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

From: Kevin Hilman <khilman@kernel.org>
To: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Sudeep Holla <Sudeep.Holla@arm.com>,
	"linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Kevin Hilman <kevin.hilman@linaro.org>,
	"nicolas.pitre@linaro.org" <nicolas.pitre@linaro.org>
Subject: Re: [PATCH] drivers: cpuidle: don't initialize big.LITTLE driver if MCPM is unavailable
Date: Fri, 09 Jan 2015 09:34:46 -0800	[thread overview]
Message-ID: <7hoaq7vow9.fsf@deeprootsystems.com> (raw)
In-Reply-To: <54AEEDDC.3070609@linaro.org> (Daniel Lezcano's message of "Thu, 08 Jan 2015 21:51:40 +0100")

Daniel Lezcano <daniel.lezcano@linaro.org> writes:

> On 01/08/2015 09:27 PM, Kevin Hilman wrote:
>> Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> writes:
>>
>>> 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.
>>>
>>> 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.
>>
>> This patch disables CPUidle all together, but shouldn't it just disable
>> the states that rely on MCPM?  IOW, C1 should still work just fine since
>> it doesn't use MCPM, right?  So, rather than fail the init, it should
>> just drop any MCPM states (e.g. set ->state_count = 1)
>
> Well, that means we will have a cpuidle driver with the WFI state only
> which is the default idle function when there is no cpuidle driver (+
> without the governor math).

Ah, OK.  Then it makes more sense to disable the driver all together.

Feel free to add

Acked-by: Kevin Hilman <khilman@linaro.org>

Thanks,

Kevin