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

From: Rafael J. Wysocki
Date: Tue Aug 25 2015 - 18:46:56 EST

On 8/25/2015 11:37 AM, Jon Hunter wrote:
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
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
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.

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.


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