Date: Mon, 16 Sep 2013 15:46:16 +0100
From: Graeme Gregory <graeme.gregory@linaro.org>
To: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: Mark Brown <broonie@kernel.org>, Kevin Hilman <khilman@linaro.org>,
        linux-i2c@vger.kernel.org, Wolfram Sang <wsa@the-dreams.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        linux-acpi@vger.kernel.org, linux-kernel@vger.kernel.org,
        Lv Zheng <lv.zheng@intel.com>, Aaron Lu <aaron.lu@intel.com>,
        linux-arm-kernel@lists.infradead.org,
        Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        Mauro Carvalho Chehab <m.chehab@samsung.com>,
        Samuel Ortiz <sameo@linux.intel.com>, Lee Jones <lee.jones@linaro.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Liam Girdwood <lgirdwood@gmail.com>,
        Kyungmin Park <kyungmin.park@samsung.com>
Subject: Re: [PATCH v2 1/9] i2c: prepare runtime PM support for I2C client
 devices
Message-ID: <20130916144616.GM1823@xora-yoga.xora.org.uk>
References: <20130913065434.GZ7393@intel.com>
 <87bo3whjz4.fsf@linaro.org>
 <20130913145022.GC7393@intel.com>
 <20130913173149.GE7393@intel.com>
 <87ioy4e8bw.fsf@linaro.org>
 <20130915064139.GJ7393@intel.com>
 <20130915124744.GW29403@sirena.org.uk>
 <20130915132823.GL7393@intel.com>
 <20130916101249.GX29403@sirena.org.uk>
 <20130916143811.GP7393@intel.com>
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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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