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Message-id: <1415176978.15850.11.camel@AMDC1943>
Subject: Re: [PATCH v9 2/4] amba: Don't unprepare the clocks if device driver
 wants IRQ safe runtime PM
From: Krzysztof Kozlowski <k.kozlowski@samsung.com>
To: Pavel Machek <pavel@ucw.cz>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>, Len Brown <len.brown@intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Russell King <linux@arm.linux.org.uk>,
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        Vinod Koul <vinod.koul@intel.com>,
        Ulf Hansson <ulf.hansson@linaro.org>,
        Alan Stern <stern@rowland.harvard.edu>, linux-pm@vger.kernel.org,
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        Lars-Peter Clausen <lars@metafoo.de>,
        Michal Simek <michal.simek@xilinx.com>,
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        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Kyungmin Park <kyungmin.park@samsung.com>,
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Date: Wed, 05 Nov 2014 09:42:58 +0100
In-reply-to: <20141104201834.GC15071@amd>
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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