linux-kernel - Re: [PATCH] ACPI / GPIO: Pass index to acpi_get_gpiod_by

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Date:	Mon, 03 Nov 2014 16:25:08 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Darren Hart <dvhart@...ux.intel.com>,
	Grant Likely <grant.likely@...retlab.ca>
Cc:	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Alexandre Courbot <acourbot@...dia.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Arnd Bergmann <arnd@...db.de>, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ACPI / GPIO: Pass index to acpi_get_gpiod_by_index() when using properties

On Sunday, November 02, 2014 08:49:37 PM Darren Hart wrote:
> 
> On 11/1/14 4:11, Grant Likely wrote:
> > On Tue, 28 Oct 2014 22:59:57 +0100
> > , "Rafael J. Wysocki" <rjw@...ysocki.net>
> >  wrote:
> >> On Tuesday, October 28, 2014 01:15:27 PM Mika Westerberg wrote:
> >>> acpi_dev_add_driver_gpios() makes it possible to set up mapping between
> >>> properties and ACPI GpioIo resources in a driver, so we can take index
> >>> parameter in acpi_find_gpio() into use with _DSD device properties now.
> >>>
> >>> This index can be used to select a GPIO from a property with multiple
> >>> GPIOs:
> >>>
> >>>   Package () {
> >>>   	"data-gpios",
> >>>   	Package () {
> >>>   		\_SB.GPIO, 0, 0, 0,
> >>>   		\_SB.GPIO, 1, 0, 0,
> >>>   		\_SB.GPIO, 2, 0, 1,
> >>>   	}
> >>>   }
> >>>
> >>> In order to retrieve the last GPIO from a driver we can simply do:
> >>>
> >>>   desc = devm_gpiod_get_index(dev, "data", 2);
> >>>
> >>> and so on.
> >>>
> >>> Signed-off-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> >>
> >> Cool. :-)
> >>
> >> Any objections anyone?
> > 
> > Actually, I do. Not in the idea, but in the implementation. The way this gets encoded:
> > 
> > 	Package () {
> > 		\_SB.GPIO, 0, 0, 0,
> > 		\_SB.GPIO, 1, 0, 0,
> > 		\_SB.GPIO, 2, 0, 1,
> > 	}
> > 
> > Means that decoding each GPIO tuple requires the length of a tuple to be
> > fixed, or to implement a DT-like #gpio-cells. If it is fixed, then there
> > is no way to expand the binding later. Can this be done in the following
> > way instead?
> > 
> > 	Package () {
> > 		Package () { \_SB.GPIO, 0, 0, 0 },
> > 		Package () { \_SB.GPIO, 1, 0, 0 },
> > 		Package () { \_SB.GPIO, 2, 0, 1 },
> > 	}
> > 
> > This is one of the biggest pains in device tree. We don't have any way
> > to group tuples so it requires looking up stuff across the tree to
> > figure out how to parse each multi-item property.
> > 
> > I know that last year we talked about how bios vendors would get
> > complicated properties wrong, but I think there is little risk in this
> > case. If the property is encoded wrong, the driver simply won't work and
> > it is unlikely to get shipped before being fixed.
> 
> This particular nesting of Packages is expressly prohibited by the
> Device Properties UUID for the reasons you mention.
> 
> http://www.uefi.org/sites/default/files/resources/_DSD-device-properties-UUID.pdf

Also we don't use properties where single name is assigned to multiple GPIOs
anywhere in the current device-properties patchset, so this is not relevant at
the moment.

Moreover, even if we were to use them, we would need to ensure that this:

	Package () {
		\_SB.GPIO, 0, 0, 0
	}

was equivalent to

	Package () {
 		Package () { \_SB.GPIO, 0, 0, 0 }
	}

This is not impossible to do and I suppose we could even explain that in the
implementation guide document in a sensible way, but that would require the
document linked above to be changed first and *then* we can think about writing
kernel code to it.  Not the other way around, please.

So Grant, do you want us to proceed with that?

Rafael

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