Date:   Tue, 10 Oct 2017 23:19:47 +0200 (CEST)
From:   David Kozub <zub@...ux.fjfi.cvut.cz>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: Re: PROBLEM: Kernel BUG in mfgpt_tick (cs5535-clockevt.c) on ALIX
 2c3 - null call

On Tue, 10 Oct 2017, Daniel Lezcano wrote:

> On 09/10/2017 21:33, David Kozub wrote:
>> On Mon, 9 Oct 2017, Daniel Lezcano wrote:
>>
>>> On 07/10/2017 23:26, David Kozub wrote:
>>>> Hi all,
>>>>
>>>> booting up kernel 4.14-rc3 with CS5535_CLOCK_EVENT_SRC on an ALIX 2c3
>>>> (http://pcengines.ch/alix2c3.htm) dies with:
>>>>
>>>> [    2.313086] cs5535-smb cs5535-smb: SCx200 device 'CS5535 ACB0'
>>>> registered
>>>> [    2.338711] cs5535-mfgpt cs5535-mfgpt: registered timer 0
>>>> [    2.355676] ledtrig-cpu: registered to indicate activity on CPUs
>>>> [    2.373745] cs5535-mfgpt cs5535-mfgpt: registered timer 1
>>>> [    2.389976] cs5535-clockevt: Registering MFGPT timer as a clock
>>>> event, using IRQ 7
>>>> [    2.412687] BUG: unable to handle kernel NULL pointer dereference
>>>> at   (null)
>>>> [    2.412698] IP:   (null)
>>>> [    2.412702] *pde = 00000000
>>>> [    2.412713] Oops: 0000 [#1]
>>>> [    2.412716] Modules linked in:
>>>> [    2.412732] CPU: 0 PID: 1 Comm: swapper Not tainted
>>>> 4.14.0-rc3-humel-test17 #36
>>>> [    2.412739] task: c0010000 task.stack: c008e000
>>>> [    2.412744] EIP:   (null)
>>>> [    2.412749] EFLAGS: 00210093 CPU: 0
>>>> [    2.412758] EAX: c05fb8c0 EBX: c05fb880 ECX: 00000000 EDX: 0000620c
>>>> [    2.412769] ESI: c0009fd4 EDI: c014e14e EBP: c0009fac ESP: c0009fa8
>>>> [    2.412780]  DS: 007b ES: 007b FS: 0000 GS: 0000 SS: 0068
>>>> [    2.412790] CR0: 80050033 CR2: 00000000 CR3: 0064f000 CR4: 00000090
>>>> [    2.412793] Call Trace:
>>>> [    2.412800]  <IRQ>
>>>> [    2.412825]  ? mfgpt_tick+0x5d/0x81
>>>> [    2.412845]  __handle_irq_event_percpu+0x56/0xb6
>>>> [    2.412864]  ? handle_fasteoi_irq+0xf3/0xf3
>>>> [    2.412878]  handle_irq_event_percpu+0x17/0x3f
>>>> [    2.412892]  handle_irq_event+0x1d/0x29
>>>> [    2.412905]  handle_level_irq+0x57/0xc6
>>>> [    2.412924]  handle_irq+0x47/0x52
>>>> [    2.412929]  </IRQ>
>>>> [    2.412934]  <SOFTIRQ>
>>>> [    2.412949]  do_IRQ+0x32/0x9b
>>>> [    2.412963]  ? __irqentry_text_end+0x7/0x7
>>>> [    2.412976]  common_interrupt+0x2e/0x34
>>>> ...
>>>>
>>>> The problem seems to be in drivers/clocksource/cs5535-clockevt.c in
>>>> mfgpt_tick() on line 132:
>>>> ...
>>>>     cs5535_clockevent.event_handler(&cs5535_clockevent);
>>>> ...
>>>> cs5535_clockevent.event_handler is null.
>>>>
>>>> Adding some more traces I see mfgpt_tick() gets called before
>>>> clockevents_config_and_register() finishes (invoked from
>>>> cs5535_mfgpt_init() on line 178). So when mfgpt_tick() accessess the
>>>> event_handler, it's NULL. Wrapping the event_handler call on line 132 in
>>>> a null pointer check results in a working system.
>>>>
>>>> The issue is present at least also in kernel 4.13.5. In kernel versions
>>>> <= 4.1-rc6 the cs5535_clockevent worked OK. Kernels >= 4.1-rc7 never
>>>> booted at all on my ALIX 2c3 (last I tried 4.5, then gave up and tried
>>>> 4.13.5 recently), so I don't know when exactly this issue appeared.
>>>>
>>>> My guess is that the timer interrupt is enabled too early. If I change
>>>> the order of clockevents_config_and_register() and
>>>> cs5535_mfgpt_write(cs5535_event_clock, MFGPT_REG_SETUP, val) in
>>>> cs5535_mfgpt_init() like this:
>>>> ...
>>>>     /* Set up the clock event */
>>>>     printk(KERN_INFO DRV_NAME
>>>>         ": Registering MFGPT timer as a clock event, using IRQ %d\n",
>>>>         timer_irq);
>>>>     clockevents_config_and_register(&cs5535_clockevent, MFGPT_HZ,
>>>>                     0xF, 0xFFFE);
>>>>
>>>>     /* Set the clock scale and enable the event mode for CMP2 */
>>>>     val = MFGPT_SCALE | (3 << 8);
>>>>     cs5535_mfgpt_write(cs5535_event_clock, MFGPT_REG_SETUP, val);
>>>> ...
>>>> the system boots and seems to be OK.
>>>
>>> What happens if instead of inverting those two lines, you add
>>> mfgpt_shutdown() early in the init function ?
>>
>> Hi Daniel,
>>
>> I can't call mfgpt_shutdown() before clockevents_config_and_register()
>> as mfgpt_shutdown() wants a struct clock_event_device* which is only
>> available after clockevents_config_and_register(). But I tried calling
>> disable_timer():
>>
>> @@ -152,6 +152,8 @@ static int __init cs5535_mfgpt_init(void)
>>         }
>>         cs5535_event_clock = timer;
>>
>> +       disable_timer(cs5535_event_clock);
>> +
>>         /* Set up the IRQ on the MFGPT side */
>>         if (cs5535_mfgpt_setup_irq(timer, MFGPT_CMP2, &timer_irq)) {
>>                 printk(KERN_ERR DRV_NAME ": Could not set up IRQ %d\n",
>>
>> which should do the same thing. Unfortunately I still get the same BUG.
>>
>> I added some more traces and I think the IRQ comes just after:
>>     ret = setup_irq(timer_irq, &mfgptirq);
>
> Mmh, the interrupt flag tells it is a shared interrupt, bad :/

While there is IRQF_SHARED, I think on the ALIX 2c3 the IRQ is not really 
shared:

$ cat /proc/interrupts
            CPU0
   0:       9959    XT-PIC      timer
   2:          0    XT-PIC      cascade
   4:       1763    XT-PIC      ttyS0
   7:       4648    XT-PIC      cs5535-clockevt
   9:          0    XT-PIC      ath
  10:        151    XT-PIC      eth0
  11:          0    XT-PIC      eth1
  12:          1    XT-PIC      ehci_hcd:usb1, ohci_hcd:usb2
  14:       4063    XT-PIC      pata_cs5536
  15:          0    XT-PIC      eth2

(if this is sufficient proof)

> The interrupt handler checks if the interrupt was for the timer.
>
> So here, there are two options:
>
> - the timer is in an inconsistent state, with an interrupt line up and
> enabled. In this case, it should be reseted correctly before going
> further in the init function (disable, ack irq if any, and then request
> irq and register).
>
> .. or ..
>
> - the check at the beginning of the interrupt handler:
>        /* See if the interrupt was for us */
>        if (!(val & (MFGPT_SETUP_SETUP | MFGPT_SETUP_CMP2 |
> 		MFGPT_SETUP_CMP1)))
>                return IRQ_NONE;
>
>   does not work and we try to handle an interrupt which is not for us.

If this interrupt is not shared, this option can be ruled out, right?

> That could be any of these two (or both) and I suspect the commit
> 8f9327cbb brings to light this problem as before the interrupt handler
> had the check:
> 	if (cs5535_tick_mode == CLOCK_EVT_MODE_SHUTDOWN)
> 		return IRQ_HANDLED;
>
> with:
>
> static unsigned int cs5535_tick_mode = CLOCK_EVT_MODE_SHUTDOWN;

I agree, in the previous version any early calls would be ignored.

I see in http://pcengines.ch/pdf/alix2.pdf on page 10 in "Setup options": 
"M: toggles MFGPT workaround – may be required to support high speed 
timer. See AMD CS5536 data book section 5.16.3 for the gory details. The 
system may hang during boot if you get it wrong…"

I tried toggling the option and it didn't seem to have any effect on this 
issue. Furthermore I did a BIOS update and now the menu option is gone.

The CD5536 Data Book is here: 
https://support.amd.com/TechDocs/33238G_cs5536_db.pdf and that 5.16.3 
issue doesn't seem to be relevant here.

So I think the following must be happening:

> - the timer is in an inconsistent state, with an interrupt line up and
> enabled. In this case, it should be reseted correctly before going
> further in the init function (disable, ack irq if any, and then request
> irq and register).

Should I do something more to verify that it is this case?

Could you please give me more detailed info on how to reset the IRQ to a 
sane sate? Being a kernel noob that summary is not sufficient for me.

Best regards,
David

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