Subject: Re: [RFC PATCH 1/7] DMA: tegra-apb: Correct runtime-pm usage
To: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Vinod Koul <vinod.koul@intel.com>
References: <1439905755-25150-1-git-send-email-jonathanh@nvidia.com>
 <55DB1AA9.7090906@nvidia.com> <20150824142143.GK13546@localhost>
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 <55DCF056.40004@intel.com>
CC: Laxman Dewangan <ldewangan@nvidia.com>,
        Stephen Warren <swarren@wwwdotorg.org>,
        Thierry Reding <thierry.reding@gmail.com>,
        "Alexandre Courbot" <gnurou@gmail.com>, <dmaengine@vger.kernel.org>,
        <linux-tegra@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <devicetree@vger.kernel.org>
From: Jon Hunter <jonathanh@nvidia.com>
Message-ID: <55E0385A.6070203@nvidia.com>
Date: Fri, 28 Aug 2015 11:30:50 +0100
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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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