linux-kernel - Re: [PATCH] ACPI / PM: Fix potential problem in acpi_device_get

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Date:	Mon, 25 Mar 2013 16:01:35 +0800
From:	Aaron Lu <aaron.lu@...el.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
CC:	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, Len Brown <lenb@...nel.org>
Subject: Re: [PATCH] ACPI / PM: Fix potential problem in acpi_device_get_power()

On 03/24/2013 07:57 AM, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> Theoretically, in some situations acpi_device_get_power() may return
> an incorrect result, because the settings of the power resources
> depended on by the device may indicate a power state shallower than
> the actual power state of the device.
> 
> Say that two devices, A and B, depend on two power resources, X and
> Y, in such a way that _PR0 for both A and B list both X and Y and
> _PR3 for both A and B list power resource Y alone.  Also suppose
> that _PS0 and _PS3 are present for both A and B.  Then, if devices
> A and B are initially in D0, power resources X and Y are initially
> "on" and their reference counters are equal to 2.  To put device A
> into power state D3hot the kernel will decrement the reference
> counter of power resource X, but that power resource won't be turned
> off, because it is still in use by device B (its reference counter is
> equal to 1).  Next, _PS3 will be executed for device A.  Afterward
> the configuration of the power resources will indicate that device
> A is in power state D0 (both X and Y are "on"), but in fact it is
> in D3hot (because _PS3 has been executed for it).

I'm not sure if D3hot is correct here, since the power resource X is
still on?

I agree that, at least from OSPM's perspective, D3hot is better than D0
here.

Thanks,
Aaron

> 
> In that situation, if acpi_device_get_power() is called to get the
> power state of device A, it will first execute _PSC for it which
> should return 3.  That will cause acpi_device_get_power() to run
> acpi_power_get_inferred_state() for device A and the resultant power
> state will be D0, which is incorrect.
> 
> To fix that change acpi_device_get_power() to first execute
> acpi_power_get_inferred_state() for the given device (if it
> depends on power resources) and to evaluate _PSC for it subsequently,
> so that the result inferred from the power resources configuration
> can be amended by the _PSC return value.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>  drivers/acpi/device_pm.c |   39 ++++++++++++++++++++++++---------------
>  1 file changed, 24 insertions(+), 15 deletions(-)
> 
> Index: linux-pm/drivers/acpi/device_pm.c
> ===================================================================
> --- linux-pm.orig/drivers/acpi/device_pm.c
> +++ linux-pm/drivers/acpi/device_pm.c
> @@ -145,27 +145,36 @@ int acpi_device_get_power(struct acpi_de
>  	}
>  
>  	/*
> -	 * Get the device's power state either directly (via _PSC) or
> -	 * indirectly (via power resources).
> +	 * Get the device's power state from power resources settings and _PSC,
> +	 * if available.
>  	 */
> +	if (device->power.flags.power_resources) {
> +		int error = acpi_power_get_inferred_state(device, &result);
> +		if (error)
> +			return error;
> +	}
>  	if (device->power.flags.explicit_get) {
> +		acpi_handle handle = device->handle;
>  		unsigned long long psc;
> -		acpi_status status = acpi_evaluate_integer(device->handle,
> -							   "_PSC", NULL, &psc);
> +		acpi_status status;
> +
> +		status = acpi_evaluate_integer(handle, "_PSC", NULL, &psc);
>  		if (ACPI_FAILURE(status))
>  			return -ENODEV;
>  
> -		result = psc;
> -	}
> -	/* The test below covers ACPI_STATE_UNKNOWN too. */
> -	if (result <= ACPI_STATE_D2) {
> -	  ; /* Do nothing. */
> -	} else if (device->power.flags.power_resources) {
> -		int error = acpi_power_get_inferred_state(device, &result);
> -		if (error)
> -			return error;
> -	} else if (result == ACPI_STATE_D3_HOT) {
> -		result = ACPI_STATE_D3;
> +		/*
> +		 * The power resources settings may indicate a power state
> +		 * shallower than the actual power state of the device.
> +		 *
> +		 * Moreover, on systems predating ACPI 4.0, if the device
> +		 * doesn't depend on any power resources and _PSC returns 3,
> +		 * that means "power off".  We need to maintain compatibility
> +		 * with those systems.
> +		 */
> +		if (psc > result && psc < ACPI_STATE_D3_COLD)
> +			result = psc;
> +		else if (result == ACPI_STATE_UNKNOWN)
> +			result = psc > ACPI_STATE_D2 ? ACPI_STATE_D3_COLD : psc;
>  	}
>  
>  	/*
> 

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