linux-kernel - Re: [PATCH v12 5/6] dmaengine: pl330: Add PM sleep support

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Message-id: <1415976646.5705.8.camel@AMDC1943>
Date:	Fri, 14 Nov 2014 15:50:46 +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 v12 5/6] dmaengine: pl330: Add PM sleep support

On pią, 2014-11-14 at 15:22 +0100, Ulf Hansson wrote:
> On 14 November 2014 14:47, Krzysztof Kozlowski <k.kozlowski@...sung.com> wrote:
> > On pią, 2014-11-14 at 14:31 +0100, Ulf Hansson wrote:
> >> On 14 November 2014 09:50, 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 | 41 +++++++++++++++++++++++++++++++++++++++++
> >> >  1 file changed, 41 insertions(+)
> >> >
> >> > diff --git a/drivers/dma/pl330.c b/drivers/dma/pl330.c
> >> > index c3bd3584f261..e499bb118f0a 100644
> >> > --- a/drivers/dma/pl330.c
> >> > +++ b/drivers/dma/pl330.c
> >> > @@ -2627,6 +2627,46 @@ 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);
> >> > +
> >> > +       pm_runtime_disable(dev);
> >> > +
> >> > +       if (!pm_runtime_status_suspended(dev)) {
> >> > +               /* amba did not disable the clock */
> >> > +               amba_pclk_disable(pcdev);
> >> > +       }
> >> > +       amba_pclk_unprepare(pcdev);
> >>
> >> I would also invoke pm_runtime_set_suspended() here, to reflect that's
> >> the current runtime PM state of the device.
> >
> > Strictly speaking the device is not runtime suspended in that moment. PM
> > runtime callbacks were not called. Although the device status looks like
> > runtime suspended (clocks disabled) but I think the whole goal here was
> > to avoid touching runtime PM because this is system sleep.
> 
> If someone outside, like a PM domain monitors the runtime PM status of
> the device it would get the wrong impression of the device.
> 
> This is quite similar to why the amba bus needs to update the runtime
> PM status during probe(), using pm_runtime_set_active(). Shouldn't we
> do that either?

Hmm... I got your point. However now device is not runtime resumed if it
was runtime suspended before (clock is not enabled). The runtime status
is preserved between system sleep.

Going your way I would have to either:
1. store runtime status somewhere and use it during resume,
2. always runtime resume during resuming (which would be error-prone
because there won't be a real runtime resume).

Current solution looks nice and simple to me.

> >> I guess I sounds like broken record :-), but using
> >> pm_runtime_force_suspend() would help to prevent some code duplication
> >> here.
> >>
> >> Something like this:
> >>
> >> pm_runtime_force_suspend()
> >> if (pm_runtime_is_irq_safe())
> >>    amba_pclk_unprepare(pcdev);
> >
> > With !CONFIG_PM_RUNTIME this would leave clocks prepared...
> 
> No.
> 
> pm_runtime_is_irq_safe() returns false, and thus the amba bus do a
> clk_disable_unprepare() from it's runtime PM suspend callback when
> it's invoked via pm_runtime_force_suspend().

Aaa, right! But during system resume that would leave the device always
runtime resumed. Now we can avoid this :). Anyway this is a conceptual
discussion. I don't mind the force-suspend way but I got an impression
that current way was preferred (separated sleep and runtime paths).

Best regards,
Krzysztof



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