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

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Message-ID: <Pine.LNX.4.44L0.1608311420010.1889-100000@iolanthe.rowland.org>
Date:   Wed, 31 Aug 2016 14:23:13 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Rafał Miłecki <zajec5@...il.com>
cc:     Greg KH <gregkh@...uxfoundation.org>,
        Richard Purdie <rpurdie@...ys.net>,
        Jacek Anaszewski <j.anaszewski@...sung.com>,
        Felipe Balbi <balbi@...nel.org>,
        Peter Chen <hzpeterchen@...il.com>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        Rafał Miłecki <rafal@...ecki.pl>,
        Jonathan Corbet <corbet@....net>,
        Ezequiel Garcia <ezequiel@...guardiasur.com.ar>,
        Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Stephan Linz <linz@...pro.net>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        "open list:LED SUBSYSTEM" <linux-leds@...r.kernel.org>
Subject: Re: [PATCH V4] leds: trigger: Introduce an USB port trigger

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

then what?

Alan Stern