PCI: Rework ASPM disable code

Right now we forcibly clear ASPM state on all devices if the BIOS indicates
that the feature isn't supported. Based on the Microsoft presentation
"PCI Express In Depth for Windows Vista and Beyond", I'm starting to think
that this may be an error. The implication is that unless the platform
grants full control via _OSC, Windows will not touch any PCIe features -
including ASPM. In that case clearing ASPM state would be an error unless
the platform has granted us that control.

This patch reworks the ASPM disabling code such that the actual clearing
of state is triggered by a successful handoff of PCIe control to the OS.
The general ASPM code undergoes some changes in order to ensure that the
ability to clear the bits isn't overridden by ASPM having already been
disabled. Further, this theoretically now allows for situations where
only a subset of PCIe roots hand over control, leaving the others in the
BIOS state.

It's difficult to know for sure that this is the right thing to do -
there's zero public documentation on the interaction between all of these
components. But enough vendors enable ASPM on platforms and then set this
bit that it seems likely that they're expecting the OS to leave them alone.

Measured to save around 5W on an idle Thinkpad X220.

Signed-off-by: Matthew Garrett <[e-mail]>
Signed-off-by: Jesse Barnes <[e-mail]>

drivers/acpi/pci_root.c

···
 		if (ACPI_SUCCESS(status)) {
 			dev_info(root->bus->bridge,
 				"ACPI _OSC control (0x%02x) granted\n", flags);
+			if (acpi_gbl_FADT.boot_flags & ACPI_FADT_NO_ASPM) {
+				/*
+				 * We have ASPM control, but the FADT indicates
+				 * that it's unsupported. Clear it.
+				 */
+				pcie_clear_aspm(root->bus);
+			}
 		} else {
 			dev_info(root->bus->bridge,
 				"ACPI _OSC request failed (%s), "

drivers/pci/pci-acpi.c

···
 
 	if (acpi_gbl_FADT.boot_flags & ACPI_FADT_NO_ASPM) {
 		printk(KERN_INFO"ACPI FADT declares the system doesn't support PCIe ASPM, so disable it\n");
-		pcie_clear_aspm();
 		pcie_no_aspm();
 	}
 

drivers/pci/pcie/aspm.c

···
 	struct aspm_latency acceptable[8];
 };
 
-static int aspm_disabled, aspm_force, aspm_clear_state;
+static int aspm_disabled, aspm_force;
 static bool aspm_support_enabled = true;
 static DEFINE_MUTEX(aspm_lock);
 static LIST_HEAD(link_list);
···
 	int pos;
 	u32 reg32;
 
-	if (aspm_clear_state)
-		return -EINVAL;
-
 	/*
 	 * Some functions in a slot might not all be PCIe functions,
 	 * very strange. Disable ASPM for the whole slot
···
 	    pdev->pcie_type != PCI_EXP_TYPE_DOWNSTREAM)
 		return;
 
-	if (aspm_disabled && !aspm_clear_state)
-		return;
-
 	/* VIA has a strange chipset, root port is under a bridge */
 	if (pdev->pcie_type == PCI_EXP_TYPE_ROOT_PORT &&
 	    pdev->bus->self)
···
 	 * the BIOS's expectation, we'll do so once pci_enable_device() is
 	 * called.
 	 */
-	if (aspm_policy != POLICY_POWERSAVE || aspm_clear_state) {
+	if (aspm_policy != POLICY_POWERSAVE) {
 		pcie_config_aspm_path(link);
 		pcie_set_clkpm(link, policy_to_clkpm_state(link));
 	}
···
 	struct pci_dev *parent = pdev->bus->self;
 	struct pcie_link_state *link, *root, *parent_link;
 
-	if ((aspm_disabled && !aspm_clear_state) || !pci_is_pcie(pdev) ||
-	    !parent || !parent->link_state)
+	if (!pci_is_pcie(pdev) || !parent || !parent->link_state)
 		return;
 	if ((parent->pcie_type != PCI_EXP_TYPE_ROOT_PORT) &&
 	    (parent->pcie_type != PCI_EXP_TYPE_DOWNSTREAM))
···
  * pci_disable_link_state - disable pci device's link state, so the link will
  * never enter specific states
  */
-static void __pci_disable_link_state(struct pci_dev *pdev, int state, bool sem)
+static void __pci_disable_link_state(struct pci_dev *pdev, int state, bool sem,
+				     bool force)
 {
 	struct pci_dev *parent = pdev->bus->self;
 	struct pcie_link_state *link;
 
-	if (aspm_disabled || !pci_is_pcie(pdev))
+	if (aspm_disabled && !force)
 		return;
+
+	if (!pci_is_pcie(pdev))
+		return;
+
 	if (pdev->pcie_type == PCI_EXP_TYPE_ROOT_PORT ||
 	    pdev->pcie_type == PCI_EXP_TYPE_DOWNSTREAM)
 		parent = pdev;
···
 
 void pci_disable_link_state_locked(struct pci_dev *pdev, int state)
 {
-	__pci_disable_link_state(pdev, state, false);
+	__pci_disable_link_state(pdev, state, false, false);
 }
 EXPORT_SYMBOL(pci_disable_link_state_locked);
 
 void pci_disable_link_state(struct pci_dev *pdev, int state)
 {
-	__pci_disable_link_state(pdev, state, true);
+	__pci_disable_link_state(pdev, state, true, false);
 }
 EXPORT_SYMBOL(pci_disable_link_state);
+
+void pcie_clear_aspm(struct pci_bus *bus)
+{
+	struct pci_dev *child;
+
+	/*
+	 * 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)
 {
···
 static int __init pcie_aspm_disable(char *str)
 {
 	if (!strcmp(str, "off")) {
+		aspm_policy = POLICY_DEFAULT;
 		aspm_disabled = 1;
 		aspm_support_enabled = false;
 		printk(KERN_INFO "PCIe ASPM is disabled\n");
···
 
 __setup("pcie_aspm=", pcie_aspm_disable);
 
-void pcie_clear_aspm(void)
-{
-	if (!aspm_force)
-		aspm_clear_state = 1;
-}
-
 void pcie_no_aspm(void)
 {
-	if (!aspm_force)
+	/*
+	 * Disabling ASPM is intended to prevent the kernel from modifying
+	 * existing hardware state, not to clear existing state. To that end:
+	 * (a) set policy to POLICY_DEFAULT in order to avoid changing state
+	 * (b) prevent userspace from changing policy
+	 */
+	if (!aspm_force) {
+		aspm_policy = POLICY_DEFAULT;
 		aspm_disabled = 1;
+	}
 }
 
 /**

include/linux/pci-aspm.h

···
 extern void pcie_aspm_powersave_config_link(struct pci_dev *pdev);
 extern void pci_disable_link_state(struct pci_dev *pdev, int state);
 extern void pci_disable_link_state_locked(struct pci_dev *pdev, int state);
-extern void pcie_clear_aspm(void);
+extern void pcie_clear_aspm(struct pci_bus *bus);
 extern void pcie_no_aspm(void);
 #else
 static inline void pcie_aspm_init_link_state(struct pci_dev *pdev)
···
 static inline void pci_disable_link_state(struct pci_dev *pdev, int state)
 {
 }
-static inline void pcie_clear_aspm(void)
+static inline void pcie_clear_aspm(struct pci_bus *bus)
 {
 }
 static inline void pcie_no_aspm(void)