linux-kernel - Re: [PATCH] ACPI / PM: Infer parent power state from child if unknown, v2

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Date:	Wed, 12 Sep 2012 14:59:04 +0800
From:	Aaron Lu <aaron.lu@...el.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Len Brown <lenb@...nel.org>, linux-acpi@...r.kernel.org,
	linux-pm@...r.kernel.org, Aaron Lu <aaron.lwe@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ACPI / PM: Infer parent power state from child if
 unknown, v2

On Tue, Sep 11, 2012 at 10:38:06PM +0200, Rafael J. Wysocki wrote:
> It turns out that there are ACPI BIOSes defining device objects with
> _PSx and without either _PSC or _PRx.  For devices corresponding to
> those ACPI objetcs __acpi_bus_get_power() returns ACPI_STATE_UNKNOWN
> and their initial power states are regarded as unknown as a result.
> If such a device is a parent of another power-manageable device, the
> child cannot be put into a low-power state through ACPI, because
> __acpi_bus_set_power() refuses to change power states of devices
> whose parents' power states are unknown.
> 
> To work around this problem, observe that the ACPI power state of
> a device cannot be higher-power (lower-number) than the power state
> of its parent.  Thus, if the device's _PSC method or the
> configuration of its power resources indicates that the device is
> in D0, the device's parent has to be in D0 as well.  Consequently,
> if the parent's power state is unknown when we've just learned that
> its child's power state is D0, we can safely set the parent's
> power.state field to ACPI_STATE_D0.
> 
> Signed-off-by: Rafael J. Wysocki <rjw@...k.pl>

Tested-by: Aaron Lu <aaron.lu@...el.com>

-Aaron

> ---
>  drivers/acpi/bus.c |   10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> Index: linux/drivers/acpi/bus.c
> ===================================================================
> --- linux.orig/drivers/acpi/bus.c
> +++ linux/drivers/acpi/bus.c
> @@ -237,6 +237,16 @@ static int __acpi_bus_get_power(struct a
>  	} else if (result == ACPI_STATE_D3_HOT) {
>  		result = ACPI_STATE_D3;
>  	}
> +
> +	/*
> +	 * If we were unsure about the device parent's power state up to this
> +	 * point, the fact that the device is in D0 implies that the parent has
> +	 * to be in D0 too.
> +	 */
> +	if (device->parent && device->parent->power.state == ACPI_STATE_UNKNOWN
> +	    && result == ACPI_STATE_D0)
> +		device->parent->power.state = ACPI_STATE_D0;
> +
>  	*state = result;
>  
>   out:
--
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