PCI: Cache offset of Resizable BAR capability

Previously most resizable BAR interfaces (pci_rebar_get_possible_sizes(),
pci_rebar_set_size(), etc) as well as pci_restore_state() searched config
space for a Resizable BAR capability.  Most devices don't have such a
capability, so this is wasted effort, especially for pci_restore_state().

Search for a Resizable BAR capability once at enumeration-time and cache
the offset so we don't have to search every time we need it.  No functional
change intended.

Link: https://lore.kernel.org/r/20250215000301.175097-3-helgaas@kernel.org
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

drivers/pci/pci.c

@@ -1871,7 +1871,7 @@ static void pci_restore_rebar_state(struct pci_dev *pdev)
 	unsigned int pos, nbars, i;
 	u32 ctrl;
 
-	pos = pci_find_ext_capability(pdev, PCI_EXT_CAP_ID_REBAR);
+	pos = pdev->rebar_cap;
 	if (!pos)
 		return;
 
@@ -3718,6 +3718,11 @@ void pci_acs_init(struct pci_dev *dev)
 	pci_enable_acs(dev);
 }
 
+void pci_rebar_init(struct pci_dev *pdev)
+{
+	pdev->rebar_cap = pci_find_ext_capability(pdev, PCI_EXT_CAP_ID_REBAR);
+}
+
 /**
  * pci_rebar_find_pos - find position of resize ctrl reg for BAR
  * @pdev: PCI device
@@ -3732,7 +3737,7 @@ static int pci_rebar_find_pos(struct pci_dev *pdev, int bar)
 	unsigned int pos, nbars, i;
 	u32 ctrl;
 
-	pos = pci_find_ext_capability(pdev, PCI_EXT_CAP_ID_REBAR);
+	pos = pdev->rebar_cap;
 	if (!pos)
 		return -ENOTSUPP;
 

drivers/pci/pci.h

@@ -799,6 +799,7 @@ static inline int acpi_get_rc_resources(struct device *dev, const char *hid,
 }
 #endif
 
+void pci_rebar_init(struct pci_dev *pdev);
 int pci_rebar_get_current_size(struct pci_dev *pdev, int bar);
 int pci_rebar_set_size(struct pci_dev *pdev, int bar, int size);
 static inline u64 pci_rebar_size_to_bytes(int size)

drivers/pci/probe.c

@@ -2566,6 +2566,7 @@ static void pci_init_capabilities(struct pci_dev *dev)
 	pci_rcec_init(dev);		/* Root Complex Event Collector */
 	pci_doe_init(dev);		/* Data Object Exchange */
 	pci_tph_init(dev);		/* TLP Processing Hints */
+	pci_rebar_init(dev);		/* Resizable BAR */
 
 	pcie_report_downtraining(dev);
 	pci_init_reset_methods(dev);

include/linux/pci.h

@@ -353,6 +353,7 @@ struct pci_dev {
 	struct pci_dev  *rcec;          /* Associated RCEC device */
 #endif
 	u32		devcap;		/* PCIe Device Capabilities */
+	u16		rebar_cap;	/* Resizable BAR capability offset */
 	u8		pcie_cap;	/* PCIe capability offset */
 	u8		msi_cap;	/* MSI capability offset */
 	u8		msix_cap;	/* MSI-X capability offset */