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From: Mika Westerberg <mika.westerberg-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
Subject: Re: [PATCH v2 1/9] i2c: prepare runtime PM support for I2C client
 devices
Date: Mon, 16 Sep 2013 17:38:12 +0300
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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.

> Even though the controller power consumption is going to be minimal the
> power domain it is in may be relatively large.  Can't the power domains
> for ACPI deal with this requirement, for example by making the I2C slave
> power domains children of the controller power domain?

We'll look into this. Thanks for the suggestion.