PCI: Add pci_find_dvsec_capability to find designated VSEC

Add pci_find_dvsec_capability to locate a Designated Vendor-Specific
Extended Capability with the specified Vendor ID and Capability ID.

The Designated Vendor-Specific Extended Capability (DVSEC) allows one or
more "vendor" specific capabilities that are not tied to the Vendor ID
of the PCI component. Where the DVSEC Vendor may be a standards body
like CXL.

Cc: David E. Box <[e-mail]>
Cc: Jonathan Cameron <[e-mail]>
Cc: Bjorn Helgaas <[e-mail]>
Cc: Dan Williams <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: Andrew Donnellan <[e-mail]>
Cc: Lu Baolu <[e-mail]>
Reviewed-by: Frederic Barrat <[e-mail]>
Signed-off-by: Ben Widawsky <[e-mail]>
Reviewed-by: Andrew Donnellan <[e-mail]>
Acked-by: Bjorn Helgaas <[e-mail]>
Tested-by: Kan Liang <[e-mail]>
Signed-off-by: Kan Liang <[e-mail]>
Reviewed-by: Jonathan Cameron <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dan Williams <[e-mail]>

drivers/pci/pci.c

···
 EXPORT_SYMBOL_GPL(pci_find_vsec_capability);
 
 /**
+ * pci_find_dvsec_capability - Find DVSEC for vendor
+ * @dev: PCI device to query
+ * @vendor: Vendor ID to match for the DVSEC
+ * @dvsec: Designated Vendor-specific capability ID
+ *
+ * If DVSEC has Vendor ID @vendor and DVSEC ID @dvsec return the capability
+ * offset in config space; otherwise return 0.
+ */
+u16 pci_find_dvsec_capability(struct pci_dev *dev, u16 vendor, u16 dvsec)
+{
+	int pos;
+
+	pos = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_DVSEC);
+	if (!pos)
+		return 0;
+
+	while (pos) {
+		u16 v, id;
+
+		pci_read_config_word(dev, pos + PCI_DVSEC_HEADER1, &v);
+		pci_read_config_word(dev, pos + PCI_DVSEC_HEADER2, &id);
+		if (vendor == v && dvsec == id)
+			return pos;
+
+		pos = pci_find_next_ext_capability(dev, pos, PCI_EXT_CAP_ID_DVSEC);
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(pci_find_dvsec_capability);
+
+/**
  * pci_find_parent_resource - return resource region of parent bus of given
  *			      region
  * @dev: PCI device structure contains resources to be searched

include/linux/pci.h

···
 u16 pci_find_next_ext_capability(struct pci_dev *dev, u16 pos, int cap);
 struct pci_bus *pci_find_next_bus(const struct pci_bus *from);
 u16 pci_find_vsec_capability(struct pci_dev *dev, u16 vendor, int cap);
+u16 pci_find_dvsec_capability(struct pci_dev *dev, u16 vendor, u16 dvsec);
 
 u64 pci_get_dsn(struct pci_dev *dev);