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

["Rafał Miłecki" <zajec5@gmail.com>]

On 31 August 2016 at 20:23, Alan Stern <stern@rowland.harvard.edu> 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
>
> then what?

Well, it sounds like a new feature then. Shall we add an extra API
with a request function for turning LED on? It could internally count
how many requests were raised and keep LED on as long as there is at
least 1 left. I guess we should implement it in trigger "subsystem"
(if I can call it so). Does it sound like a good plan?

-- 
Rafał