Re: [PATCH] drivers: cpuidle: don't initialize big.LITTLE driver if MCPM is unavailable

[Sudeep Holla <sudeep.holla@arm.com>]

Hi Lorenzo,

On Thursday 08 January 2015 05:59 PM, Lorenzo Pieralisi wrote:
> On Thu, Jan 08, 2015 at 11:11:40AM +0000, Daniel Lezcano wrote:
>
> [...]
>
>>>> IMO, it would be better to be more strict with the mcpm
>>>> initialization and not let the system boot if something is wrong with
>>>> it which I believe is coming from the firmware and let the user to
>>>> figure out what is really happening by letting him to disable mcpm in
>>>> the kernel configuration (which in turn will disable cpuidle).
>>>
>>> Again I fully agree, but in this case I manually switched to legacy boot
>>> mode on TC2 and used same kernel with MCPM config enabled. Do you mean
>>> to say we should not support that even when developer understand the
>>> consequence of that ?
>>
>> Well, I see there are the exynos5410/5420/5422. For the 5422 on
>> chromebook2 MCPM works well, IIUC. But for the 5422 on odroid-xu3, MCPM
>> does not work, hence cpuidle neither because of the firmware.
>>
>> Silently disabling cpuidle because mcpm did not initialize will hide the
>> issue.
>
> No. MCPM *will* initialize, Sudeep's patch does not silently disable
> CPUidle.
> To put it differently MCPM will initialize if CCI is in the DT and it
> is "available", so unless defined differently in the dts mcpm will be
> available and CPUidle will be initialized (and break if there is an issue
> with the platform FW/HW).
>
> I agree the mechanism to define if MCPM is available can be improved
> but that's what it is at the moment.
>
> The problem here is to boot a platform with different boot methods
> and still have a single kernel image.
>
>> I understand your point about switching to legacy without recompiling
>> the kernel.
>>
>> I suggest we add a big fat WARN_ON when the mcpm initialization fails
>> with your patch.
>
> I think there are multiple facets we are tackling at once here and they
> should not be mixed.
>

I agree, I could have been more clear on which problem I was fixing.

> 1) We left static idle states there to cope with legacy DTBs that were
>     published before we introduced idle states bindings. If we want to
>     boot eg vexpress in legacy mode but single kernel image with MCPM on,
>     we could remove the idle states in DT and the problem would be
>     solved; we can't do that since we were forced to leave the static
>     idle tables. Overall I think this is not the way to fix the issue.
> 2) The idle driver should be initialized if there is an idle state entry
>     method, which in this case is MCPM. If I boot vexpress with MCPM
>     enabled but legacy boot method (ie spin table) with a single kernel image
>     I do not want to warn if the idle states entry method (MCPM) can't be
>     initialized (and I do not want to get a warning if the idle driver is
>     triggering a mcpm_cpu_suspend), so Sudeep's patch is valid and I am
>     against adding a:
>
>     if (WARN_ON(!mcpm_is_available())
>
> 3) Sudeep's patch is not hiding anything. If CCI is in DT, CCI is
>     probed so mcpm_is_available() == true. If the firmware is borked
>     the idle states will be entered and we will notice there is something
>     wrong
>
> So overall I think Sudeep's patch is sound. I also think we should
> improve the way we detect if MCPM is available, and again, I think the
> CPU operations on arm64 are a good example that we can and we should
> replicate.

Thanks for the providing clarification in details.

Regards,
Sudeep