linux-kernel - Re: [PATCH v11 5/6] dmaengine: pl330: Add PM sleep support

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Date:	Wed, 12 Nov 2014 16:53:32 +0100
From:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
To:	Ulf Hansson <ulf.hansson@...aro.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Russell King <linux@....linux.org.uk>,
	Dan Williams <dan.j.williams@...el.com>,
	Vinod Koul <vinod.koul@...el.com>,
	Alan Stern <stern@...land.harvard.edu>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	dmaengine@...r.kernel.org, Kevin Hilman <khilman@...nel.org>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [PATCH v11 5/6] dmaengine: pl330: Add PM sleep support

On śro, 2014-11-12 at 16:28 +0100, Ulf Hansson wrote:
> On 12 November 2014 15:32, Krzysztof Kozlowski <k.kozlowski@...sung.com> wrote:
> > Add system suspend/resume capabilities to the pl330 driver so the amba
> > bus clock could be also unprepared to conserve energy.
> >
> > Signed-off-by: Krzysztof Kozlowski <k.kozlowski@...sung.com>
> > ---
> >  drivers/dma/pl330.c | 37 +++++++++++++++++++++++++++++++++++++
> >  1 file changed, 37 insertions(+)
> >
> > diff --git a/drivers/dma/pl330.c b/drivers/dma/pl330.c
> > index c3bd3584f261..fd71777fc565 100644
> > --- a/drivers/dma/pl330.c
> > +++ b/drivers/dma/pl330.c
> > @@ -2627,6 +2627,42 @@ static int pl330_dma_device_slave_caps(struct dma_chan *dchan,
> >         return 0;
> >  }
> >
> > +/*
> > + * Runtime PM callbacks are provided by amba/bus.c driver.
> > + *
> > + * It is assumed here that IRQ safe runtime PM is chosen in probe and amba
> > + * bus driver will only disable/enable the clock in runtime PM callbacks.
> > + */
> > +static int __maybe_unused pl330_suspend(struct device *dev)
> > +{
> > +       struct amba_device *pcdev = to_amba_device(dev);
> > +
> > +       if (!pm_runtime_status_suspended(dev)) {
> 
> No, this wont work. Consider this scenario:
> 
> pm_runtime_status_suspended() returns "true", which means you consider
> the device to be runtime PM suspended, thus you will unprepare the
> clock below.
> 
> Later, a call to pm_runtime_get_sync() is invoked from "somewhere" and
> before this driver's system PM resume callback has been invoked. That
> will trigger the runtime PM resume callback to be invoked, causing
> clock unbalance issues.

If during system resume the device runtime resume is called BEFORE
normal resume of device, then you're right: clock will be unbalanced. 

But such case does not make sense. pm_runtime_get_sync() is called when
someone wants to use the dmaengine/pl330... but the pl330 driver is
still suspended in that time!

If runtime resume is called AFTER normal resume of device - then it is
fine with my code.

> You need a pm_runtime_disable() prior checking the runtime PM status
> using pm_runtime_status_suspended(). That will prevent this from
> happen.
> 
> I guess you may see where this is leading. I think
> pm_runtime_force_suspend|resume() is well suited for this situation.

I've been there... :)



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