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

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  Re: [RFC PATCH 1/7] DMA: tegra-apb: Correct runtime-pm usage Vinod Koul <vinod.koul@intel.com> - 2015-08-23 16:20 +0200
    Re: [RFC PATCH 1/7] DMA: tegra-apb: Correct runtime-pm usage Jon Hunter <jonathanh@nvidia.com> - 2015-08-24 10:50 +0200
      Re: [RFC PATCH 1/7] DMA: tegra-apb: Correct runtime-pm usage Vinod Koul <vinod.koul@intel.com> - 2015-08-24 11:30 +0200
        Re: [RFC PATCH 1/7] DMA: tegra-apb: Correct runtime-pm usage Jon Hunter <jonathanh@nvidia.com> - 2015-08-24 15:30 +0200
          Re: [RFC PATCH 1/7] DMA: tegra-apb: Correct runtime-pm usage Vinod Koul <vinod.koul@intel.com> - 2015-08-24 16:20 +0200
            Re: [RFC PATCH 1/7] DMA: tegra-apb: Correct runtime-pm usage "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-08-25 01:40 +0200
              Re: [RFC PATCH 1/7] DMA: tegra-apb: Correct runtime-pm usage Jon Hunter <jonathanh@nvidia.com> - 2015-08-25 11:40 +0200
                Re: [RFC PATCH 1/7] DMA: tegra-apb: Correct runtime-pm usage "Rafael J. Wysocki" <rafael.j.wysocki@intel.com> - 2015-08-26 00:50 +0200
                  Re: [RFC PATCH 1/7] DMA: tegra-apb: Correct runtime-pm usage Jon Hunter <jonathanh@nvidia.com> - 2015-08-28 12:40 +0200

#1211635 — Re: [RFC PATCH 1/7] DMA: tegra-apb: Correct runtime-pm usage

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 ?

-- 
~Vinod

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#1211915

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.

Cheers
Jon

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#1211989

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

-- 
~Vinod
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#1212175

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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#1212233

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.

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

-- 
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#1212578

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

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#1212907

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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#1213417

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
>>>>> 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.

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.

Cheers,
Rafael

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#1215268

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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