linux-kernel - Re: [PATCH v2 1/9] i2c: prepare runtime PM support for I2C client devices

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Date:	Mon, 16 Sep 2013 15:46:16 +0100
From:	Graeme Gregory <graeme.gregory@...aro.org>
To:	Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:	Mark Brown <broonie@...nel.org>, Kevin Hilman <khilman@...aro.org>,
	linux-i2c@...r.kernel.org, Wolfram Sang <wsa@...-dreams.de>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
	Lv Zheng <lv.zheng@...el.com>, Aaron Lu <aaron.lu@...el.com>,
	linux-arm-kernel@...ts.infradead.org,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Mauro Carvalho Chehab <m.chehab@...sung.com>,
	Samuel Ortiz <sameo@...ux.intel.com>,
	Lee Jones <lee.jones@...aro.org>,
	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Liam Girdwood <lgirdwood@...il.com>,
	Kyungmin Park <kyungmin.park@...sung.com>
Subject: Re: [PATCH v2 1/9] i2c: prepare runtime PM support for I2C client
 devices

On Mon, Sep 16, 2013 at 05:38:12PM +0300, Mika Westerberg wrote:
> On Mon, Sep 16, 2013 at 11:12:49AM +0100, Mark Brown wrote:
> > That's definitely an ACPI specific (probably x86 specific ACPI?)
> > requirement not a generic one, on some systems it would increase power
> > consumption since the controller will need to sit on while the device is
> > functioning autonomously.
> 
> Yes, the ACPI 5.0 spec says that the device cannot be in higher D-state
> than its parent. This is not x86 specific, though I'm not sure if this is
> implemented elsewhere.
> 
I do not think this stops the OS fine controlling the power of the device
though. It is only a mechanism to make sure the tree of D states is vaguely
sane from the ACPI point of view. What happens in each D state is never
actually defined in the ACPI spec.

Graeme

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