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From: peter@hurleysoftware.com (Peter Hurley)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH 03/16] tty: serial: 8250_core: read only RX if there is something in the FIFO
Date: Wed, 11 Feb 2015 15:01:53 -0500
Message-ID: <54DBB531.2030504@hurleysoftware.com> (raw)
In-Reply-To: <54DA43F8.9090904@hurleysoftware.com>

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@linutronix.de>
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.

Regards,
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               |
                                     |

  reply index

Thread overview: 97+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-09-10 19:29 [PATCH 00/16 v9] omap 8250 based uart + DMA Sebastian Andrzej Siewior
2014-09-10 19:29 ` [PATCH 01/16] tty: serial: 8250_core: allow to set ->throttle / ->unthrottle callbacks Sebastian Andrzej Siewior
2014-09-10 19:29 ` [PATCH 02/16] tty: serial: 8250_core: add run time pm Sebastian Andrzej Siewior
2014-09-29  9:46   ` Frans Klaver
2014-09-29 13:39     ` Sebastian Andrzej Siewior
2014-09-10 19:29 ` [PATCH 03/16] tty: serial: 8250_core: read only RX if there is something in the FIFO Sebastian Andrzej Siewior
2015-02-09 13:34   ` Nicolas Schichan
2015-02-09 23:34     ` Peter Hurley
2015-02-10  9:32       ` Sebastian Andrzej Siewior
2015-02-10 12:04       ` Nicolas Schichan
2015-02-10 17:46         ` Peter Hurley
2015-02-11 20:01           ` Peter Hurley [this message]
2015-02-11 20:03             ` Tony Lindgren
2015-02-11 20:42               ` Peter Hurley
2015-02-12  8:45                 ` Sebastian Andrzej Siewior
2015-02-12  9:40                   ` Russell King - ARM Linux
2015-02-12 16:32                   ` Peter Hurley
2015-02-12 19:23                     ` Sebastian Andrzej Siewior
2015-02-12 19:55                       ` Peter Hurley
2015-02-12 20:34                         ` Sebastian Andrzej Siewior
2015-02-13 18:51                         ` Sebastian Andrzej Siewior
2015-02-13 23:15                           ` Russell King - ARM Linux
2015-02-15 17:32                             ` [PATCH] serial: 8250: Revert "tty: serial: 8250_core: read only RX if there is something in the FIFO" Sebastian Andrzej Siewior
2015-05-12 20:25                               ` Tony Lindgren
2014-09-10 19:29 ` [PATCH 04/16] tty: serial: 8250_core: use the ->line argument as a hint in serial8250_find_match_or_unused() Sebastian Andrzej Siewior
2014-09-10 19:30 ` [PATCH 05/16] tty: serial: 8250_core: remove UART_IER_RDI in serial8250_stop_rx() Sebastian Andrzej Siewior
2014-09-11 11:19   ` Heikki Krogerus
2014-09-10 19:30 ` [PATCH 06/16] tty: serial: Add 8250-core based omap driver Sebastian Andrzej Siewior
2014-09-11 11:57   ` Peter Hurley
2014-09-16 17:01     ` Sebastian Andrzej Siewior
2014-09-29  9:38   ` Frans Klaver
2014-09-29 13:27     ` Sebastian Andrzej Siewior
2014-09-29 13:34       ` Frans Klaver
2014-09-10 19:30 ` [PATCH 07/16] tty: serial: 8250_dma: handle error on TX submit Sebastian Andrzej Siewior
2014-09-10 19:30 ` [PATCH 08/16] tty: serial: 8250_dma: enqueue RX dma again on completion Sebastian Andrzej Siewior
2014-09-10 19:30 ` [PATCH 09/16] tty: serial: 8250_dma: Add a TX trigger workaround for AM33xx Sebastian Andrzej Siewior
2014-09-11 11:17   ` Heikki Krogerus
2014-09-11 11:42     ` Sebastian Andrzej Siewior
2014-09-11 12:32       ` Peter Hurley
2014-09-11 12:50         ` Sebastian Andrzej Siewior
2014-09-11 14:35           ` Peter Hurley
2014-09-15 17:01             ` Sebastian Andrzej Siewior
2014-09-16 16:55               ` Sebastian Andrzej Siewior
2014-09-17 12:20                 ` Peter Hurley
2014-09-17 16:25                   ` Sebastian Andrzej Siewior
2014-09-29 16:15                   ` Sebastian Andrzej Siewior
2014-09-11 15:11           ` Frans Klaver
2014-09-11 16:04             ` Sebastian Andrzej Siewior
2014-09-11 17:04               ` Frans Klaver
2014-09-12  7:23                 ` Sebastian Andrzej Siewior
     [not found]                   ` <20140912094010.GE28458@ci00147.xsens-tech.local>
2014-09-12  9:51                     ` Sebastian Andrzej Siewior
2014-09-12 10:28                       ` Frans Klaver
2014-09-15 16:42                         ` Sebastian Andrzej Siewior
2014-09-16  9:05                           ` Frans Klaver
2014-09-16 12:42                             ` Frans Klaver
2014-09-16 14:23                               ` Frans Klaver
2014-09-17 10:28                           ` Frans Klaver
2014-09-21 20:41                             ` Sebastian Andrzej Siewior
2014-09-22  9:28                               ` Frans Klaver
2014-09-24  7:56                                 ` Sebastian Andrzej Siewior
2014-09-25 15:14                                 ` Sebastian Andrzej Siewior
2014-09-25 15:18                                   ` Frans Klaver
2014-09-29  8:50                                   ` Frans Klaver
2014-09-29  9:54                                     ` Sebastian Andrzej Siewior
2014-09-29 10:30                                       ` Frans Klaver
2014-09-30  8:44                                         ` Frans Klaver
2014-10-02 10:27                                           ` Sebastian Andrzej Siewior
2014-10-13 14:55                                             ` Frans Klaver
2014-09-23 17:03                               ` Peter Hurley
2014-09-24  7:53                                 ` Sebastian Andrzej Siewior
2014-09-25 10:42                                   ` Sebastian Andrzej Siewior
2014-09-25 11:31                                     ` Peter Hurley
2014-09-25 13:11                                       ` Sebastian Andrzej Siewior
2014-09-17 16:34       ` Sebastian Andrzej Siewior
2014-09-19 10:22         ` Heikki Krogerus
2014-09-19 10:58           ` Sebastian Andrzej Siewior
2014-09-19 11:25             ` Peter Hurley
2014-09-22  7:46             ` Heikki Krogerus
2014-09-25  9:24               ` Sebastian Andrzej Siewior
2014-09-10 19:30 ` [PATCH 10/16] tty: serial: 8250_dma: optimize the xmit path due to UART_BUG_DMA_TX Sebastian Andrzej Siewior
2014-09-10 19:30 ` [PATCH 11/16] tty: serial: 8250_dma: keep own book keeping about RX transfers Sebastian Andrzej Siewior
2014-09-10 19:30 ` [PATCH 12/16] tty: serial: 8250_dma: handle the UART RDI event while DMA remains idle Sebastian Andrzej Siewior
2014-09-29  9:23   ` Frans Klaver
2014-09-10 19:30 ` [PATCH 13/16] tty: serial: 8250_dma: add pm runtime Sebastian Andrzej Siewior
2014-09-29  9:26   ` Frans Klaver
2014-09-29  9:56     ` Sebastian Andrzej Siewior
2014-09-10 19:30 ` [PATCH 14/16] arm: dts: am33xx: add DMA properties for UART Sebastian Andrzej Siewior
2014-09-10 19:30 ` [PATCH 15/16] arm: dts: dra7: " Sebastian Andrzej Siewior
2014-09-10 19:30 ` [PATCH 16/16] tty: serial: 8250: omap: add dma support Sebastian Andrzej Siewior
2014-09-12 22:43 ` [PATCH 00/16 v9] omap 8250 based uart + DMA Tony Lindgren
2014-09-15 11:50   ` Sebastian Andrzej Siewior
2014-09-16 12:57     ` Sebastian Andrzej Siewior
2014-09-16 16:48       ` Tony Lindgren
2014-09-16 21:30         ` Tony Lindgren
2014-09-17  8:38           ` Sebastian Andrzej Siewior
2014-09-17  9:05 ` Sebastian Andrzej Siewior
2014-09-26 16:02   ` Greg KH

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