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

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Date:   Mon, 12 Sep 2016 23:25:36 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     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>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        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 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.

Thanks,
Rafael