linux-kernel - Re: GPIO bindings guidelines (Was: Re: [PATCH v5 10/12] gpio: Support for unified device properties interface)

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Date:	Thu, 23 Oct 2014 14:08:50 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Alexandre Courbot <gnurou@...il.com>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Grant Likely <grant.likely@...aro.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Aaron Lu <aaron.lu@...el.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Bryan Wu <cooloney@...il.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Mark Rutland <mark.rutland@....com>
Subject: Re: GPIO bindings guidelines (Was: Re: [PATCH v5 10/12] gpio: Support for unified device properties interface)

On Thursday 23 October 2014 15:10:55 Alexandre Courbot wrote:
> >
> > Then, the driver needs to do something like:
> >
> >         if (!device_property_present(dev, "known_property_that_should_be_present")
> >             && ACPI_COMPANION(dev))
> >                 acpi_probe_gpios(dev);
> >
> > and in the acpi_probe_gpios() routine there may be checks like:
> >
> >         if (device_has_id(dev, "MARY0001")) {
> >                 The first pin in the first GpioIo resource in _CRS is "fred" and
> >                 it is active-low.
> >                 The third pin in the second GpioIo resource in _CRS is "steve"
> >                 and it is not active-low.
> >         } else if (device_has_id(dev, "JANE0002")) {
> >                 The first pin in the second GpioIo resource in _CRS is "fred" and
> >                 it is not active-low.
> >                 The second pin in the first GpioIo resource in _CRS is "steve"
> >                 and it is active-low.
> >         }
> >
> > and so on.  Of course, there may be drivers knowing that the meaning of the
> > GpioIo resources in _CRS is the same for all devices handled by them, in which
> > case they will not need to check device IDs, but the core has now way of
> > knowing that.  Only the drivers have that information and the core has now
> > way to figure out what to do for a given specific device.
> >
> > So here's a radical idea: Why don't we introduce something like
> >
> >         acpi_enumerate_gpio(dev, name, GpioIo_index, pin_index, active_low)
> >
> > such that after calling, say, acpi_enumerate_gpio(dev, "fred", 0, 0, true) the
> > driver can do something like:
> >
> >         desc = get_gpiod_by_name(dev, "fred");
> >
> > and it'll all work.  Then, the only part of the driver that really needs to be
> > ACPI-specific will be the acpi_probe_gpios() function calling acpi_enumerate_gpio()
> > in accordance with what the device ID is.
> 
> I like this idea. It doesn't complicate the GPIO interface (i.e. no
> "if you are on ACPI and no _DSD is present then gpiod_get() will
> behave that way...") and does the plumbing behind the scenes.
> 
> I will also allow us to finally push the use of names instead of
> indexes in the GPIO API. I'm all for it.

Yes, sounds good.

	Arnd
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