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:	Fri, 07 Nov 2014 13:18:03 +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 pią, 2014-11-07 at 13:13 +0100, Pavel Machek wrote:
> On Wed 2014-11-05 09:42:58, Krzysztof Kozlowski wrote:
> > 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.
> 
> I still don't get it. You say that you don't know how to handle
> !pcdev->irq_safe case... Yet have code above that tries to handle it.
> 
> If that case can't be sanely handled, I'd expect
> BUG_ON(!pcdev->irq_safe).

Hmmm... I could misunderstand your question. The amba/bus.c driver can
handle both cases. However this varies for child drivers (which use
these runtime PM callbacks too). For pl330 cannot handle non-irq-safe.
Other drivers can.

Is it the answer for your question?

Best regards,
Krzysztof


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