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Date:   Fri, 12 Mar 2021 10:03:27 +0100
From:   Marek Behun <marek.behun@nic.cz>
To:     Hermes Zhang <Hermes.Zhang@axis.com>
Cc:     Alexander Dahl <ada@thorsis.com>,
        "linux-leds@vger.kernel.org" <linux-leds@vger.kernel.org>,
        Pavel Machek <pavel@ucw.cz>, Dan Murphy <dmurphy@ti.com>,
        kernel <kernel@axis.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] leds: leds-dual-gpio: Add dual GPIO LEDs driver
Message-ID: <20210312100327.3c3c47ba@nic.cz>
In-Reply-To: <e56946d1d85948c7b6ce94982a221981@XBOX03.axis.com>
References: <20210311130408.10820-1-chenhui.zhang@axis.com>
        <2315048.uTtSMl1LR1@ada>
        <e56946d1d85948c7b6ce94982a221981@XBOX03.axis.com>
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On Fri, 12 Mar 2021 08:48:55 +0000
Hermes Zhang <Hermes.Zhang@axis.com> wrote:

> Hi Alexander,
> 
> > Am Donnerstag, 11. März 2021, 14:04:08 CET schrieb Hermes Zhang:  
> > > From: Hermes Zhang <chenhuiz@axis.com>
> > >
> > > Introduce a new Dual GPIO LED driver. These two GPIOs LED will act as
> > > one LED as normal GPIO LED but give the possibility to change the
> > > intensity in four levels: OFF, LOW, MIDDLE and HIGH.  
> > 
> > Interesting use case. Is there any real world hardware wired like that you
> > could point to?
> >   
> 
> Yes, we have the HW, it's not a chip but just some circuit to made of.
>  
> > > +config LEDS_DUAL_GPIO
> > > +	tristate "LED Support for Dual GPIO connected LEDs"
> > > +	depends on LEDS_CLASS
> > > +	depends on GPIOLIB || COMPILE_TEST
> > > +	help
> > > +	  This option enables support for the two LEDs connected to GPIO
> > > +	  outputs. These two GPIO LEDs act as one LED in the sysfs and
> > > +	  perform different intensity by enable either one of them or both.  
> > 
> > Well, although I never had time to implement that, I suspect that could
> > conflict if someone will eventually write a driver for two pin dual color LEDs
> > connected to GPIO pins.  We actually do that on our hardware and I know
> > others do, too.
> > 
> > I asked about that back in 2019, see this thread:
> > 
> > https://www.spinics.net/lists/linux-leds/msg11665.html
> > 
> > At the time the multicolor framework was not yet merged, so today I would
> > probably make something which either uses the multicolor framework or at
> > least has a similar interface to userspace. However, it probably won't surprise
> > you all, this is not highest priority on my ToDo list. ;-)
> > 
> > (What we actually do is pretend those are separate LEDs and ignore the
> > conflicting case where both GPIOs are on and the LED is dark then.)
> >   
> 
> Yes, that case seems conflict with mine, the pattern for me is like:
> 
> P1 | P2 | LED
> -- + -- + -----
>  0 |  0 | off
>  0 |  1 | Any color
>  1 |  0 | Any color
>  1 |  1 | both on
> 
> Now I'm investigate another way from Marek's suggestion by using REGULATOR_GPIO, to see if could meet my requirement. If yes, then I do think no new  driver is needed.

Maybe you could even implement multicolor-gpio, now that we have
multicolor LED class :)

Marek