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

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Date:   Thu, 1 Sep 2016 07:25:11 +0200
From:   Rafał Miłecki <zajec5@...il.com>
To:     Alan Stern <stern@...land.harvard.edu>
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 31 August 2016 at 21:00, Rafał Miłecki <zajec5@...il.com> wrote:
> On 31 August 2016 at 20:23, Alan Stern <stern@...land.harvard.edu> wrote:
>> On Tue, 30 Aug 2016, Rafał Miłecki 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

-- 
Rafał