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

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Date:	Mon, 24 Aug 2015 19:51:43 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Jon Hunter <jonathanh@...dia.com>,
	Rafael <rafael.j.wysocki@...el.com>
Cc:	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 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.

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

-- 
~Vinod
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