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SubjectRe: [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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