PCI: Introduce pci_get_dsn

Several device drivers read their Device Serial Number from the PCIe
extended config space.

Introduce a new helper function, pci_get_dsn(). This function reads the
eight bytes of the DSN and returns them as a u64. If the capability does not
exist for the device, the function returns 0.
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Cc: Michael Chan <michael.chan@broadcom.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/pci/pci.c

@@ -580,6 +580,40 @@ int pci_find_ext_capability(struct pci_dev *dev, int cap)
 }
 EXPORT_SYMBOL_GPL(pci_find_ext_capability);
 
+/**
+ * pci_get_dsn - Read and return the 8-byte Device Serial Number
+ * @dev: PCI device to query
+ *
+ * Looks up the PCI_EXT_CAP_ID_DSN and reads the 8 bytes of the Device Serial
+ * Number.
+ *
+ * Returns the DSN, or zero if the capability does not exist.
+ */
+u64 pci_get_dsn(struct pci_dev *dev)
+{
+	u32 dword;
+	u64 dsn;
+	int pos;
+
+	pos = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_DSN);
+	if (!pos)
+		return 0;
+
+	/*
+	 * The Device Serial Number is two dwords offset 4 bytes from the
+	 * capability position. The specification says that the first dword is
+	 * the lower half, and the second dword is the upper half.
+	 */
+	pos += 4;
+	pci_read_config_dword(dev, pos, &dword);
+	dsn = (u64)dword;
+	pci_read_config_dword(dev, pos + 4, &dword);
+	dsn |= ((u64)dword) << 32;
+
+	return dsn;
+}
+EXPORT_SYMBOL_GPL(pci_get_dsn);
+
 static int __pci_find_next_ht_cap(struct pci_dev *dev, int pos, int ht_cap)
 {
 	int rc, ttl = PCI_FIND_CAP_TTL;

include/linux/pci.h

@@ -1052,6 +1052,8 @@ int pci_find_ht_capability(struct pci_dev *dev, int ht_cap);
 int pci_find_next_ht_capability(struct pci_dev *dev, int pos, int ht_cap);
 struct pci_bus *pci_find_next_bus(const struct pci_bus *from);
 
+u64 pci_get_dsn(struct pci_dev *dev);
+
 struct pci_dev *pci_get_device(unsigned int vendor, unsigned int device,
 			       struct pci_dev *from);
 struct pci_dev *pci_get_subsys(unsigned int vendor, unsigned int device,
@@ -1707,6 +1709,9 @@ static inline int pci_find_next_capability(struct pci_dev *dev, u8 post,
 static inline int pci_find_ext_capability(struct pci_dev *dev, int cap)
 { return 0; }
 
+static inline u64 pci_get_dsn(struct pci_dev *dev)
+{ return 0; }
+
 /* Power management related routines */
 static inline int pci_save_state(struct pci_dev *dev) { return 0; }
 static inline void pci_restore_state(struct pci_dev *dev) { }