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

[Krzysztof Kozlowski]

On sob, 2014-11-01 at 02:29 +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.
> 
> 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

That makes sense. However I am not familiar with DMA core code as much
as I think it would be needed to make such generic changes :). I'll
stick to one driver for now.

Thanks for comments!
Krzysztof



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