[PATCH] arm64: acpi/pci: WARN() when ignoring _DSM request to preserve PCI resources

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On arm64 ACPI systems, we unconditionally reconfigure the entire PCI
hierarchy at boot. This is a departure from what is customary on ACPI
systems, and may break assumptions in some places (e.g., CCIX), that
the OS will honour the PCI resource allocation as configured by the
firmware.

ACPI already specifies a device specific ACPI method (_DSM) for PCI
root bridge nodes that tells us whether the firmware thinks the
configuration should be ignored or preserved, but unfortunately, we
cannot simply change the behavior of the OS and start honouring
these requests unconditionally, given that firmware implementations
in the field may issue the _DSM request inadvertently and still rely
on the OS to create the resource allocation from scratch.

So as a preliminary step, let's issue a warning about this case,
increasing the likelihood that we can fix this behavior at some
point in the future.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 arch/arm64/kernel/pci.c  | 15 +++++++++++++++
 include/linux/pci-acpi.h |  7 ++++---
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/pci.c b/arch/arm64/kernel/pci.c
index bb85e2f4603f..318ca865c41f 100644
--- a/arch/arm64/kernel/pci.c
+++ b/arch/arm64/kernel/pci.c
@@ -168,6 +168,7 @@ struct pci_bus *pci_acpi_scan_root(struct acpi_pci_root *root)
 	struct acpi_pci_generic_root_info *ri;
 	struct pci_bus *bus, *child;
 	struct acpi_pci_root_ops *root_ops;
+	union acpi_object *obj;
 
 	ri = kzalloc(sizeof(*ri), GFP_KERNEL);
 	if (!ri)
@@ -193,6 +194,20 @@ struct pci_bus *pci_acpi_scan_root(struct acpi_pci_root *root)
 	if (!bus)
 		return NULL;
 
+	/*
+	 * Invoke the PCI device specific method (_DSM) #5 'Ignore PCI Boot
+	 * Configuration', which tells us whether the firmware wants us to
+	 * preserve or ignore the firmware's configuration of the PCI resource
+	 * tree for this root bridge.
+	 */
+	obj = acpi_evaluate_dsm(ACPI_HANDLE(bus->bridge), &pci_acpi_dsm_guid, 1,
+				IGNORE_PCI_BOOT_CONFIG_DSM, NULL);
+	if (obj && obj->type == ACPI_TYPE_INTEGER)
+		WARN_ONCE(obj->integer.value == 0,
+			  "Firmware did not request its PCI resource allocation to be ignored, but ignoring it anyway\n");
+
+	ACPI_FREE(obj);
+
 	pci_bus_size_bridges(bus);
 	pci_bus_assign_resources(bus);
 
diff --git a/include/linux/pci-acpi.h b/include/linux/pci-acpi.h
index 8082b612f561..62b7fdcc661c 100644
--- a/include/linux/pci-acpi.h
+++ b/include/linux/pci-acpi.h
@@ -107,9 +107,10 @@ static inline void acpiphp_check_host_bridge(struct acpi_device *adev) { }
 #endif
 
 extern const guid_t pci_acpi_dsm_guid;
-#define DEVICE_LABEL_DSM	0x07
-#define RESET_DELAY_DSM		0x08
-#define FUNCTION_DELAY_DSM	0x09
+#define IGNORE_PCI_BOOT_CONFIG_DSM	0x05
+#define DEVICE_LABEL_DSM		0x07
+#define RESET_DELAY_DSM			0x08
+#define FUNCTION_DELAY_DSM		0x09
 
 #else	/* CONFIG_ACPI */
 static inline void acpi_pci_add_bus(struct pci_bus *bus) { }
-- 
2.17.1