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Date:   Sat, 4 Jul 2020 14:59:00 +0200
From:   Pavel Machek <>
To:     Marek BehĂșn <>
Cc:     Ondrej Jirman <>,,
        Jacek Anaszewski <>,
        Dan Murphy <>,
        "open list:LED SUBSYSTEM" <>
Subject: Re: [PATCH RFC] leds: Add support for per-LED device triggers


> Some criticism to this approach to HW triggers:
> - every hw trigger for each LED has to be registered via current trigger
>   API. This will grow code size and memory footprint once this API is
>   widely used
> - one HW trigger can only master one LED device (via private_led
>   member). So if I have, for example an ethernet switch with 8 ports,
>   and each port has 2 LEDs, and each LED has 10 possible HW triggering
>   mechanisms, with your proposed API one would need to register 8*2*10
>   = 160 triggers

Well, code is simple, and so far I have seen 2 HW triggering
mechanisms, not 10. Maybe we should have a function to regiter a hw
trigger for a LED, so that internal implementation can be changed
more easily.

Ondrej: You already have code using this, right? Can we get an example?

> I too have been thinking about an API for HW LED triggers, and I
> tinkered with it a little. Some time ago I sent some emails, with
> subjects:
>   "RFC: LED hw triggering API"
>   "about my trigger-sources work"

Perhaps it is time to send them one more time, so Ondrej can say if it
works for him/looks okay for him?

> My current thoughts about how HW LED triggers could work nicely is as
> such:
>   - these members (maybe with different names) shall be added to struct
>     led_classdev:
>       available_hw_triggers()
>         - shall return a NULL terminated list of HW trigger names
>           available for this LED
>       set_hw_trigger()
>         - sets HW trigger for this LED. The LED triggering API shall
>           call this method after previous LED trigger is unset. If
>           called with NULL parameter, unsets HW trigger
>       current_hw_trigger
>         - name of the currently set HW LED trigger for this LED
>   - the driver registering the LED cdev informs abouth the LED being
>     capable of HW triggering - members available_hw_triggers and
>     set_hw_trigger must be set
>   - SW LED trigger and HW LED trigger are mutualy exclusive on one LED
>   - the trigger file in sysfs (/sys/class/leds/LED/trigger) shall first
>     list the available SW triggers, and then available hw triggers for
>     this LED, prefixed with "hw:"
>     When written, if the written trigger name starts with "hw:",
>     instead of setting SW trigger, a HW trigger is set via
>     set_hw_trigger() method

This does not sound bad, either.

Best regards,
(cesky, pictures)

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