linux-kernel - Re: [RFC PATCH v1 1/1] leds: support to use own workqueue for each LED

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Date:   Mon, 31 Oct 2022 07:01:28 +0000
From:   Arseniy Krasnov <AVKrasnov@...rdevices.ru>
To:     Pavel Machek <pavel@....cz>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-leds@...r.kernel.org" <linux-leds@...r.kernel.org>,
        kernel <kernel@...rdevices.ru>
Subject: Re: [RFC PATCH v1 1/1] leds: support to use own workqueue for each
 LED

On 30.10.2022 23:15, Pavel Machek wrote:
> Hi!
> 
>>>> This allows to set own workqueue for each LED. This may be useful, because
>>>> default 'system_wq' does not guarantee execution order of each work_struct,
>>>> thus for several brightness update requests (for multiple leds), real
>>>> brightness switch could be in random order.
>>>
>>> So.. what?
>>>
>>> Even if execution order is switched, human eye will not be able to
>>> tell the difference.
>> Hello,
>>
>> Problem arises on one of our boards where we have 14 triples of leds(each
>> triple contains R G B). Test case is to play complex animation on all leds:
>> smooth switch from on RGB state to another. Sometimes there are glitches in
>> this process - divergence from expectable RGB state. We fixed this by using
>> ordered workqueue.
> 
> Are there other solutions possible? Like batch and always apply _all_
> the updates you have queued from your the worker code?

IIUC You, it is possible to do this if brightness update requests are performed using
write to "brightness" file in /sys/class/led/. But if pattern trigger mode is used(in my
case) - I can't synchronize these requests as they are created internally in kernel on
timer tick.

Thanks, Arseniy
> 
> Best regards,
> 								Pavel
>