linux-kernel - Re: [RFC PATCH 1/7] DMA: tegra-apb: Correct runtime-pm usage

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Date:	Tue, 25 Aug 2015 02:04:50 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Vinod Koul <vinod.koul@...el.com>
Cc:	Jon Hunter <jonathanh@...dia.com>,
	Rafael <rafael.j.wysocki@...el.com>,
	Laxman Dewangan <ldewangan@...dia.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Alexandre Courbot <gnurou@...il.com>,
	dmaengine@...r.kernel.org, linux-tegra@...r.kernel.org,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [RFC PATCH 1/7] DMA: tegra-apb: Correct runtime-pm usage

On Monday, August 24, 2015 07:51:43 PM Vinod Koul wrote:
> On Mon, Aug 24, 2015 at 02:22:49PM +0100, Jon Hunter wrote:
> > 
> > On 24/08/15 10:22, Vinod Koul wrote:
> > > On Mon, Aug 24, 2015 at 09:47:13AM +0100, Jon Hunter wrote:
> > >>
> > >> On 23/08/15 15:17, Vinod Koul wrote:
> > >>> On Tue, Aug 18, 2015 at 02:49:09PM +0100, Jon Hunter wrote:
> > >>>
> > >>>> @@ -1543,7 +1531,7 @@ static int tegra_dma_pm_suspend(struct device *dev)
> > >>>>  	int ret;
> > >>>>  
> > >>>>  	/* Enable clock before accessing register */
> > >>>> -	ret = tegra_dma_runtime_resume(dev);
> > >>>> +	ret = pm_runtime_get_sync(dev);
> > >>>
> > >>> why is this required ?
> > >>
> > >> Because the clock could be disabled when this function is called. This
> > >> function saves the DMA context so that if the context is lost during
> > >> suspend, it can be restored.
> > > 
> > > Have you verified this? Coz my understanding is that when PM does suspend it
> > > will esnure you are runtime resume if runtime suspended and then will do
> > > suspend.
> > > So you do not need to do above
> > 
> > I see what you are saying. I did some testing with ftrace today to trace
> > rpm and suspend/resume calls. If the dma controller is runtime suspended
> > and I do not call pm_runtime_get_sync() above then I do not see any
> > runtime resume of the dma controller prior to suspend. Now I was hoping
> > that this would cause a complete kernel crash but it did not and so the
> > DMA clock did not appear to be needed here (at least on the one board I
> > tested). However, I would not go as far as to remove this and prefer to
> > keep as above.
> 
> Okay am adding Rafael here for his recommendations.

Well, and what is the question I'm supposed to answer, exactly?

I was in Seattle last week, so haven't been following this closely.

> I have tested in past and if my driver was runtime suspended we were resumed
> prior to being suspended. So I am not sure why you did not see that
> behaviour, and if that is right we don't need to force resume here

We're adding code for skipping runtime-resume-before-system-suspend, because
it is not desirable in general.

The rule of thumb is that if you know you need to change the device's settings
(eg. because of wakeup being enabled or not) for system suspend and that
requires the device to be resumed, resume it.  It can stay suspended
otherwise.

Thanks,
Rafael

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