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

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Message-ID: <CACxGe6sOhvRC4YFxKmDSc1xJ3Q6DomPpAKjtk3_gdY4XpfONRg@mail.gmail.com>
Date:	Fri, 30 Nov 2012 11:09:54 +0000
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Thierry Reding <thierry.reding@...onic-design.de>
Cc:	Peter Ujfalusi <peter.ujfalusi@...com>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Linus Walleij <linus.walleij@...aro.org>,
	Rob Landley <rob@...dley.net>,
	devicetree-discuss <devicetree-discuss@...ts.ozlabs.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>
Subject: Re: [PATCH] gpio: New driver for GPO emulation using PWM generators

On Fri, Nov 30, 2012 at 11:04 AM, Thierry Reding
<thierry.reding@...onic-design.de> wrote:

>> > One other problem is that some PWM devices cannot be setup to achieve a
>> > 0% or 100% duty-cycle but instead will toggle for at least one period.
>> > This would be another argument in favour of moving the functionality to
>> > the individual drivers, perhaps with some functionality provided by the
>> > core to do the gpio_chip registration (a period could be passed to that
>> > function at registration time), which will likely be the same for all
>> > hardware that can and wants to support this feature.
>>
>> It is a bit of an oddball case where if the hardware engineer wires up a
>> PWM to use as a GPIO, then he better be sure that it is actually fit for
>> the purpose.
>
> Yes, I agree. So what we really want is to make this configurable in
> some way. For DT this could just be controlled by the gpio-controller
> property. The PWM core could easily setup the gpio_chip in the presence
> of that property.
>
> For non-DT it could probably be done via a flag that is passed to the
> driver via platform data, in which case the driver would have to call
> the helper explicitly based on the setting of this flag.
>
>> That doesn't prevent the PWM core having some gpio-emulation helper
>> functionality, but do need to be careful about it.
>
> On the other hand, if we turn that support into a helper maybe it is
> easier to leave it up to the driver whether to call it or not. A big
> advantage of this would be that the driver could pass a period along
> that it can choose sensibly according to the chip's capabilities.
>
> Something as simple as:
>
>         int pwmchip_emulate_gpio(struct pwm_chip *chip, unsigned long period);
>
> could do. Cleanup could be done automatically in pwmchip_remove().

Looks reasonable.

g.
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