linux-kernel - Re: [PATCH] ACPI: Disable ASPM if the platform won't provide

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Message-Id: <201006222017.30023.rjw@sisk.pl>
Date:	Tue, 22 Jun 2010 20:17:29 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Matthew Garrett <mjg@...hat.com>
Cc:	jbarnes@...tuousgeek.org, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ACPI: Disable ASPM if the platform won't provide _OSC control for PCIe

On Tuesday, June 22, 2010, Matthew Garrett wrote:
> The PCI SIG documentation for the _OSC OS/firmware handshaking interface
> states:
> 
> "If the _OSC control method is absent from the scope of a host bridge
> device, then the operating system must not enable or attempt to use any
> features defined in this section for the hierarchy originated by the host
> bridge."
> 
> The obvious interpretation of this is that the OS should not attempt to use
> PCIe hotplug, PME or AER - however, the specification also notes that an
> _OSC method is *required* for PCIe hierarchies, and experimental validation
> with An Alternative OS indicates that it doesn't use any PCIe functionality
> if the _OSC method is missing. That arguably means we shouldn't be using
> MSI or extended config space, but right now our problems seem to be limited
> to vendors being surprised when ASPM gets enabled on machines when other
> OSs refuse to do so. So, for now, let's just disable ASPM if the _OSC
> method doesn't exist or refuses to hand over PCIe capability control.
> 
> Signed-off-by: Matthew Garrett <mjg@...hat.com>

Acked-by: Rafael J. Wysocki <rjw@...k.pl>

> ---
>  drivers/acpi/pci_root.c |    9 +++++++++
>  1 files changed, 9 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/acpi/pci_root.c b/drivers/acpi/pci_root.c
> index 1af8081..7167213 100644
> --- a/drivers/acpi/pci_root.c
> +++ b/drivers/acpi/pci_root.c
> @@ -32,6 +32,7 @@
>  #include <linux/pm.h>
>  #include <linux/pci.h>
>  #include <linux/pci-acpi.h>
> +#include <linux/pci-aspm.h>
>  #include <linux/acpi.h>
>  #include <acpi/acpi_bus.h>
>  #include <acpi/acpi_drivers.h>
> @@ -576,6 +577,14 @@ static int __devinit acpi_pci_root_add(struct acpi_device *device)
>  	if (flags != base_flags)
>  		acpi_pci_osc_support(root, flags);
>  
> +	status = acpi_pci_osc_control_set(root->device->handle,
> +					  OSC_PCI_EXPRESS_CAP_STRUCTURE_CONTROL);
> +
> +	if (ACPI_FAILURE(status)) {
> +		printk(KERN_INFO "Unable to assume PCIe control: Disabling ASPM\n");
> +		pcie_no_aspm();
> +	}
> +
>  	return 0;
>  
>  end:
> 

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