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Date: Wed, 01 Jul 2015 11:52:11 +0200
From: Jacek Anaszewski <j.anaszewski@samsung.com>
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To: Sakari Ailus <sakari.ailus@iki.fi>
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Subject: Re: [PATCH/RFC v2 1/5] leds: Use set_brightness_work for
 brightness_set ops that can sleep
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 <20150630222450.GR5904@valkosipuli.retiisi.org.uk>
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Hi Sakari,

On 07/01/2015 12:24 AM, Sakari Ailus wrote:
> Hi Jacek,
>
> On Tue, Jun 30, 2015 at 03:59:26PM +0200, Jacek Anaszewski wrote:
>> This patch rearranges the core LED subsystem code, so that it
>> now shifts the responsibility for using work queues from drivers,
>> in case their brightness_set ops can sleep, onto the LED core
>> 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 allows 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.
>
> Nice patch! Thanks!
>
> Looks like this is the favourite topic nowadays. ;-)

Yeah, this allows to believe that we will manage to tackle the issue
finally :)

> The documentation should be improved to tell how the API is expected to be
> have, e.g. which functions may block. I think this is out of scope for this
> patch though.

Yes, I planned to cover this after the patch is accepted.

> I think all the existing drivers that implement the set_brightness()
> callback have a fast (and non-blocking) implementation, and some of these
> drivers use a work queue. In order to avoid modifying those drivers right
> now, how about adding a flag for slow devices instead? "Slow" handlers
> should be those that do at least one of the following: 1) sleep and 2) take
> excessive amount of time to run.

As Andrew Lunn mentioned, he was also working on this issue and he did
the vast majority of work [1] needed to remove work queues from existing
drivers. Only new flags would have to be added.

> How about splitting the patch as follows:
>
> - set_brightness()/set_brightness_sync() -> set_brightness() +
>    LED_BRIGHTNESS_FAST + slow handlers in a work queue,
> - add LED_BLINK_DISABLE flag,
> - fix the heartbeat trigger (it's sleeping in a timer if LED_BRIGHTNESS_SYNC
>    is set).

With my solution heartbeat trigger is not sleeping even if
LED_BRIGHTNESS_SYNC is set as all triggers use the new function -
led_set_brightness_nosleep.

> I'd propose to drop led_set_brightness_async() and just make
> led_set_brightness() asynchronous (or non-blocking if you wish) as it was
> before the LED flash class patches. Considering the nature and tradition of
> the framework, that's probably how most users want it to be. One can always
> use led_set_brightness_sync() if needed.
>
> The caller should indeed decide whether the operation is synchronous or not,
> that's not really a property of the LED. I requested that for the V4L2
> framework due to the very different use cases that are typical for the LED
> class devices.
>
> I have some patches along these lines, but I probably won't have much time
> to work on them, and I can rebase mine on yours later on. If you're
> interested in taking a peek they're here:
>
> <URL:http://salottisipuli.retiisi.org.uk/cgi-bin/gitweb.cgi?p=~sailus/linux.git;a=shortlog;h=refs/heads/leds-as3645a>

I've skimmed through the patches and I find them valuable.
Could you elaborate on the patch
"leds: Serialise access to LED framework data and drivers",
what problem does it solve?

>
> As the result the as3645a driver is quite a bit smaller than the V4L2
> sub-device one, which is a good sign. :-)
>

[1] https://github.com/lunn/linux.git 3.19-rc4-led-devel-workqueue

-- 
Best Regards,
Jacek Anaszewski
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