linux-kernel - Re: [PATCH/RFC RESEND] leds: Use set_brightness_work for brightness

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Message-ID: <20150701074327.GB11385@amd>
Date:	Wed, 1 Jul 2015 09:43:27 +0200
From:	Pavel Machek <pavel@...x.de>
To:	Jacek Anaszewski <j.anaszewski@...sung.com>
Cc:	linux-leds@...r.kernel.org, linux-kernel@...r.kernel.org,
	Bryan Wu <cooloney@...il.com>,
	Richard Purdie <rpurdie@...ys.net>,
	Stas Sergeev <stsp@...rs.sourceforge.net>,
	Sakari Ailus <sakari.ailus@...ux.intel.com>,
	Andreas Werner <andreas.werner@....de>,
	Andrew Lunn <andrew@...n.ch>,
	Antonio Ospite <ospite@...denti.unina.it>,
	Atsushi Nemoto <anemo@....ocn.ne.jp>,
	Ben Dooks <ben@...tec.co.uk>, Chris Boot <bootc@...tc.net>,
	Dan Murphy <dmurphy@...com>,
	Daniel Jeong <daniel.jeong@...com>,
	Daniel Mack <daniel@...que.org>,
	"David S. Miller" <davem@...emloft.net>,
	Fabio Baltieri <fabio.baltieri@...il.com>,
	Felipe Balbi <balbi@...com>,
	Florian Fainelli <florian@...nwrt.org>,
	"G.Shark Jeong" <gshark.jeong@...il.com>,
	Guennadi Liakhovetski <g.liakhovetski@....de>,
	Ingi Kim <ingi2.kim@...sung.com>,
	Jan-Simon Moeller <dl9pf@....de>,
	Johan Hovold <johan@...nel.org>, John Lenz <lenz@...wisc.edu>,
	Jonas Gorski <jogo@...nwrt.org>,
	Kim Kyuwon <q1.kim@...sung.com>,
	Kristian Kielhofner <kris@...sk.org>,
	Kristoffer Ericson <kristoffer.ericson@...il.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Mark Brown <broonie@...nel.org>,
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	Milo Kim <milo.kim@...com>,
	Márton Németh <nm127@...email.hu>,
	Nate Case <ncase@...-inc.com>, NeilBrown <neilb@...e.de>,
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	Peter Meerwald <p.meerwald@...-electronic.com>,
	Phil Sutter <n0-1@...ewrt.org>,
	Philippe Retornaz <philippe.retornaz@...l.ch>,
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	Rod Whitby <rod@...tby.id.au>, Dave Hansen <dave@...1.net>,
	Rodolfo Giometti <giometti@...ux.it>,
	"Sebastian A. Siewior" <bigeasy@...utronix.de>,
	Shuah Khan <shuahkhan@...il.com>,
	Simon Guinot <sguinot@...ie.com>,
	Álvaro Fernández Rojas <noltari@...il.com>
Subject: Re: [PATCH/RFC RESEND] leds: Use set_brightness_work for
 brightness_set ops that can sleep

On Wed 2015-07-01 09:28:52, Jacek Anaszewski wrote:
> On 06/30/2015 07:46 PM, Pavel Machek wrote:
> >On Tue 2015-06-30 15:06:19, Jacek Anaszewski wrote:
> >>On 06/30/2015 01:58 PM, Pavel Machek wrote:
> >>>On Tue 2015-06-30 10:01:08, Jacek Anaszewski wrote:
> >>>>This patch rearranges the core LED subsystem code, so that it
> >>>>now removes from drivers the responsibility of using work queues
> >>>>internally in case their brightness_set ops can sleep.
> >>>>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 allow 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.
> >>>
> >>>Are you sure this is good idea?
> >>>
> >>>You'll now use single callback for blocking and non-blocking
> >>>behaviour. I'm pretty sure stuff like lockdep will have some fun with
> >>>that.
> >>
> >>I enabled "Lock Debugging" options and didn't get any warning.
> >>Could you describe the use case you are thinking of?
> >
> >You may get one when one of the sleeping functions uses some lock...
> 
> Drivers which use spin_lock in their brightness_set op will have to
> set LED_BRIGHTNESS_FAST flag, which will instruct the LED core to
> call the op synchronously. On the other hand drivers which can sleep
> in their brightness_set op won't set the flag, which will make LED core
> delegating the op to the work queue task. It is also possible that
> driver with brightness_set op that can sleep set SET_BRIGHTNESS_SYNC
> flag - then LED core will call it in a synchronous way from
> led_brightness_set and it will schedule work queue task in case
> the op is called from triggers.

I understand this "works".

> If you want to NAK the patch, please come up with detailed analysis
> on how it can cause problems. Without this I infer that you didn't
> spend a second on analyzing the code. This is counterproductive.

NAK.

Because calling two functions with different semantics through same
function pointer is extremely ugly, and _will_ cause lockdep
problems. Talk to the lockdep people for details.

								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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