linux-kernel - Re: [PATCH] gpio: New driver for GPO emulation using PWM generators

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Message-ID: <50B344D6.8030608@metafoo.de>
Date:	Mon, 26 Nov 2012 11:30:46 +0100
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Peter Ujfalusi <peter.ujfalusi@...com>
CC:	Grant Likely <grant.likely@...retlab.ca>,
	Linus Walleij <linus.walleij@...aro.org>,
	Rob Landley <rob@...dley.net>,
	devicetree-discuss@...ts.ozlabs.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	linux-omap@...r.kernel.org
Subject: Re: [PATCH] gpio: New driver for GPO emulation using PWM generators

On 11/23/2012 10:44 AM, Peter Ujfalusi wrote:
> Hi Grant,
> 
> On 11/23/2012 10:13 AM, Peter Ujfalusi wrote:
>> Hi Grant,
>>
>> On 11/23/2012 08:55 AM, Grant Likely wrote:
>>> Ugh. and this is why I wanted the PWM and GPIO subsystems to use the
>>> same namespace and binding. <grumble, mutter> But that's not your fault.
>>>
>>> It's pretty horrible to have a separate translator node to convert a PWM
>>> into a GPIO (with output only of course). The gpio properties should
>>> appear directly in the PWM node itself and the translation code should
>>> be in either the pwm or the gpio core. I don't think it should look like
>>> a separate device.
>>
>> Let me see if I understand your suggestion correctly. In the DT you suggest
>> something like this:
>>
>> twl_pwmled: pwmled {
>> 	compatible = "ti,twl4030-pwmled";
>> 	#pwm-cells = <2>;
>> 	#gpio-cells = <2>;
>> 	gpio-controller;
>> };
> 
> After I thought about this.. Is this what we really want?
> After all what we have here is a PWM generator used to emulate a GPIO signal.
> The PWM itself can be used for driving a LED (standard LED or backlight and we
> have DT bindings for these already), vibra motor, or other things.
> If we combine the PWM with GPIO we should go and 'bloat' the DT node to also
> include all sort of other uses of PWM at once?
> 
> IMHO it is better to keep them as separate things.
> pwm node to describe the PWM generator,
> separate nodes to describe it's uses like led, backlight, motor and gpio.
> 

The difference here is that the LED, backlight, etc are all different
physical devices begin driven by the pwm pin, so it makes sense to have a
device tree node for them, while using the pwm as gpio is just a different
function of the same physical pin.  So in a sense the pwm controller also
becomes a gpio controller. I like the idea of the pwm core automatically
instantiating a pwm-gpo device if it sees a gpio-controller property in the
pwm device devicetree node.

- Lars
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