linux-kernel - Re: [PATCH v4 4/4] dma: pl330: add Power Management support

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Date:	Wed, 24 Sep 2014 12:09:23 +0200
From:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
To:	Ulf Hansson <ulf.hansson@...aro.org>
Cc:	Russell King <linux@....linux.org.uk>,
	Dan Williams <dan.j.williams@...el.com>,
	Vinod Koul <vinod.koul@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	dmaengine@...r.kernel.org, Grant Likely <grant.likely@...aro.org>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Michal Simek <michal.simek@...inx.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Randy Dunlap <rdunlap@...radead.org>,
	Alan Stern <stern@...land.harvard.edu>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [PATCH v4 4/4] dma: pl330: add Power Management support

On wto, 2014-09-23 at 17:39 +0200, Ulf Hansson wrote:
> On 18 September 2014 12:09, Krzysztof Kozlowski <k.kozlowski@...sung.com> wrote:
> > On śro, 2014-09-17 at 20:42 +0200, Ulf Hansson wrote:
> >> On 16 September 2014 10:51, Krzysztof Kozlowski <k.kozlowski@...sung.com> wrote:
> >
> > [...]
> >
> >> >
> >> > @@ -2585,6 +2620,34 @@ static int pl330_dma_device_slave_caps(struct dma_chan *dchan,
> >> >         return 0;
> >> >  }
> >> >
> >> > +/*
> >> > + * Assume that IRQ safe runtime PM is chosen in probe and amba bus driver
> >> > + * will only disable/enable the clock in runtime PM suspend/resume.
> >> > + */
> >> > +static int __maybe_unused pl330_suspend(struct device *dev)
> >> > +{
> >> > +       struct amba_device *pcdev = to_amba_device(dev);
> >> > +
> >> > +       if (!pm_runtime_suspended(dev))
> >> > +               amba_pclk_disable(pcdev);
> >>
> >> I would suggest to use pm_runtime_force_suspend() instead of the above.
> >
> > Sure, I can change it but... (see below)
> >
> >>
> >> > +       amba_pclk_unprepare(pcdev);
> >> > +
> >> > +       return 0;
> >> > +}
> >> > +
> >> > +static int __maybe_unused pl330_resume(struct device *dev)
> >> > +{
> >> > +       struct amba_device *pcdev = to_amba_device(dev);
> >> > +
> >> > +       amba_pclk_prepare(pcdev);
> >> > +       if (!pm_runtime_suspended(dev))
> >> > +               return amba_pclk_enable(pcdev);
> >>
> >> The above if statement could be replaced with pm_runtime_force_resume().
> >
> > But that would lead to runtime resuming the device even when it is not
> > needed. We don't have to fully wakeup the device during resume operation
> > when the device was runtime suspended before suspend.
> 
> That's true - but that's not a big issue I believe. I think the below
> problems are worse. :-)
> 
> 1) You don't disable runtime PM, thus not preventing the runtime PM
> callbacks from being invoked.
> 2) You don't update the runtime PM status, thus not reflecting the
> actual state of the device.
> 
> Both the above may cause unbalanced clk_enable|disable calls.
> 
> Don't get me wrong, I certainly think the "don't do runtime PM resume
> during system PM resume" is interesting to solve. But that it's a
> common problem and should be supported by the runtime PM API somehow
> instead. I have been thinking of extending the
> pm_runtime_force_resume() API to support this, let's see when that
> happens. :-)

OK, I get the point. I'll fix this.


> >
> >>
> >> Doing that, means the amba_pclk_enable|disable() don't need to be
> >> exported from the AMBA bus header, but entirely handled by the AMBA
> >> bus itself.
> >>
> >> > +
> >> > +       return 0;
> >> > +}
> >> > +
> >> > +static SIMPLE_DEV_PM_OPS(pl330_pm, pl330_suspend, pl330_resume);
> >> > +
> >> >  static int
> >> >  pl330_probe(struct amba_device *adev, const struct amba_id *id)
> >> >  {
> >> > @@ -2738,6 +2801,9 @@ pl330_probe(struct amba_device *adev, const struct amba_id *id)
> >> >                 pcfg->data_buf_dep, pcfg->data_bus_width / 8, pcfg->num_chan,
> >> >                 pcfg->num_peri, pcfg->num_events);
> >> >
> >>
> >> You need pm_runtime_set_active() here as well.
> >
> > This is done by amba/bus.c. Do I have to do it again?
> 
> Ohh, you are right! No need to add it!
> 
> >
> >>
> >> > +       pm_runtime_irq_safe(&adev->dev);
> >> > +       pm_runtime_put_noidle(&adev->dev);
> >>
> >> Why pm_runtime_put_noidle(), that seems like you might end up leaving
> >> the device in active state - unless you get some request. Likely not
> >> what you want?
> >
> > Hmmm... I am sorry but I do not get the point here. It could be
> > pm_runtime_put() as well because initially we start with counter
> > incremented by amba/bus.c.
> 
> The "no_idle" version will prevent the device from going inactive,
> unless a new cycle of pm_runtime_get() to pm_runtime_put() happens,
> which there are no guarantees of.

Right.

Thanks for reviewing the code. I'll send the next version removing also
the amba_pclk_enable|disable() macros.

Best regards,
Krzysztof

> 
> Kind regards
> Uffe

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