[PATCH] ARM: imx6q: work around faulty PMU irq routing

[Frank Li]

On Tue, Apr 29, 2014 at 6:17 AM, Dirk Behme <dirk.behme at de.bosch.com> wrote:
> On 29.04.2014 11:55, Lucas Stach wrote:
>>
>> Am Dienstag, den 29.04.2014, 13:28 +0800 schrieb Shawn Guo:
>>>
>>> On Fri, Apr 25, 2014 at 11:13:25AM +0200, Lucas Stach wrote:
>>>>
>>>> Am Freitag, den 25.04.2014, 07:37 +0200 schrieb Dirk Behme:
>>>>>
>>>>> On 24.04.2014 22:23, Lucas Stach wrote:
>>>>>>
>>>>>> The i.MX6 PMU has a design errata where the interrupts of all cores
>>>>>> are
>>>>>> wired together into a single irq line. To work around this we have to
>>>>>> bounce the interrupt around all cores until we find the one where the
>>>>>> PMU
>>>>>> counter overflow has happened.
>>>>>>
>>>>>> This causes the perf measurements to be less accurate and we can't
>>>>>> really
>>>>>> handle the case where two cores fire a PMU irq at the same time. The
>>>>>> implemented woraround makes perf at least somewhat useable on imx6
>>>>>> SoCs
>>>>>> with more than one core.
>>>>>>
>> [...]
>>>>>
>>>>> Do you have anything like a test case which shows that it works (at
>>>>> least better) on a !single core with this patch? Compared to a
>>>>> non-patched system?
>>>>>
>>>> Without this patch, running perf top completely kills the system on
>>>> i.MX6q, most likely because of the sheer number of spurious interrupts
>>>> hitting the system from 4 cores. Even the spurious killer doesn't work
>>>> sometimes, so perf is completely busted right now.
>>>>
>>>> With this patch perf has to reduce the sample frequency in order to
>>>> compensate the added irq latency, but at least we get some plausible
>>>> numbers out. Though I won't take any blame if the amount of salt you
>>>> have to apply while looking at those numbers is already a deadly
>>>> dose. ;)
>>>>
>>>> I don't yet have any numbers on how accurate the measurement is, but at
>>>> least things didn't look completely off.
>>>
>>>
>>> If it cannot provide correct/accurate data, I'd say let's not fake it
>>> to, and just let it be completely broken there, so that people can be
>>> aware of the brokenness, and not take inaccurate data as accurate one.
>>>
>> The data isn't bogus, it just isn't as accurate as it could be with a
>> properly working PMU. I'll run some tests with a defined load on
>> Solo/Quad to see how far the measurements are off. I'm fine with holding
>> this patch until then.
>>
>> But the thing is this patch also fixes a serious userspace triggerable
>> DoS on i.MX6q. Just running perf top completely locks up the system
>> because of the sheer number of stray irqs. This isn't the case anymore
>> with this patch applied.

I am very strange. Why PMU can work because below errata?

ERR006259 ARM: Debug/trace functions (PMU, PTM and ETB) are disabled
with absence of JTAG_TCK clock after POR

Default it should no any PMU irqs happen.

best regards
Frank Li

>>
>> Maybe we can just print a warning into dmesg to make the users aware of
>> the imprecise measurement.
>
>
> Yes, I think this sounds like a good compromise :)
>
> Best regards
>
> Dirk
>
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