Re: Async runtime put in __device_release_driver()

[Tomi Valkeinen <tomi.valkeinen@ti.com>]

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On 2013-11-05 23:29, Ulf Hansson wrote:
> On 23 October 2013 12:11, Tomi Valkeinen <tomi.valkeinen@ti.com> wrote:
>> Hi,
>>
>> I was debugging why clocks were left enabled after removing omapdss
>> driver, and I found this commit:
>>
>> fa180eb448fa263cf18dd930143b515d27d70d7b (PM / Runtime: Idle devices
>> asynchronously after probe|release)
>>
>> I don't understand how that is supposed to work.
>>
>> When a driver is removed, instead of using pm_runtime_put_sync() the
>> commit uses pm_runtime_put(), so the runtime_suspend call is queued. But
>> who is going to handle the queued suspend call, as the driver is already
>> removed? At least in my case, obviously nobody, as I only get
>> runtime_resume call in my driver, never the runtime_suspend.
>>
>> Is there something I need to add to my driver to make this work, or
>> should that part of the patch be reverted?
> 
> I believe it is quite common that a device driver calls
> pm_runtime_get_sync as a part of it's remove callback, then it
> explicitly returns it's resources that has been fetched during probe.
> Like a clk_disable_unprepare for example.

I guess you mean the driver calls pm_runtime_get_sync _and_
pm_runtime_put_sync as part of its remove callback?

Probably bus drivers need to do that, but for memory mapped devices in a
SoC, I don't think there's normally any need to do
pm_runtime_get/put_sync during the remove callback.

> The idea behind the change in __device_release_driver, was to try to
> prevent  devices from going active->idle->active and instead just
> remain active (if possible).
>
> In your case, which seems like a more modern way of implementing
> "remove", you shall call "pm_runtime_suspend" to make sure the
> runtime_suspend callbacks gets called.

And as far as I understand, the change creates an explicit requirement
to do either pm_runtime_get/put_sync or pm_runtime_suspend inside
driver's remove callback. If so, that should be mentioned in big red
letters in the pm-runtime documentation.

The runtime_pm.txt doc does mention something related to this (and btw,
the doc says pm_runtime_put_sync is being called, which is no longer
true), but nothing clear about how the driver remove callback must be
implemented.

I tried grepping the kernel sources to find out if pm_runtime_suspend is
widely used to get SoC platform devices to suspend, but it doesn't seem
like it is. I didn't see pm_runtime_get/put_sync being used in remove
callbacks widely either, but that was more difficult one to grep.

 Tomi



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