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

[RafaÅ MiÅecki]

On 31 August 2016 at 21:00, RafaÅ MiÅecki <zajec5@xxxxxxxxx> wrote:
> On 31 August 2016 at 20:23, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
>> On Tue, 30 Aug 2016, RafaÅ MiÅecki wrote:
>>> 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?

I'm pretty sure noone ever planned to have more than 1 trigger
assigned to a single LED. I just realized there will be a problem with
proposed solution: sysfs files conflict.

Consider 2 existing triggers for a moment:
1) oneshot: it creates following sysfs files:
/sys/class/leds/foo/delay_on
/sys/class/leds/foo/delay_off
/sys/class/leds/foo/invert
/sys/class/leds/foo/shot
2) timer: it creates following sysfs files:
/sys/class/leds/foo/delay_on
/sys/class/leds/foo/delay_off

Activating both of them will probably cause a WARNING in sysfs. They
can't coexist :(

We should probably have per-trigger subdirs, e.g.:
/sys/class/leds/foo/trigger-oneshot/delay_on
/sys/class/leds/foo/trigger-oneshot/delay_off
/sys/class/leds/foo/trigger-oneshot/invert
/sys/class/leds/foo/trigger-oneshot/shot
/sys/class/leds/foo/trigger-timer/delay_on
/sys/class/leds/foo/trigger-timer/delay_off
but implementing it now would break the ABI.

One workaround I can see is doing triggers V2, they:
1) Would put sysfs files in /sys/class/leds/foo/trigger-bar/
2) Use new API for *requesting* LED to be on/off
3) There would be a counter of requests in V2 API
4) Multiple triggers V2 would be allowed to be used (assigned) at the same time

-- 
RafaÅ