa525d11859c1a4d5b78fdc704148e2ae03ae13
Author: Sebastian Andrzej Siewior <>
Date: Wed Sep 10 21:29:58 2014 +0200

tty: serial: 8250_core: read only RX if there is something in the FIFO

which just means OMAP3630 will be limited to using the omap_serial driver.

Peter Hurley

[1] To clear the RX timeout interrupt requires reading the rx fifo even though
LSR[data ready] indicates no data. However, this could result in dropped data
if the data became available just before clearing the RX timeout. For example,

CPU | Device
irq handler (simplified) |
read IIR |
is interrupt? yes |
read LSR |
is data ready? no |
is IIR == Rx timeout? yes | new data arrives
| rx_fifo[0] = new data
| lsr[data ready] = 1
read RX and discard |

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