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: <3066752.zjH0tZW1Ns@vostro.rjw.lan>
Date:   Wed, 14 Sep 2016 01:59:41 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     Lukas Wunner <lukas@...ner.de>,
        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

On Tuesday, September 13, 2016 09:21:23 AM Marek Szyprowski wrote:
> 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.

OK, thanks!