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

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Date:	Fri, 28 Aug 2015 11:30:50 +0100
From:	Jon Hunter <jonathanh@...dia.com>
To:	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	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 25/08/15 23:46, Rafael J. Wysocki wrote:
> On 8/25/2015 11:37 AM, Jon Hunter wrote:

[snip]

>> 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.
> 
> One caveat here: system suspend may be invoked at any time, so you need
> to ensure that the device is properly suspended when that happens.
> 
> I believe you at least need a ->suspend callback for that.

Thanks, makes sense.

On a related note, I see a few drivers, including this DMA driver doing
the following in the driver ->remove callback.

    pm_runtime_disable(&pdev->dev);
        !pm_runtime_status_suspended(&pdev->dev))
            tegra_dma_runtime_suspend(&pdev->dev);

I understand that the code is trying to ensure that the device is
suspended regardless of whether rpm is enabled or not in the kernel
config. However, looking at the pm_runtime_status_suspended() function,
AFAICT, it will always return false above as the disable_depth will be
greater than 0. So I am concerned that the tegra_dma_runtime_suspend()
is called even when not needed? However, I could also be missing
something here.

Cheers
Jon


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