Re: [PATCH V4] leds: trigger: Introduce an USB port trigger

From: Pavel Machek <pavel@ucw.cz>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: "Rafał Miłecki" <zajec5@gmail.com>,
	"Greg KH" <gregkh@linuxfoundation.org>,
	"Richard Purdie" <rpurdie@rpsys.net>,
	"Jacek Anaszewski" <j.anaszewski@samsung.com>,
	"Felipe Balbi" <balbi@kernel.org>,
	"Peter Chen" <hzpeterchen@gmail.com>,
	"linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
	"Rafał Miłecki" <rafal@milecki.pl>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Ezequiel Garcia" <ezequiel@vanguardiasur.com.ar>,
	"Boris Brezillon" <boris.brezillon@free-electrons.com>,
	"Geert Uytterhoeven" <geert@linux-m68k.org>,
	"Stephan Linz" <linz@li-pro.net>,
	"open list" <linux-kernel@vger.kernel.org>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	"open list:LED SUBSYSTEM" <linux-leds@vger.kernel.org>
Subject: Re: [PATCH V4] leds: trigger: Introduce an USB port trigger
Date: Mon, 10 Oct 2016 16:25:30 +0200	[thread overview]
Message-ID: <20161010142530.GB26643@amd> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1608311420010.1889-100000@iolanthe.rowland.org>

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On Wed 2016-08-31 14:23:13, Alan Stern wrote:
> On Tue, 30 Aug 2016, Rafał Miłecki wrote:
> 
> > >> As you quite often need more complex LED management, there are
> > >> triggers that were introduced in 2006 by c3bc9956ec52f ("[PATCH] LED:
> > >> add LED trigger tupport"). Some triggers are trivial and could be
> > >> implemented in userspace as well (e.g. "timer"). Some had to be
> > >> implemented in kernelspace (CPU activity, MTD activity, etc.). Having
> > >> few triggers compiled, you can assign them to LEDs at it pleases you.
> > >> Your hardware may have generic LED (not labeled) and you can
> > >> dynamically assign various triggers to it, depending e.g. on user
> > >> actions. E.g. if user (using GUI or whatever) wants to see flash
> > >> activity, your userspace script should do:
> > >> echo mtd > /sys/class/leds/foo/trigger
> > >
> > > So for example, you might want to do:
> > >
> > >         echo usb1-4 >/sys/class/leds/foo/trigger
> > >
> > > and then have the "foo" LED toggle whenever an URB was submitted or
> > > completed for a device attached to the 1-4 port.  Right?
> > 
> > Not really as it won't cover some pretty common use cases. Many home
> > routers have few USB ports (2-5) and only 1 USB LED. It has to be
> > possible to assign few USB ports to a single LED (trigger). That way
> > LED should be turned on (and kept on) if there is at least 1 USB
> > device connected. You obviously can't do:
> > echo "usb1-1 usb1-2 usb2-1" > /sys/class/leds/foo/trigger
> > 
> > This was already brought up by Rob (who mentioned CPU trigger) and I
> > replied him pretty much the same way in:
> > https://lkml.org/lkml/2016/7/29/38
> > (reply starts with "Anyway, the serious limitation I see").
> 
> The code for a bunch of triggers must already be written.  What would 
> the user do if he wanted to flash a single LED in response to both
> CPU activity and MTD activity?  If not
> 
> 	echo "cpu mtd" >/sys/class/leds/foo/trigger

Lets not overcomplicate this... What if user wanted to blink only when
there's both cpu and mtd activity?

I mean, there are way too many possible combinations, but we should
not implement everything. "Heartbeat" for example is nice demo and
nice test case, but ...

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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