Re: [PATCH] leds: core: use deferred probing if default trigger isn't available yet

[Jacek Anaszewski <jacek.anaszewski@gmail.com>]

On 02/23/2017 10:23 PM, Heiner Kallweit wrote:
> Am 23.02.2017 um 22:08 schrieb Pavel Machek:
>> On Wed 2017-02-22 21:35:52, Heiner Kallweit wrote:
>>> When registering a LED device we have the option to set a default trigger.
>>> Depending on load order of drivers this trigger may not be available yet.
>>> (affected LED device in my case: a DT-configured GPIO LED)
>>> So far if the default trigger can't be found this error is silently
>>> ignored.
>>>
>>> Let's change this to return EPROBE_DEFER if the default trigger can't be
>>> found. This gives the system the chance to probe the LED device later
>>> once the trigger is available.
>>
>> I see a lot of EPROBE_DEFERs on N900, and it is quite nasty, as it
>> spams a log a lot.
>>
> Usually error messages are printed only if there is an error and it is not
> EPROBE_DEFER. However indeed there still may be several drivers not
> taking into account that a subsystem they depend on may return
> EPROBE_DEFER and this should not be treated as "hard error".
> 
>> Rather then re-trying LED registration few times, could we make sure
>> leds are always registered after triggers or something like that?
>>
> I'm afraid if guaranteeing a particular order would be possible w/o
> significant effort then the whole deferred probing concept wouldn't exist.
> 
> I could imagine that we can try reordering definitions in the DTS to
> ensure a certain load order. But this might be somewhat fragile.
> So using the existing concept of deferred probing seems to me to be
> the cleaner solution.

We could go for a solution similar to v4l2-async.c mechanism which
allows for deferring video pipeline linking until all video pipeline
entities are probed, but in the LED subsystem case I think it would be
an overengineering. V4L2 media device has to wait for completion of
probing of several v4l2 drivers, whereas here we've got to wait for
a single driver. EPROBE_DEFER seems to fit for that ideally.

-- 
Best regards,
Jacek Anaszewski