On 02/11/2015 03:03 PM, Tony Lindgren wrote: > * Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> [150211 12:05]: >> 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 >> serial8250_do_startup). >> >> Unfortunately, I think this means we'll have to revert Sebastian's commit: >> >> commit 0aa525d11859c1a4d5b78fdc704148e2ae03ae13 >> Author: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> >> 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. > > Reverting makes sense to me if it has caused a regression. Maybe Sebastian > can update his patch to do this based on some quirk flag instead? That's fine with me. There's a 'bugs' field in struct 8250_uart_port and UART_BUG_* defines in 8250/8250.h for that purpose. Regards, Peter Hurley -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
