Re: [PATCH/RFC RESEND] leds: Use set_brightness_work for brightness_set ops that can sleep

Jacek Anaszewski
2015-07-01T10:47:18Z

On 07/01/2015 09:43 AM, Pavel Machek wrote:
> On Wed 2015-07-01 09:28:52, Jacek Anaszewski wrote:
>> On 06/30/2015 07:46 PM, Pavel Machek wrote:
>>> On Tue 2015-06-30 15:06:19, Jacek Anaszewski wrote:
>>>> On 06/30/2015 01:58 PM, Pavel Machek wrote:
>>>>> On Tue 2015-06-30 10:01:08, Jacek Anaszewski wrote:
>>>>>> This patch rearranges the core LED subsystem code, so that it
>>>>>> now removes from drivers the responsibility of using work queues
>>>>>> internally in case their brightness_set ops can sleep.
>>>>>> Addition of two flags: LED_BRIGHTNESS_FAST and LED_BLINK_DISABLE
>>>>>> as well as new_brightness_value property to the struct led_classdev
>>>>>> allows for employing existing set_brightness_work to do the job.
>>>>>> The modifications allow also to get rid of brightness_set_sync op,
>>>>>> as flash LED devices can now be handled properly only basing on the
>>>>>> SET_BRIGHTNESS_SYNC flag.
>>>>>
>>>>> Are you sure this is good idea?
>>>>>
>>>>> You'll now use single callback for blocking and non-blocking
>>>>> behaviour. I'm pretty sure stuff like lockdep will have some fun with
>>>>> that.
>>>>
>>>> I enabled "Lock Debugging" options and didn't get any warning.
>>>> Could you describe the use case you are thinking of?
>>>
>>> You may get one when one of the sleeping functions uses some lock...
>>
>> Drivers which use spin_lock in their brightness_set op will have to
>> set LED_BRIGHTNESS_FAST flag, which will instruct the LED core to
>> call the op synchronously. On the other hand drivers which can sleep
>> in their brightness_set op won't set the flag, which will make LED core
>> delegating the op to the work queue task. It is also possible that
>> driver with brightness_set op that can sleep set SET_BRIGHTNESS_SYNC
>> flag - then LED core will call it in a synchronous way from
>> led_brightness_set and it will schedule work queue task in case
>> the op is called from triggers.
>
> I understand this "works".
>
>> If you want to NAK the patch, please come up with detailed analysis
>> on how it can cause problems. Without this I infer that you didn't
>> spend a second on analyzing the code. This is counterproductive.
>
> NAK.
>
> Because calling two functions with different semantics through same
> function pointer is extremely ugly, and _will_ cause lockdep
> problems. Talk to the lockdep people for details.

Which two functions are you thinking of? There is a single
brightness_set op to call.