Re: [PATCH v8 1/5] PM / Runtime: Add getter for querying the IRQ safe option

[Vinod Koul]

On Sat, Nov 01, 2014 at 02:29:42AM +0200, Laurent Pinchart wrote:
> Hi Krzysztof,
> 
> On Friday 31 October 2014 15:40:16 Krzysztof Kozlowski wrote:
> > On piÄ, 2014-10-31 at 15:22 +0100, Pavel Machek wrote:
> > > On Fri 2014-10-31 10:14:55, Krzysztof Kozlowski wrote:
> > >> On pon, 2014-10-20 at 11:04 +0200, Krzysztof Kozlowski wrote:
> > >>> Add a simple getter pm_runtime_is_irq_safe() for querying whether
> > >>> runtime PM IRQ safe was set or not.
> > >>> 
> > >>> Various bus drivers implementing runtime PM may use choose to suspend
> > >>> differently based on IRQ safeness status of child driver (e.g. do not
> > >>> unprepare the clock if IRQ safe is not set).
> > >>> 
> > >>> Signed-off-by: Krzysztof Kozlowski <k.kozlowski@xxxxxxxxxxx>
> > >>> Reviewed-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
> > >> 
> > >> Rafael, Len, Pavel,
> > >> 
> > >> Is proposed API ok? Do you have any comments?
> > >> 
> > >> I'll upload whole patchset to Russell's patch tracking system. However
> > >> an ack from PM maintainer is probably needed.
> > > 
> > > I don't like the API. Having callbacks work in different context (irq
> > > / noirq) based on what another function reports is ugly.
> > > 
> > > What is the penalty if we always decide callbacks are not IRQ safe?
> > 
> > Then pm_runtime_get_sync() could not be called in atomic context. The
> > pl330 runtime PM would have to be completely reworked because one
> > pm_runtime_get_sync() is called in device_issue_pending which cannot
> > sleep (at least in non preemptible kernels). Probably this can be solved
> > some way...
> 
> Many other drivers suffer from the same problem. While I won't reject your 
> proposed fix, I would prefer a more generic approach.
> 
> One option that has been discussed previously was to use a work queue to delay 
> starting the DMA transfer to an interruptible context where 
> pm_runtime_get_sync() could be called. However, as Russell pointed out [1], 
> even that won't work in all cases as the DMA slave might need the transfer to 
> be started before enabling part of its hardware (OMAP audio seem to be such a 
> case).
> 
> I've heard a rumor of a possible DMA engine rework to forbid calling the 
> descriptor preparation API from atomic context. This could be used as a base 
> to implement runtime PM, as DMA slave drivers should not prepare descriptors 
> if they don't need to use them. However that's a long term plan, and we need a 
> solution sooner than that.

Well it is not a rumour :)

I have been contemplating that now that async_tx will be killed so we dont
have to worry about that usage. For the slave dma usage, we can change the
prepare API to be non atomic. I think the users will be okay with approach.
This way drivers can use runtime pm calls in prepare.

-- 
~Vinod

> 
> I've been toying with the idea of adding explicit open/close (or whatever we 
> would call them) operations to the DMA engine API. Those would be used by DMA 
> slave drivers to signal that they will start/stop using the DMA engine.
> 
> If (1) we must start the DMA synchronously with a DMA slave call, (2) need to 
> sleep to handle PM, and (3) don't want to keep the DMA engine powered for as 
> long as one channel is requested, then we need to turn at least preparation as 
> not callable in atomic context, or introduce a new operation.
> 
> [1] http://www.spinics.net/lists/dmaengine/msg01548.html
> 
> > >>> --- a/Documentation/power/runtime_pm.txt
> > >>> +++ b/Documentation/power/runtime_pm.txt
> > >>> 
> > >>> @@ -468,6 +468,10 @@ drivers/base/power/runtime.c and
> > >>> include/linux/pm_runtime.h:
> > >>>      - set the power.irq_safe flag for the device, causing the
> > >>>      runtime-PM
> > >>>      
> > >>>        callbacks to be invoked with interrupts off
> > >>> 
> > >>> +  bool pm_runtime_is_irq_safe(struct device *dev);
> > >>> +    - return true if power.irq_safe flag was set for the device,
> > >>> causing
> > >>> +      the runtime-PM callbacks to be invoked with interrupts off
> > >>> +
> > >>> 
> > >>>    void pm_runtime_mark_last_busy(struct device *dev);
> > >>>    
> > >>>      - set the power.last_busy field to the current time
> 
> -- 
> Regards,
> 
> Laurent Pinchart
> 

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