PCI/ACPI: Report ASPM support to BIOS if not disabled from command line

We need to distinguish the situation in which ASPM support is
disabled from the command line or through .config from the situation
in which it is disabled, because the hardware or BIOS can't handle
it. In the former case we should not report ASPM support to the BIOS
through ACPI _OSC, but in the latter case we should do that.

Introduce pcie_aspm_support_enabled() that can be used by
acpi_pci_root_add() to determine whether or not it should report ASPM
support to the BIOS through _OSC.

Cc: [e-mail]
References: https://bugzilla.kernel.org/show_bug.cgi?id=29722
References: https://bugzilla.kernel.org/show_bug.cgi?id=20232
Reported-and-tested-by: Ortwin Glück <[e-mail]>
Reviewed-by: Kenji Kaneshige <[e-mail]>
Tested-by: Kenji Kaneshige <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>
Signed-off-by: Jesse Barnes <[e-mail]>

drivers/acpi/pci_root.c

···
 	/* Indicate support for various _OSC capabilities. */
 	if (pci_ext_cfg_avail(root->bus->self))
 		flags |= OSC_EXT_PCI_CONFIG_SUPPORT;
-	if (pcie_aspm_enabled())
+	if (pcie_aspm_support_enabled())
 		flags |= OSC_ACTIVE_STATE_PWR_SUPPORT |
 			OSC_CLOCK_PWR_CAPABILITY_SUPPORT;
 	if (pci_msi_enabled())

drivers/pci/pcie/aspm.c

···
 };
 
 static int aspm_disabled, aspm_force, aspm_clear_state;
+static bool aspm_support_enabled = true;
 static DEFINE_MUTEX(aspm_lock);
 static LIST_HEAD(link_list);
 
···
 {
 	if (!strcmp(str, "off")) {
 		aspm_disabled = 1;
+		aspm_support_enabled = false;
 		printk(KERN_INFO "PCIe ASPM is disabled\n");
 	} else if (!strcmp(str, "force")) {
 		aspm_force = 1;
···
 }
 EXPORT_SYMBOL(pcie_aspm_enabled);
 
+bool pcie_aspm_support_enabled(void)
+{
+	return aspm_support_enabled;
+}
+EXPORT_SYMBOL(pcie_aspm_support_enabled);

include/linux/pci.h

···
 #endif
 
 #ifndef CONFIG_PCIEASPM
-static inline int pcie_aspm_enabled(void)
-{
-	return 0;
-}
+static inline int pcie_aspm_enabled(void) { return 0; }
+static inline bool pcie_aspm_support_enabled(void) { return false; }
 #else
 extern int pcie_aspm_enabled(void);
+extern bool pcie_aspm_support_enabled(void);
 #endif
 
 #ifdef CONFIG_PCIEAER