[PATCH] PM / Domains: Power on the PM domain right after attach completes

[Ulf Hansson]

On 20 November 2014 13:17, Grygorii Strashko <grygorii.strashko at ti.com> wrote:
> On 11/19/2014 10:54 AM, Ulf Hansson wrote:
>> [...]
>>
>>>>
>>>> Scenario 5), a platform driver with/without runtime PM callbacks.
>>>> ->probe()
>>>> - do some initialization
>>>> - may fetch handles to runtime PM resources
>>>> - pm_runtime_enable()
>>>
>>> Well, and now how the driver knows if the device is "on" before accessing it?
>>
>> In this case the driver don't need to access the device during
>> ->probe(). That's postponed until sometime later.
>>
>>>
>>>> Note 1)
>>>> Scenario 1) and 2), both relies on the approach to power on the PM
>>>> domain by using pm_runtime_get_sync(). That approach didn't work when
>>>> CONFIG_PM_RUNTIME was unset, but we recently decided to fixed that by
>>>> the below patch, so that's good!
>>>> "[PATCH] PM / domains: Kconfig: always enable PM_RUNTIME when genpd enabled"
>>>>
>>>> Note 2)
>>>> Scenario 3) and 4) use the same principles for managing runtime PM.
>>>> These scenarios needs a way to power on the generic PM domain prior
>>>> probing the device. The call to pm_runtime_set_active(), prevents an
>>>> already powered PM domain from power off until after probe, but that's
>>>> not enough.
>>>>
>>>> Note 3)
>>>> The $subject patch, tried to address the issues for scenario 3) and
>>>> 4). It does so, but will affect scenario 5) which was working nicely
>>>> before. In scenario 5), the $subject patch will cause the generic PM
>>>> domain to potentially stay powered after ->probe() even if the device
>>>> is runtime PM suspended.
>>>
>>> Why would it?  If the device is runtime-suspended, the domain will know
>>> that, because its callbacks will be used for that.  At least, that's
>>> what I'd expect to happen, so is there a problem here?
>>
>> Genpd do knows about the device but it doesn’t get a "notification" to
>> power off. There are no issues whatsoever for driver.
>>
>> This is a somewhat special case. Let's go through an example.
>>
>> 1. The PM domain is initially in powered off state.
>> 2. The bus ->probe() invokes dev_pm_domain_attach() and then the PM
>> domain gets attached to the device.
>> 3. $subject patch causes the PM domain to power on.
>> 4. A driver ->probe() sequence start, following the Scenario 5).
>> 5. The device is initially in runtime PM suspended state and it will
>> remain so during ->probe().
>> 6. The pm_request_idle() invoked after really_probe() in driver core,
>> won't trigger a runtime PM suspend callback to be invoked. In other
>> words, genpd don't get a "notification" that it may power off.
>>
>> In this state, genpd will either power off from the late_initcall,
>> genpd_poweroff_unused() (depending on when the driver was probed) or
>> wait until some device's runtime PM suspend callback is invoked at any
>> later point.
>
> if I understand things right (thanks to Russell), the Power domain may not
>  be powered off not only in above case, but also in some cases when
> driver is unloaded.
>
> AMBA bus for example:
> static int amba_remove(struct device *dev)
> {
>         pm_runtime_get_sync(dev); <---------- GPD=on, dev is active, usage_count >= 1
>         ret = drv->remove(pcdev); <----------- GPD=on, should do balancing put() to compensate all get() made by driver, usage_count == 1
>                                   <----------- GPD=on, should do balancing get() to compensate put() in probe, usage_count == 2
>         pm_runtime_put_noidle(dev); <---------- GPD=on, dev is active, usage_count == 1
>
>         /* Undo the runtime PM settings in amba_probe() */
>         pm_runtime_disable(dev);  <---------- GPD=on, dev is active, usage_count == 1
>         pm_runtime_set_suspended(dev); <---------- GPD=on, dev is suspended, usage_count == 1
>         pm_runtime_put_noidle(dev); <---------- GPD=on, dev is suspended, usage_count == 0
>
>         amba_put_disable_pclk(pcdev);
>         dev_pm_domain_detach(dev, true); <---------- GPD=on, dev is suspended, usage_count == 0

For the generic OF-based PM domain look-up case:
->dev_pm_domain_detach()
    ->genpd_dev_pm_detach() - to remove the device from the PM domain.
       ->genpd_queue_power_off_work() - to power off unused PM domains.

That does the trick, right!?

Kind regards
Uffe