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From: Michael Welling <mwelling@ieee.org>
Subject: Re: [PATCH v2] gpio: add userspace ABI for GPIO line information
Date: Fri, 19 Feb 2016 04:26:14 -0600
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To: Linus Walleij <linus.walleij@linaro.org>
Cc: Jonathan Cameron <jic23@kernel.org>, "linux-gpio@vger.kernel.org" <linux-gpio@vger.kernel.org>, Alexandre Courbot <acourbot@nvidia.com>, Johan Hovold <johan@kernel.org>, Markus Pargmann <mpa@pengutronix.de>, Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>, Grant Likely <grant.likely@linaro.org>, Amit Kucheria <amit.kucheria@linaro.org>

On Fri, Feb 19, 2016 at 10:47:45AM +0100, Linus Walleij wrote:
> On Fri, Feb 19, 2016 at 10:46 AM, Michael Welling <mwelling@ieee.org> wrote:
> 
> > I am looking forward to see how the GPIOs will be modified from userspace.
> > The fact that single controllers can host hundreds of GPIOs is going to
> > make it an interesting problem.
> >
> > Do you have a sketch of how the accesses with be performed?
> > ioctl, read, write.
> 
> No. Suggestions welcome. Looking at drivers/iio for inspiration.
>

Well one thing I never liked about the iio userspace is the mixed API.
You have to use sysfs to setup buffered reads and character device for
the reads.

I suppose individual GPIOs could be configured by simply calling
specific ioctls passing the (offset, value) as the argument.

GPIO_EXPORT_LINE_IOCTL - Allocate the GPIO to userspace.
GPIO_SET_DIRECTION_IOCTL - Set GPIO direction.
GPIO_SET_VALUE_IOCTL - Set GPIO value.
GPIO_GET_VALUE_IOCTL - Get GPIO value.
GPIO_SET_ACTIVE_IOCTL - Set GPIO active high/low.
GPIO_SET_OPEN_DRAIN_IOCTL - Set GPIO open drain.
GPIO_SET_OPEN_SOURCE_IOCTL - Set GPIO open source.

Then all exported GPIOs could be accessible using read and write callbacks.

Handling interrupts may prove a little trickier.
The poll could be used on the character device but then there would have to
be a way to resolve which GPIO triggered the interrupt.

What do you think?

> Yours,
> Linus Walleij