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

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Date:	Sun, 3 May 2015 12:34:36 +0200
From:	Pavel Machek <pavel@....cz>
To:	Stas Sergeev <stsp@...t.ru>
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


> >What about simply "echo 0.001 > delay_on"?
> This is possible.
> But please consider the following reservations:
> - There is already 2 files, so you are not going to write settings
> atomically anyway. When resolution changes, it might be better
> to just reset to the sane defaults (not in my current patch).

Sane defaults would be mandatory, but lets get reasonable interface.

Someone left 1 in delay_on. You want 100 nsec. You echo nsec >
delay_on_units, bang, dead machine, looping in kernel.

Someone left 100 / nsec in delay on. You want one usec. Echo 1 >
delay_on, bang, dead machine.

> - As was already discussed in the same thread, not all drivers
> can support sub-ms delays. For these drivers such resolutions
> should not be available. With separate file this is naturally
> achieved: you either don't create it at all, or list only the possible
> resolutions. With your approach you never know whether you
> can write 0.0001 or not.

Well, so you get back einval. Knowing "unit" is not enough to know how
short delays hw can support.

> - You will set the delay in ms units. For example for 100us you'll
> write 0.1. IMHO it is counter-intuitive: people will make a mistake
> and try 0.0001 instead, wrongly assuming that this is in seconds.
> And nanoseconds should then better be removed, as writing
> nanosecond delay will just require too much zeros.

This is machine-to-machine interface. And users can handle this.
									Pavel
-- 
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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