linux-kernel - Re: [PATCH v2 0/3] leds: blink resolution improvements

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Date:	Thu, 30 Apr 2015 20:11:41 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Jacek Anaszewski <j.anaszewski@...sung.com>
CC:	linux-leds@...r.kernel.org,
	Linux kernel <linux-kernel@...r.kernel.org>,
	Stas Sergeev <stsp@...rs.sourceforge.net>
Subject: Re: [PATCH v2 0/3] leds: blink resolution improvements

28.04.2015 15:58, Jacek Anaszewski пишет:
>>> I tried it with Samsung M0 board and
>>> my leds-aat1290 driver. It didn't work well. And for small delay
>>> intervals it will not have a chance to work reliably with all drivers,
>>> especially the ones which use mutex in their brightness_set op,
>>> since mutex can sleep.
>> OK, I can remove the nsec resolution.
> 
> usec also didn't work, please look at my use case and warning:
> 
> echo "timer" > trigger
> echo 1 > delay_on
> echo 1 > delay_off
> echo usec > delay_unit
> [  178.584433] hrtimer: interrupt took 300747 ns
> 
> Only some time later I realized that for AAT1290 brightness is set
> through ASCwire pulse protocol, which takes few ms.
> 
> Please note that with this approach users would have to wonder why
> they are getting the warnings and why they can't get their LEDs to work
> with given settings.
I've now found that the drivers itself use a work queue
when needed. And some drivers, like leds_gpio, even do this:
---
        if (led_dat->can_sleep) {
                led_dat->new_level = level;
                schedule_work(&led_dat->work);
        } else {
                set_brightness_now();
        }
---
So it seems the problem is already solved on the per-driver
basis. I don't have leds-aat1290 driver, it is probably not
in the kernel. It is likely forgetting to use the work-queue
the way all other drivers do. So I think my patch is good for
the in-kernel drivers.

There is also a led_cdev->set_brightness_work, and it looks
unused. I could use it for my patch, but for what, if the
drivers already use the work queue when needed?
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