PCI/PTM: Save/restore Precision Time Measurement Capability for suspend/resume

The PCI subsystem does not currently save and restore the configuration
space for the Precision Time Measurement (PTM) Extended Capability leading
to the possibility of the feature returning disabled on S3 resume.  This
has been observed on Intel Coffee Lake desktops. Add save/restore of the
PTM control register. This saves the PTM Enable, Root Select, and Effective
Granularity bits.

Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/20201207223951.19667-1-david.e.box@linux.intel.com
Signed-off-by: David E. Box <david.e.box@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/pci.c

@@ -1566,6 +1566,7 @@ int pci_save_state(struct pci_dev *dev)
 	pci_save_ltr_state(dev);
 	pci_save_dpc_state(dev);
 	pci_save_aer_state(dev);
+	pci_save_ptm_state(dev);
 	return pci_save_vc_state(dev);
 }
 EXPORT_SYMBOL(pci_save_state);
@@ -1677,6 +1678,7 @@ void pci_restore_state(struct pci_dev *dev)
 	pci_restore_vc_state(dev);
 	pci_restore_rebar_state(dev);
 	pci_restore_dpc_state(dev);
+	pci_restore_ptm_state(dev);
 
 	pci_aer_clear_status(dev);
 	pci_restore_aer_state(dev);

drivers/pci/pci.h

@@ -516,6 +516,14 @@ static inline int pci_iov_bus_range(struct pci_bus *bus)
 
 #endif /* CONFIG_PCI_IOV */
 
+#ifdef CONFIG_PCIE_PTM
+void pci_save_ptm_state(struct pci_dev *dev);
+void pci_restore_ptm_state(struct pci_dev *dev);
+#else
+static inline void pci_save_ptm_state(struct pci_dev *dev) { }
+static inline void pci_restore_ptm_state(struct pci_dev *dev) { }
+#endif
+
 unsigned long pci_cardbus_resource_alignment(struct resource *);
 
 static inline resource_size_t pci_resource_alignment(struct pci_dev *dev,

drivers/pci/pcie/ptm.c

@@ -29,6 +29,47 @@ static void pci_ptm_info(struct pci_dev *dev)
 		 dev->ptm_root ? " (root)" : "", clock_desc);
 }
 
+void pci_save_ptm_state(struct pci_dev *dev)
+{
+	int ptm;
+	struct pci_cap_saved_state *save_state;
+	u16 *cap;
+
+	if (!pci_is_pcie(dev))
+		return;
+
+	ptm = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_PTM);
+	if (!ptm)
+		return;
+
+	save_state = pci_find_saved_ext_cap(dev, PCI_EXT_CAP_ID_PTM);
+	if (!save_state) {
+		pci_err(dev, "no suspend buffer for PTM\n");
+		return;
+	}
+
+	cap = (u16 *)&save_state->cap.data[0];
+	pci_read_config_word(dev, ptm + PCI_PTM_CTRL, cap);
+}
+
+void pci_restore_ptm_state(struct pci_dev *dev)
+{
+	struct pci_cap_saved_state *save_state;
+	int ptm;
+	u16 *cap;
+
+	if (!pci_is_pcie(dev))
+		return;
+
+	save_state = pci_find_saved_ext_cap(dev, PCI_EXT_CAP_ID_PTM);
+	ptm = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_PTM);
+	if (!save_state || !ptm)
+		return;
+
+	cap = (u16 *)&save_state->cap.data[0];
+	pci_write_config_word(dev, ptm + PCI_PTM_CTRL, *cap);
+}
+
 void pci_ptm_init(struct pci_dev *dev)
 {
 	int pos;
@@ -65,6 +106,8 @@ void pci_ptm_init(struct pci_dev *dev)
 	if (!pos)
 		return;
 
+	pci_add_ext_cap_save_buffer(dev, PCI_EXT_CAP_ID_PTM, sizeof(u16));
+
 	pci_read_config_dword(dev, pos + PCI_PTM_CAP, &cap);
 	local_clock = (cap & PCI_PTM_GRANULARITY_MASK) >> 8;