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