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

[Jacek Anaszewski]

On 06/30/2015 01:41 PM, Stas Sergeev wrote:
> 30.06.2015 11:27, Jacek Anaszewski ÐÐÑÐÑ:
> > On 06/29/2015 05:17 PM, Stas Sergeev wrote:> 29.06.2015 17:05, Jacek Anaszewski ÐÐÑÐÑ:
> > > > +         * 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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