[PATCH] PCI: Don't clear ASPM bits when the FADT declares it's unsupported

[Colin Ian King]

On 24/04/15 07:59, Alex Hung wrote:
> From: Matthew Garrett <mjg59 at coreos.com>
> 
> Communications with a hardware vendor confirm that the expected behaviour
> on systems that set the FADT ASPM disable bit but which still grant full
> PCIe control is for the OS to leave any BIOS configuration intact and
> refuse to touch the ASPM bits.  This mimics the behaviour of Windows.
> 
> BugLink: http://bugs.launchpad.net/bugs/1441335
> 
> Signed-off-by: Matthew Garrett <mjg59 at coreos.com>
> Signed-off-by: Bjorn Helgaas <bhelgaas at google.com>
> (cherry picked from commit 387d37577fdd05e9472c20885464c2a53b3c945f)
> Signed-off-by: Alex Hung <alex.hung at canonical.com>
> ---
>  drivers/acpi/pci_root.c  | 19 ++++++++-----------
>  drivers/pci/pcie/aspm.c  | 18 ------------------
>  include/linux/pci-aspm.h |  4 ----
>  3 files changed, 8 insertions(+), 33 deletions(-)
> 
> diff --git a/drivers/acpi/pci_root.c b/drivers/acpi/pci_root.c
> index c6bcb8c..2b69fc9 100644
> --- a/drivers/acpi/pci_root.c
> +++ b/drivers/acpi/pci_root.c
> @@ -423,8 +423,7 @@ out:
>  }
>  EXPORT_SYMBOL(acpi_pci_osc_control_set);
>  
> -static void negotiate_os_control(struct acpi_pci_root *root, int *no_aspm,
> -				 int *clear_aspm)
> +static void negotiate_os_control(struct acpi_pci_root *root, int *no_aspm)
>  {
>  	u32 support, control, requested;
>  	acpi_status status;
> @@ -495,10 +494,12 @@ static void negotiate_os_control(struct acpi_pci_root *root, int *no_aspm,
>  		decode_osc_control(root, "OS now controls", control);
>  		if (acpi_gbl_FADT.boot_flags & ACPI_FADT_NO_ASPM) {
>  			/*
> -			 * We have ASPM control, but the FADT indicates
> -			 * that it's unsupported. Clear it.
> +			 * We have ASPM control, but the FADT indicates that
> +			 * it's unsupported. Leave existing configuration
> +			 * intact and prevent the OS from touching it.
>  			 */
> -			*clear_aspm = 1;
> +			dev_info(&device->dev, "FADT indicates ASPM is unsupported, using BIOS configuration\n");
> +			*no_aspm = 1;
>  		}
>  	} else {
>  		decode_osc_control(root, "OS requested", requested);
> @@ -525,7 +526,7 @@ static int acpi_pci_root_add(struct acpi_device *device,
>  	int result;
>  	struct acpi_pci_root *root;
>  	acpi_handle handle = device->handle;
> -	int no_aspm = 0, clear_aspm = 0;
> +	int no_aspm = 0;
>  	bool hotadd = system_state != SYSTEM_BOOTING;
>  
>  	root = kzalloc(sizeof(struct acpi_pci_root), GFP_KERNEL);
> @@ -584,7 +585,7 @@ static int acpi_pci_root_add(struct acpi_device *device,
>  
>  	root->mcfg_addr = acpi_pci_root_get_mcfg_addr(handle);
>  
> -	negotiate_os_control(root, &no_aspm, &clear_aspm);
> +	negotiate_os_control(root, &no_aspm);
>  
>  	/*
>  	 * TBD: Need PCI interface for enumeration/configuration of roots.
> @@ -607,10 +608,6 @@ static int acpi_pci_root_add(struct acpi_device *device,
>  		goto remove_dmar;
>  	}
>  
> -	if (clear_aspm) {
> -		dev_info(&device->dev, "Disabling ASPM (FADT indicates it is unsupported)\n");
> -		pcie_clear_aspm(root->bus);
> -	}
>  	if (no_aspm)
>  		pcie_no_aspm();
>  
> diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> index e1e7026..0f32f6c 100644
> --- a/drivers/pci/pcie/aspm.c
> +++ b/drivers/pci/pcie/aspm.c
> @@ -782,24 +782,6 @@ void pci_disable_link_state(struct pci_dev *pdev, int state)
>  }
>  EXPORT_SYMBOL(pci_disable_link_state);
>  
> -void pcie_clear_aspm(struct pci_bus *bus)
> -{
> -	struct pci_dev *child;
> -
> -	if (aspm_force)
> -		return;
> -
> -	/*
> -	 * Clear any ASPM setup that the firmware has carried out on this bus
> -	 */
> -	list_for_each_entry(child, &bus->devices, bus_list) {
> -		__pci_disable_link_state(child, PCIE_LINK_STATE_L0S |
> -					 PCIE_LINK_STATE_L1 |
> -					 PCIE_LINK_STATE_CLKPM,
> -					 false, true);
> -	}
> -}
> -
>  static int pcie_aspm_set_policy(const char *val, struct kernel_param *kp)
>  {
>  	int i;
> diff --git a/include/linux/pci-aspm.h b/include/linux/pci-aspm.h
> index 8af4610..207c561 100644
> --- a/include/linux/pci-aspm.h
> +++ b/include/linux/pci-aspm.h
> @@ -29,7 +29,6 @@ void pcie_aspm_pm_state_change(struct pci_dev *pdev);
>  void pcie_aspm_powersave_config_link(struct pci_dev *pdev);
>  void pci_disable_link_state(struct pci_dev *pdev, int state);
>  void pci_disable_link_state_locked(struct pci_dev *pdev, int state);
> -void pcie_clear_aspm(struct pci_bus *bus);
>  void pcie_no_aspm(void);
>  #else
>  static inline void pcie_aspm_init_link_state(struct pci_dev *pdev)
> @@ -47,9 +46,6 @@ static inline void pcie_aspm_powersave_config_link(struct pci_dev *pdev)
>  static inline void pci_disable_link_state(struct pci_dev *pdev, int state)
>  {
>  }
> -static inline void pcie_clear_aspm(struct pci_bus *bus)
> -{
> -}
>  static inline void pcie_no_aspm(void)
>  {
>  }
> 

Given that new information has come to light from a hardware vendor that
this strategy makes sense to follow the Windows way of doing things that
this seems sensible. I recall previously that the previous policy was
based on inference on what was best given that there was no written
policy on what to do in this particular case.

Has this been tested on H/W with this FADT setting with this patch on
the target kernel?