Re: [PATCH] sh_eth: pm_runtime should not need null operations

[Ben Dooks <ben.dooks@codethink.co.uk>]

On 21/03/14 14:24, Geert Uytterhoeven wrote:
> Hi Ben,
>
> (dropping netdev and davem, adding Felipe, Kevin, linux-pm, and
> linux-arm-kernel)
>
> On Fri, Mar 21, 2014 at 11:57 AM, Ben Dooks <ben.dooks@codethink.co.uk> wrote:
>> On 21/03/14 11:30, Geert Uytterhoeven wrote:
>>> On Fri, Mar 21, 2014 at 11:15 AM, Ben Dooks <ben.dooks@codethink.co.uk>
>>> wrote:
>>>> The driver has a no-op for the pm_runtime callbacks but
>>>> the pm_runtime core should correctly ignore drivers without
>>>> any pm_rumtime callback ops.
>>>
>>> The pm_runtime core doesn't ignore non-existing runtime_{suspend,resume}
>>> callbacks, it turns them into a failure withv -ENOSYS.
>>> Only non-existing runtime_idle callbacks are ignored.
>>
>> I've added Rafael Wysocki as he may be able to add better
>> feedback to this issue.
>>
>> [snip rpm_susend code block]
>>
>> I got very confused here. The clock_ops sets dev->pm_domain
>> which over-rides the use of the dev->driver->pm entry as the
>> primary pm for the device. The code above the bit you snipped
>> does a ladder looking for the pm_runtime entry it calls and
>> would stop at finding dev->pm_domain as so:
>>
>> from drivers/base/power/runtime.c:
>>      495         if (dev->pm_domain)
>>      496                 callback = dev->pm_domain->ops.runtime_suspend;
>>   ...
>>      502                 callback = dev->bus->pm->runtime_suspend;
>>      503         else
>>      504                 callback = NULL;
>>
>>
>> So for drivers on shmobile with drivers/sh/pm_runtime.c enabled
>> we would never call the drivers' entry as the ops that this code
>> introduces just calls the pm_clk calls and does not send the
>> events on.
>
> Yes, that's also my understanding.
>
> Commit 4d27e9dcff00a6425d779b065ec8892e4f391661 ("PM: Make
> power domain callbacks take precedence over subsystem ones") explains
> the rationale behind this.
>
> Now, this doesn't prevent your power domain from delegating to other
> callbacks...
>
>> If we send the events on, then we would use pm_generic_runtime_suspend()
>> to send it. This call treats the lack of runtime_pm driver entry as a
>> do nothing and return 0 which means in this case the code in sh_eth
>> is not necessary to have any pm_runtime functions.
>
> If the power domain just calls pm_generic_runtime_suspend(), it will only
> consider the driver-specific callback, bypassing type, class, and
> bus-specific callbacks.
>
> So should the power domain delegate it further using a similar ladder
> strategy like RPM_GET_CALLBACK() at the core pm level, i.e.
> try type/class/bus/driver?
> And type should delegate to class/bus/driver, class to bus/driver, bus to
> driver?
>
>> This means depending on if we have a pm_domain in the path we get
>> different treatment of NULL runtime pm ops pointer. I am not sure
>> how to handle this, as IIRC a number of other drivers for Renesas
>> currently assume that the NULL case is going to be fine for them.
>>
>> Changing pm_generic_runtime_suspend() to return ENOSYS would end
>> up breaking davinci and probably a number of other platforms.
>>
>> So questions:
>>
>> - Should rpm_suspend() ignore the lack of pm_runtime operations?
>> - Do we need to add a generic `ignore pm runtime callback`
>> - Are any other shmobile drivers similarly affected?
>
> The code indeed looks a bit like a mix of:
>    - Lack of callback means it's safe to suspend,
>    - Lack of callback means it's not safe to suspend.

I thought historically NULL tended to mean it did not care about
this.

The whole thing is giving me a headache as I would expect the
suspend to start with device and then work down the layers and
resume to do the opposite. However currently rpm_resume will also
start at the dev->pm_domain.

Should the rpm_resume start with the dev->bus->pm and then work
its way up to the dev->driver->pm callback? If so then the current
davinci code is also going to be wrong...

-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius