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

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Date:	Mon, 24 Aug 2015 14:22:49 +0100
From:	Jon Hunter <jonathanh@...dia.com>
To:	Vinod Koul <vinod.koul@...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 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.

Furthermore, other drivers do similar things, including the sirf dma
controller (see sirf-dma.c).

Cheers
Jon

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