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

[Jacek Anaszewski]

Hi Sakari,

On 07/01/2015 11:44 PM, Sakari Ailus wrote:
> 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.

Roughly half of the total number of drivers use work queues.
The rest of them set the brightness synchronously.

When I run following command:

find drivers/leds -name "*.c" | xargs grep INIT_WORK | awk '{print $1}' 
| uniq | wc -l

it shows 31.

> 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.

I agree. I changed the approach in the new version of the patch set.

> > > > 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.

Right. Ignore this.

> > 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).

Exactly.

--
Best Regards,
Jacek Anaszewski
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