Re: [PATCH/RFC v2 1/5] leds: Use set_brightness_work for brightness_set ops that can sleep

[Jacek Anaszewski]

On 07/01/2015 11:52 AM, Jacek Anaszewski wrote:
> 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.

led_set_brightness called brightness_set op in a synchronous way
before LED flash class patches. It was up to driver how it implemented
the op - blocking or non-blocking. API was not async by default then.

Adding public API led_set_brightness_sync would introduce ambiguity, as 
led_set_brightness also can be synchronous.

> > 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 agree that caller should decide, but we would have to have unambiguous
API for this. I am wondering if renaming led_set_brightness to
led_set_brightness_async and making it always scheduling the work queue
would be harmless solution. This would modify only kernel internal API.
We could introduce led_set_brightness_sync API then, which would call
brightness_set op in a synchronous way.

> > 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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