Date: Thu, 2 Jul 2015 00:44:56 +0300
From: Sakari Ailus <sakari.ailus@iki.fi>
To: Jacek Anaszewski <j.anaszewski@samsung.com>
Cc: linux-leds@vger.kernel.org, linux-kernel@vger.kernel.org,
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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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Hi Jacek,

On Wed, Jul 01, 2015 at 02:01:54PM +0200, Jacek Anaszewski wrote:
> 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.

The framework did not implement it but the drivers did. Quite a few drivers
actually change the LED state asynchronously, while the set_brightness()
callback does not block.

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

Well, it could be synchronous, indeed. But synchronous operation is not
guaranteed. The essence of this is that led_set_brightness() does not sleep.
Whether the LED state is changed synchronously or not is not important.

> 
> >>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 don't want to queue work if we just want to write to a register. The work
queue should only be used for slow handlers that are better not called e.g.
from interrupt context.

> We could introduce led_set_brightness_sync API then, which would call
> brightness_set op in a synchronous way.

Considering the pre-flash LED use cases and what the V4L2 flash API
requirements are, my understanding is that we should do with two LED class
API functions for setting LED brightness:

- led_set_brightness (which will not block and is very fast, but the LED
  brightness change may happen asynchronously) and

- led_set_brightness_sync (which is always synchronous).

-- 
Kind regards,

Sakari Ailus
e-mail: sakari.ailus@iki.fi	XMPP: sailus@retiisi.org.uk
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