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

[Marek Szyprowski]

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