Re: [PATCH] drivers: led: is31fl319x: 6/9-channel light effect led driver

[H. Nikolaus Schaller]

Hi,

> Am 07.07.2016 um 10:46 schrieb Jacek Anaszewski <j.anaszewski@xxxxxxxxxxx>:
> 
> Hi Nikolaus,
> 
> On 07/06/2016 12:02 PM, H. Nikolaus Schaller wrote:
>> Hi,
>> finally, I found the time to update the driver according to the many comments
>> received a while ago.
>> 
>> Most of them have been worked in, including the regmap idea and
>> brightness_set_blocking().
>> 
>> The driver works on our system, so that I will mail [PATCH v2] as a followup.
>> 
>> There is only one aspect of the new solution I am not sure if it is
>> really better than our old proposal (see below).
>> 
>> 
>>> Am 20.04.2016 um 23:04 schrieb Jacek Anaszewski <jacek.anaszewski@xxxxxxxxx>:
>>> 
>>> On 04/19/2016 07:21 PM, H. Nikolaus Schaller wrote:
>>>>> I believe the LEDS core now handles the workqueues generically for
>>>>> blocking operations, so it's no longer needed in the individual drivers.
>>>> 
>>>> We had a lot of trouble with locking and blocking especially if we
>>>> want to indicate CPU or (root) disk activity.
>>> 
>>> What kind of troubles you had? Could you share more details?
>>> Does it mean that current LED core design doesn't fit for your
>>> use cases?
>> 
>> The system started to flicker the LEDs irregularily and sometimes
>> the whole kernel stalled.
>> 
>>> 
>>>> So it is implemented in a way that changes can be requested faster
>>>> than the I2C bus can write new values to the chip.
>>>> 
>>>> Only after one sequence of I2C writes is done, another work function
>>>> can be scheduled. And each group of writes updates as many LEDs
>>>> in parallel if necessary.
>>> 
>>> You can serialize the operations in brightness_set_blocking with
>>> a mutex.
>> 
>> Yes, that works fine in our (incomplete) test setup.
>> 
>> But I think it assumes that the i2c bus is never congested by other i2c traffic.
>> 
>> I have not found code that obviously takes care of the situation if led
>> trigger events (e.g. mmc or cpu triggers) are coming in faster than the
>> i2c (even using regmap) can write out over i2c.
>> 
>> If I understand the led core code correctly, it will just do another schedule_work
>> for every single change of led brightness.
> 
> Please look at schedule_work documentation in include/linux/workqueue.h:
> 
> /**
> * schedule_work - put work task in global workqueue
> * @work: job to be done
> *
> * Returns %false if @work was already on the kernel-global workqueue and
> * %true otherwise.
> *
> * This puts a job in the kernel-global workqueue if it was not already

Ah, ok. I missed that. Thanks!

> * queued and leaves it in the same position on the kernel-global
> * workqueue otherwise.
> */
> static inline bool schedule_work(struct work_struct *work)
> {
>        return queue_work(system_wq, work);
> }

> 
>> 
>> So I wonder if Is there some guarantee that this work queue will not fill
>> up memory and is really processed faster than being filled? I.e. can the
>> queue overflow?
>> 
>> To reduce this risk, my original implementation strategy was different. The
>> update speed was limited by i2c writing. A new register update batch job
>> was only scheduled if the previous one is finished. If i2c was blocked/congested,
>> the writing worker thread would come to a halt.
>> 
>> All incoming led brightness changes were simply accumulated until a new
>> batch job is started, because LEDs would anyways flicker invisibly fast.
>> 
>> Tests with the new driver have shown that it seems not to run into this situation
>> on our system but it might depend on factors we have not yet tested (slow i2c,
>> other i2c traffic on the same bus, CPU speed, event types choosen).
>> 
>> So I am a little in doubt about this risk. But I may have simply missed
>> the reason why the standard approach works and can never overflow.
>> 
>> BR,
>> Nikolaus
>> 
>> 
>> 
> 
> 
> -- 
> Best regards,
> Jacek Anaszewski