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

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Message-id: <55928E7B.2020306@samsung.com>
Date:	Tue, 30 Jun 2015 14:41:31 +0200
From:	Jacek Anaszewski <j.anaszewski@...sung.com>
To:	Stas Sergeev <stsp@...t.ru>
Cc:	Stas Sergeev <stsp@...rs.sourceforge.net>,
	linux-leds@...r.kernel.org, linux-kernel@...r.kernel.org,
	Bryan Wu <cooloney@...il.com>,
	Richard Purdie <rpurdie@...ys.net>,
	Pavel Machek <pavel@....cz>,
	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>,
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	Álvaro Fernández Rojas <noltari@...il.com>
Subject: Re: [PATCH/RFC RESEND] leds: Use set_brightness_work for
 brightness_set ops that can sleep

On 06/30/2015 01:41 PM, Stas Sergeev wrote:
> 30.06.2015 11:27, Jacek Anaszewski пишет:
>> On 06/29/2015 05:17 PM, Stas Sergeev wrote:> 29.06.2015 17:05, Jacek Anaszewski пишет:
>>>> +         * If need to disable soft blinking delegate this to the
>>>> +         * work queue task to avoid problems in case we are
>>>> +         * called from hard irq context.
>>>> +         */
>>>> +        led_cdev->flags |= LED_BLINK_DISABLE;
>>> Wouldn't it be better to just enforce the callers
>>> to explicitly disable software blink, so that it to
>>> never happen from irq context? Something like in this
>>> patch:
>>> https://lkml.org/lkml/2015/5/13/491
>>>
>>
>> Blinking can be disabled not only by removing trigger explicitly,
>> but also by setting brightness to 0 and led_set_brightness
>> can be called from hard irq context. set_brightness_work
>> was originally introduced exactly for this use case.
> Could you please describe where does this happen?

This in fact doesn't take place in the mainline kernel,
however there are some out of tree users apparently [1].
We could remove this possibility, just give me a reason
why we should do that? If we removed it, we would force
any potential driver wanting to call led_set_brightness(LED_OFF)
from hard irq context to use the work queue on its own.

Modifications I am proposing also need set_brightness_work,
so there is almost no cost of keeping the support for calling 
led_set_brightness from hard irq context intact.

> I can see LED_OFF happening only in led_heartbeat_function(), but
> it doesn't use soft-blink, so it will not activate the delayed
> timer cancel.
>
> I would suggest the patch below to make it explicit that
> led_heartbeat_function() doesn't want to cancel soft blink.
> Note that led_heartbeat_function() already uses led_set_brightness_async()
> in a normal case.

With the new approach it should call led_set_brightness_nosleep there.

>
> diff --git a/drivers/leds/trigger/ledtrig-heartbeat.c b/drivers/leds/trigger/ledtrig-heartbeat.c
> index fea6871..0f89d12 100644
> --- a/drivers/leds/trigger/ledtrig-heartbeat.c
> +++ b/drivers/leds/trigger/ledtrig-heartbeat.c
> @@ -37,7 +37,7 @@ static void led_heartbeat_function(unsigned long data)
>   	unsigned long delay = 0;
>
>   	if (unlikely(panic_heartbeats)) {
> -		led_set_brightness(led_cdev, LED_OFF);
> +		led_set_brightness_async(led_cdev, LED_OFF);
>   		return;
>   	}
>
>

[1] http://www.spinics.net/lists/linux-leds/msg00006.html

-- 
Best Regards,
Jacek Anaszewski
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