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

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Message-ID: <553EA6B9.5040309@list.ru>
Date:	Tue, 28 Apr 2015 00:14:33 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Pavel Machek <pavel@....cz>
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

Hi.

27.04.2015 23:54, Pavel Machek пишет:
> Hi!
>
>> The following patches improve the precision of led
>> timer trigger and add the delay unit control.
>> That allows to make PWM brightness control with timer
>> trigger.
> Are you sure that is good idea? Doing LED pwm with main cpu is quite harsh...
Do you remember the pc-speaker driver? :)
In fact I tried the maximal possible freq on armada-xp
board (it doesn't go above approx 100KHz, perhaps this
all the HW timers can do on that board) and I haven't
measured any noticeable CPU load with htop.

> We already have "brightness" for... well... brightness level.
>
> Would it make sense to have an option (Kconfig?) to do PWM using timer on hardware
> that does not do hardware PWM?
I think it would make sense, but not for the "timer" trigger.
Maybe for "none" trigger, or inventing another trigger called
"pwm".
For the timer trigger I would pretty much like my approach to stay.
The reason is that the PWM I need to do, is not strictly a PWM -
it needs the ON period in range of tens or hundreds of milliseconds,
while the OFF period is in a couple of usecs (or vice-versa). No
generic PWM driver can afford such distribution, so I'd like to have
the "timer" trigger for the unusual things like that.
As I said, your idea can still be implemented for "none" trigger
or some new trigger, so its still all for good.
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