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

[Mark Rutland <mark.rutland@arm.com>]

On Thu, Oct 02, 2014 at 01:15:08PM +0100, Mika Westerberg wrote:
> On Thu, Oct 02, 2014 at 01:51:24PM +0200, Arnd Bergmann wrote:
> > On Thursday 02 October 2014 13:41:23 Mika Westerberg wrote:
> > > 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>
> > > 
> > > > > +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/
> > 
> > I thought when we had discussed the subsystem specific bindings, the
> > consensus there was to have subsystem specific accessors and
> > properties/tables.
> > 
> > I would argue that for everything that ACPI already has (interrupts,
> > registers, gpio, dmaengine, ...) the native method should be used,
> > possibly using _DSD to provide naming for otherwise anonymous references.
> 
> Absolutely. That's precisely what we do in the GPIO patch of this
> series. E.g we use ACPI GpioIo/GpioInt _CRS resources but give name to
> the GPIOs with the help of _DSD.
> 
> For things that don't have correspondence in ACPI but have well defined
> existing DT schema, like PWMs, we should follow that.

I'm rather concerned that while that's expedient for us, that's going to
end up in the creation of Linux-only ACPI tables. If any other OS vendor
decides they need to model this information and doesn't wnat to pick up
Linux _DSD bindings, what happens if they try to get an explicit object
model defined in ACPI for those objects?

Mark.