Add new helper pci_tph_dsm_supported() to check whether the root port
of a given PCI device exposes the TPH ST ACPI _DSM method.

Wrap all ACPI logic under CONFIG_ACPI guard, return false directly
when ACPI is disabled.

This helper is prepared for follow-up VFIO TPH virtualization patches
to uniformly judge platform CPU TPH resolving support.

Signed-off-by: Chengwen Feng <fengchengwen@huawei.com>
---
 drivers/pci/tph.c       | 28 ++++++++++++++++++++++++++++
 include/linux/pci-tph.h |  3 +++
 2 files changed, 31 insertions(+)

diff --git a/drivers/pci/tph.c b/drivers/pci/tph.c
index 8194f8b45e3c..e6a850ab2b03 100644
--- a/drivers/pci/tph.c
+++ b/drivers/pci/tph.c
@@ -627,3 +627,31 @@ void pci_tph_init(struct pci_dev *pdev)
 	save_size = sizeof(u32) + num_entries * sizeof(u16);
 	pci_add_ext_cap_save_buffer(pdev, PCI_EXT_CAP_ID_TPH, save_size);
 }
+
+/**
+ * pcie_tph_dsm_supported - Check if root port exposes TPH ST _DSM
+ * @pdev: target PCI device
+ *
+ * Return true if root port of @pdev provides TPH ST DSM function,
+ * false otherwise (ACPI disabled or DSM missing).
+ */
+bool pcie_tph_dsm_supported(struct pci_dev *pdev)
+{
+#ifdef CONFIG_ACPI
+	struct pci_dev *rp = pcie_find_root_port(pdev);
+	acpi_handle handle;
+
+	if (!rp || !rp->bus || !rp->bus->bridge)
+		return false;
+
+	handle = ACPI_HANDLE(rp->bus->bridge);
+	if (!handle)
+		return false;
+
+	return acpi_check_dsm(handle, &pci_acpi_dsm_guid, 7,
+			      BIT(TPH_ST_DSM_FUNC_INDEX));
+#else
+	return false;
+#endif
+}
+EXPORT_SYMBOL_GPL(pcie_tph_dsm_supported);
diff --git a/include/linux/pci-tph.h b/include/linux/pci-tph.h
index c26bf8bba478..aeaf701e7965 100644
--- a/include/linux/pci-tph.h
+++ b/include/linux/pci-tph.h
@@ -50,6 +50,7 @@ u16 pcie_tph_get_st_table_size(struct pci_dev *pdev);
 u32 pcie_tph_get_st_table_loc(struct pci_dev *pdev);
 u8 pcie_tph_enabled_req_type(struct pci_dev *pdev);
 u8 pcie_tph_completer_type(struct pci_dev *pdev);
+bool pcie_tph_dsm_supported(struct pci_dev *pdev);
 #else
 #define pcie_std_tph_supported(pdev) false
 #define pcie_ext_tph_supported(pdev) false
@@ -83,6 +84,8 @@ static inline u8 pcie_tph_enabled_req_type(struct pci_dev *pdev)
 { return PCI_TPH_REQ_DISABLE; }
 static inline u8 pcie_tph_completer_type(struct pci_dev *pdev)
 { return PCI_EXP_DEVCAP2_TPH_COMP_NONE; }
+static inline bool pcie_tph_dsm_supported(struct pci_dev *pdev)
+{ return false; }
 #endif
 
 #endif /* LINUX_PCI_TPH_H */
-- 
2.17.1