Re: [PATCH] leds: gpio: support multi-level brightness

[Akinobu Mita <akinobu.mita@xxxxxxxxx>]

2019年10月9日(水) 5:53 Jacek Anaszewski <jacek.anaszewski@xxxxxxxxx>:
>
> On 10/6/19 4:11 PM, Akinobu Mita wrote:
> > 2019年10月6日(日) 4:17 Jacek Anaszewski <jacek.anaszewski@xxxxxxxxx>:
> >>
> >> On 10/5/19 3:20 PM, Akinobu Mita wrote:
> >>> 2019年10月5日(土) 6:17 Jacek Anaszewski <jacek.anaszewski@xxxxxxxxx>:
> >>>>
> >>>> Hi Akinobu,
> >>>>
> >>>> Why do you think this change is needed? Does it solve
> >>>> some use case for you?
> >>>
> >>> It can be useful when using with an LED trigger that could set the
> >>> brightness values other than LED_FULL or LED_OFF.
> >>>
> >>> The LED CPU trigger for all CPUs (not per CPU) sets the brightness value
> >>> depending on the number of active CPUs.  We can define the multi brightness
> >>> level gpio LED with fewer number of GPIO LEDs than the total number of
> >>> CPUs, and the LEDs can be viewed as a level meter.
> >>
> >> Can't you achieve exactly the same effect by creating separate LED class
> >> device for each GPIO LED and registering each of them for separate cpuN
> >> trigger?
> >
> > If there are GPIO LEDs as many as the total number of CPUs, we can.
> > However, if there are only two GPIO LEDs and six CPUs, we can only know
> > the CPU activity for two CPUs out of six CPUs with cpuN trigger.
> > So it's different from using cpu (all) trigger with multi level (2-level)
> > brightness GPIO LED.
>
> OK, that's a reasonable argument. However, this is clearly
> trigger-specific functionality and we should not delegate this
> logic down to the driver.
>
> What you propose should be a responsibility of a trigger that would
> allow registering multiple LEDs for its disposal. This would have to
> be different from existing LED Trigger mechanism, that blindly
> applies trigger event to all LEDs that have registered for it.
>
> Such a trigger would have to be a separate LED (pattern?) class device.
> It would need to be told how many LEDs it is going to manage
> and create files for filling the LED names. This design could be also
> used for defining patterns spanning on multiple LEDs. Just a rough idea.
> We can dwell on it if it catches.

What do you think about introducing a new led driver and describing a
level meter in the following DT node.

led-level-meter {
        compatible = "led-level-meter";
        leds = <&led0>, <&led1>, <&led2>, <&led3>;
};

Setting the brightness of led-level-meter to LED_HALF turns on led0 and
led1 with max brightness and turns off led2 and led3.

This is inspired by led-backlight driver patchset and Bjorn Andersson's
reply in this thread.