linux-kernel - RE: [PATCH v5 05/26] leds: core: Add support for composing LED class device names

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Date:   Fri, 28 Jun 2019 13:30:30 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Walleij' <linus.walleij@...aro.org>,
        Jacek Anaszewski <jacek.anaszewski@...il.com>
CC:     Linux LED Subsystem <linux-leds@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Pavel Machek <pavel@....cz>, Rob Herring <robh@...nel.org>,
        Dmitry Torokhov <dtor@...gle.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Dan Murphy <dmurphy@...com>,
        Baolin Wang <baolin.wang@...aro.org>,
        Daniel Mack <daniel@...que.org>,
        Oleh Kravchenko <oleg@....org.ua>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Simon Shields <simon@...eageos.org>
Subject: RE: [PATCH v5 05/26] leds: core: Add support for composing LED class
 device names

From: Linus Walleij
> Sent: 28 June 2019 09:46
...
> A problem with LEDs is that it invites bikeshedding because it is too
> relateable.

Bikeshedding leds :-)

It also isn't at all clear how to handle bi-colour and tri-colour leds.
ISTR the usual interface lets you set the brightness, but more often
leds are single brightness but multi-colour.
Eg the ethernet 'speed' led which is (usually) off/orange/green.

Changing the brightness either means changing the current or using PWM.
Both really require more hardware support than changing colours.

I've done some led driving (for a front panel) from a PLD (small FPGA).
As well as the obvious things I did:
- dim: 1/8th on at 80Hz.
- flash: 1/8th on at 4Hz.
- orange: 50-50 red-green at 80Hz on an RGB led.

There was also the 'ethernet activity' led which could either be driven
by the hardware, or forced on/off/flash by the driver.
If driven by the hardware, the software could read the current state.

None of this really fitted the Linux leds interface.

	David

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