linux-kernel - Re: [RFC/RFT][PATCH v2 5/7] PM / runtime: Flag to indicate PM sleep transitions in progress

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Message-id: <ca933135-5d87-8813-f1cc-ac3bd0443aff@samsung.com>
Date:   Tue, 13 Sep 2016 09:21:23 +0200
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Lukas Wunner <lukas@...ner.de>
Cc:     Linux PM list <linux-pm@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Mark Brown <broonie@...nel.org>,
        Kevin Hilman <khilman@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        "Luis R. Rodriguez" <mcgrof@...e.com>
Subject: Re: [RFC/RFT][PATCH v2 5/7] PM / runtime: Flag to indicate PM sleep
 transitions in progress

Hi Rafael,


On 2016-09-12 23:25, Rafael J. Wysocki wrote:
> On Monday, September 12, 2016 04:07:27 PM Lukas Wunner wrote:
>> On Thu, Sep 08, 2016 at 11:29:48PM +0200, Rafael J. Wysocki wrote:
>>> Introduce a new flag in struct dev_pm_info, pm_sleep_in_progress, to
>>> indicate that runtime PM has been disabled because of a PM sleep
>>> transition in progress.
>> [...]
>>> That will allow helpers like pm_runtime_get_sync() to be called
>>> during system sleep transitions without worrying about possible
>>> error codes they may return because runtime PM is disabled at
>>> that point.
>> I have a suspicion that this patch papers over the direct_complete bug
>> I reported Sep 10 and that the patch is unnecessary once that bug is
>> fixed.
> It doesn't paper over anything, but it may not be necessary anyway.
>
>> AFAICS, runtime PM is only disabled in two places during the system
>> sleep process: In __device_suspend() for devices using direct_complete,
>> and __device_suspend_late() for all devices.
>>
>> In both of these phases (dpm_suspend() and dpm_suspend_late()), the
>> device tree is walked bottom-up. Since we've reordered consumers to
>> the back of dpm_list, they will be treated *before* their suppliers.
>> Thus, runtime PM is disabled on the consumers first, and only later
>> on the suppliers.
>>
>> Then how can it be that runtime PM is already disabled on the supplier?
> Actually, I think that this was a consequence of a bug in device_reorder_to_tail()
> that was present in the previous iteration of the patchset (it walked suppliers
> instead of consumers).
>
>> The only scenario I can imagine is that the supplier chose to exercise
>> direct_complete, thus was pm_runtime_disabled() in the __device_suspend()
>> phase, and the consumer did *not* choose to exercise direct_complete and
>> later tried to runtime resume its suppliers and itself.
>>
>> I assume this patch is a replacement for Marek's [v2 08/10].
>> @Marek, does this scenario match with what you witnessed?
> It is not strictly a replacement for it.  The Marek's patch was the
> reason to post it, but I started to think about this earlier.
>
> Some people have complained to me about having to deal with error codes
> returned by the runtime PM framework during system suspend, so I thought
> it might be useful to deal with that too.
>
> That said it probably is not necessary right now.

I've tested this patchset without this patch and system sleep with 
device link
enabled worked fine. However this might be also a consequence of 
enabling runtime
pm during system sleep since v4.8-rc1.

It looks that for now this patch can be skipped until a real use case for it
appears.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland