linux-kernel - Re: ACPI vs Device Tree

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Message-ID: <20130821160903.GA11908@srcf.ucam.org>
Date:	Wed, 21 Aug 2013 17:09:03 +0100
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Guenter Roeck <linux@...ck-us.net>,
	Darren Hart <dvhart@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: ACPI vs Device Tree - moving forward

On Wed, Aug 21, 2013 at 05:57:07PM +0200, Linus Walleij wrote:

> - The I2C address is specified in "reg" - maybe ACPI have
>   some other way to assign I2C addresses to I2C devices?
>   In any case, it *must* reference the parent I2C controller,
>   here that is done implicitly by placing this DT node inside
>   the I2C controllers DT node.

That's fine. You put the child device inside the I2C contorller's scope, 
which can be done from a separate ACPI table if you want. The address 
can be provided via _ADR().

> - Then it is using a GPIO line as interrupt, and specify that
>   this shall be configured as a falling edge IRQ.

ACPI 5 permits this.

> - It then tells the interrupt controller parent. So it needs
>   to have a reference to whatever interrupt chip device
>   will handle that IRQ.

By interrupt controller, do you mean the GPIO controller? ACPI GPIO 
definitions include the parent device.

> - Further it *is* an interrupt controller, so devices connected
>   to the GPIO lines may generate IRQs and then this
>   device should service them. Is it possible that the devices
>   connected to this expander in turn use ACPI to describe
>   themselves? Then we need a reference in the other
>   direction.

I think that's also doable.

> - Further it is a wakeup source, so each IRQ it provides
>   on its GPIO lines can be set as a wakeup. I wonder how
>   this plays with D-states and ACPI.

That's fine. GPIO lines can be described as causing ACPI events and then 
that simply referenced as a wakeup event.

> I did present the above as an extreme example, but if we
> start to combine DT and ACPI we have to have that kind of
> hardware in mind. GPIO expanders with IRQs and all are
> maybe rare on desktops and laptops but very common on
> embedded systems.

Yeah, describing complicated device topology isn't really the problem I 
think we'll end up facing - it's the wider range of device configuration 
data that worries me.

-- 
Matthew Garrett | mjg59@...f.ucam.org
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