linux-kernel - Re: [PATCH v9 2/4] amba: Don't unprepare the clocks if device driver wants IRQ safe runtime PM

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Date:	Wed, 05 Nov 2014 09:42:58 +0100
From:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
To:	Pavel Machek <pavel@....cz>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <len.brown@...el.com>,
	Jonathan Corbet <corbet@....net>,
	Russell King <linux@....linux.org.uk>,
	Dan Williams <dan.j.williams@...el.com>,
	Vinod Koul <vinod.koul@...el.com>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Alan Stern <stern@...land.harvard.edu>,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	dmaengine@...r.kernel.org, Lars-Peter Clausen <lars@...afoo.de>,
	Michal Simek <michal.simek@...inx.com>,
	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 v9 2/4] amba: Don't unprepare the clocks if device driver
 wants IRQ safe runtime PM

On wto, 2014-11-04 at 21:18 +0100, Pavel Machek wrote:
> On Tue 2014-11-04 13:52:48, Krzysztof Kozlowski wrote:
> > The AMBA bus driver defines runtime Power Management functions which
> > disable and unprepare AMBA bus clock. This is problematic for runtime PM
> > because unpreparing a clock might sleep so it is not interrupt safe.
> > 
> > However some drivers may want to implement runtime PM functions in
> > interrupt-safe way (see pm_runtime_irq_safe()). In such case the AMBA
> > bus driver should only disable/enable the clock in runtime suspend and
> > resume callbacks.
> 
> 
> 
> >  /*
> >   * Hooks to provide runtime PM of the pclk (bus clock).  It is safe to
> >   * enable/disable the bus clock at runtime PM suspend/resume as this
> > @@ -95,8 +102,14 @@ static int amba_pm_runtime_suspend(struct device *dev)
> >  	struct amba_device *pcdev = to_amba_device(dev);
> >  	int ret = pm_generic_runtime_suspend(dev);
> >  
> > -	if (ret == 0 && dev->driver)
> > -		clk_disable_unprepare(pcdev->pclk);
> > +	if (ret == 0 && dev->driver) {
> > +		pcdev->irq_safe = get_pm_runtime_irq_safe(dev);
> > +
> > +		if (pcdev->irq_safe)
> > +			clk_disable(pcdev->pclk);
> > +		else
> > +			clk_disable_unprepare(pcdev->pclk);
> > +	}
> 
> So you can handle the case of !pcdev->irq_safe. What is the penalty
> for always assuming !pcdev->irq_safe?

The penalty (for pl330 driver) would be that the runtime resume/suspend
cannot happen from atomic context
  => pm_runtime_get_sync() cannot be called from atomic context
    => complete rework of runtime PM for pl330 DMA driver because now
       one of pm_runtime_get_sync() calls is in device_issue_pending
       callback which may not sleep. And by "rework" I also mean that
       I do not know how to do this... yet.

Best regards,
Krzysztof





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