Re: [PATCH v3 04/15] ACPI: Document ACPI device specific properties

[Mika Westerberg <mika.westerberg@linux.intel.com>]

On Wed, Oct 01, 2014 at 09:59:14AM +0200, Arnd Bergmann wrote:
> On Wednesday 01 October 2014 04:11:20 Rafael J. Wysocki wrote:
> > From: Mika Westerberg <mika.westerberg@linux.intel.com>
> > 
> > This document describes the data format and interfaces of ACPI device
> > specific properties.
> > 
> > Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
> > Signed-off-by: Darren Hart <dvhart@linux.intel.com>
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> 
> Overall looks sane, but I wonder if we should try harder to not duplicate
> some of the mistakes we made in the DT bindings. Two points in particular
> stick out:
> 
> > +2.3 Strings
> > +-----------
> > +String properties can be used to describe many things like labels for GPIO
> > +buttons, compability ids, etc.
> > +
> > +A string property looks like this:
> > +
> > +	Package () {"pwm-names", "backlight"},
> 
> The way we name things in DT using separate "foos" and "foo-names" properties
> is a bit quirky. Those are always defined on a per-subsystem level, not
> a per-device level though, so it should be possible to come up with a
> better representation in ACPI.
> 
> Since the device driver should never look into the "foo-names" property
> itself but just pass down the name into the subsystem, the "foo" subsystem
> could instead have a way to add an (optional) name for each reference.
> 
> This is something the DT syntax doesn't allow because you can't have
> both a phandle and a string in a single property but I think the ACPI
> packages can do it, and it wouldn't change the basic structure.

ACPI packages can hold anything (even other packages) but one goal with
the _DSD implementation was to reuse existing DT bindings wherever it
makes sense even if they would not always be optimal for ACPI. So the
current acpi_dev_get_property_reference() is modelled after
corresponding DT function and it allows only integer arguments to
accompany the reference.

Doing that allows taking the existing DT description, package it inside
ACPI _DSD and be done with it.

> > +The referenced ACPI device is returned in args->adev if found.
> > +
> > +In addition to simple object references it is also possible to have object
> > +references with arguments. These are represented in ASL as follows:
> > +
> > +	Device (\_SB.PCI0.PWM)
> > +	{
> > +		Name (_DSD, Package () {
> > +			ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
> > +			Package () {
> > +				Package () {"#pwm-cells", 2}
> > +			}
> > +		})
> > +	}
> > +
> 
> Similarly, the "#foo-cells" syntax is an artifact of the limitations of the
> DT syntax, and I'd assume there would be a better way to encode this
> in ACPI. Also, a "cell" in Open Firmware is defined as a big-endian
> 32-bit value, which doesn't directly correspond to something in ACPI,
> and the '#' character is an artifact of the use of the Forth language
> in Open Firmware, which you also don't have here.

Same here, we tried to make it follow closely the DT description. It is
probably not the best/optimal encoding for ACPI but it is documented
well in Documentation/devicetree/bindings so why not use it.

The summary email from Darren at KS also mentions that for the existing
drivers, the existing schemas should be common for both implementations [1].

For new bindings we probably should look out if they can be better
represented using ACPI types.

[1] http://lwn.net/Articles/609373/