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

[Krzysztof Kozlowski <k.kozlowski@samsung.com>]

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