Re: [PATCH 1/3] ARM: EXYNOS: remove non-working AFTR mode support

Daniel Lezcano
2013-06-28T21:47:36Z

On 06/28/2013 06:27 PM, Bartlomiej Zolnierkiewicz wrote:
> On Friday, June 28, 2013 01:20:09 PM Daniel Lezcano wrote:
>> On 06/28/2013 12:11 PM, Tomasz Figa wrote:
>>> Hi Daniel,
>>>
>>> I've been fighting with this whole AFTR state as well, before Bartlomiej.
>>> Let me share my thoughts on this.
>>>

[ ... ]

>>>
>>> If you don't unplug all the CPUs >0 the state is obviously never reached.
>>> Otherwise the whole system hangs after it tries to enter this mode without
>>> any reaction for external events, other than reset.
>>
>> Need investigation.
>>
>> What is the exynos board version where that occurs ?
>
> Could you please tell me what exactly do you mean by that?
>
> I already wrote that we can reproduce the problem on EXYNOS4210 rev0
> and rev1.1 (we don't have rev1.0). Tomek has also reproduced the problem
> on some later SoCs (I hope that he can give you exact revisions).
>
> In our testing we didn't encounter the board on which the problem
> doesn't occur. Our current working theory is that the problem may be
> u-boot (or first stage bootloader) related.

Ok, the status for what I know:

Origen Exynos4210, board ver A: works for me
Arndale Exynos5250: works for me but only if u-boot does not enable the
hypervisor mode.
Chromebook Exynos5250: works for me

I found the following drivers:

https://github.com/AndreiLux/Perseus-UNIVERSAL5410/blob/samsung/arch/arm/mach-exynos/cpuidle.c

https://github.com/CyanogenMod/hardkernel-kernel-4412/blob/cm-10.1/arch/arm/mach-exynos/cpuidle-exynos4.c

Sounds like the num cpus > 1 is still there.

[ ... ]

>> The kernel is not a playground where you can upstream code and then
>> remove it because a feature seems broken and you don't have an idea of why.
>
> Neither me or Tomek did upstream this code and we couldn't react in
> time because we haven't noticed that it is completely unusable for us
> as EXYNOS cpuidle driver is not even used by default on EXYNOS (it is
> not enabled either in defconfig or Kconfig).
>
> Moreover the feature we are talking about (AFTR mode) is also not used
> by default (except EXYNOS4210 rev0 on which it lockups system for us)
> even with EXYNOS cpuidle driver being enabled (because this specific
> feature depends on CPU hot-unplug which is not done automatically right
> now).
>
> Such things like unused/broken code removal is not something very
> unusual in the upstream kernel (I'm speaking from the experience here
> having maintained large subsystem for a couple of years). In this
> particular case we are talking about ~130 lines of code which can
> be trivially brought back later when/if needed.
>
> Anyway if the code removal is controversial for you we can just disable
> AFTR mode by default and enable it only when special command line option
> is given (i.e. "aftr"). This would fix all the broken configuration
> while still allowing the feature to be enabled on systems that had it
> working previously (since you claim that it works on some chipset/u-boot
> configurations).

Actually, there are several reasons I am not in favor for the moment to
remove this code:

1) code can't be pushed upstream and then removed so easily
2) I asked several times what was this cpu1 hack, I had no answer
3) I tried to make both cpus entering the AFTR state, but the cpu1 never
wakes up, I asked but no answer.

I would like to have some answers :)

Before taking the decision to remove this state (btw you can remove the
driver directly, no ? the default idle function is WFI), IMO it is worth
to investigate and to spend some time to clarify what is happening. Then
we can take a decision.

I am willing to help.

>> I asked several times the reasons of why the AFTR state couldn't work
>> with multiple CPUs and I had no answer.
>
> Unfortunately I don't know the answer for your question.
>
> The AFTR mode doesn't work for us *at*all* (even with *one* CPU).
>
>> Frankly speaking I have a couple of hypothesis:
>>
>> 1. something is not correctly setup and the PMU does not wake up the CPU1
>> 2. there is a silicon bug and no one wants to tell it is the case
>>
>> In any case, this must be investigated and identified. And then we can
>> take a decision about this state.
>
> I don't have good idea currently how to investigate it further.
>
> I also don't have any prove that the actual work is worth it
> (and this work can easily take some weeks).
>
> One of main responsibilities of the maintainer it to make sure that
> the code does indeed work and that regressions (like these caused by
> AFTR mode feature) are fixed in the timely manner, not let the code
> sit in the limbo state for large periods of time. It is already very
> bad situation that the regression we are hitting was present since
> v3.4 and we are in v3.10 now, I would like to have it fixed ASAP so
> we may actually consider enabling cpuidle in our exynos_defconfig.

I agree.

Thanks
-- Daniel
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