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

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Date:	Tue, 25 Aug 2015 10:37:35 +0100
From:	Jon Hunter <jonathanh@...dia.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Vinod Koul <vinod.koul@...el.com>
CC:	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 25/08/15 01:04, Rafael J. Wysocki wrote:
> 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.

Vinod, thinking about this some more, I am wondering if it is just
better to get rid of the suspend/resume callbacks and simply handling
the state in the runtime suspend/resume callbacks. I think that would be
safe too, because once the clock has been disabled, then who knows what
the context state will be.

Cheers
Jon
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