PCI: Do not use bus number zero from EA capability

As per PCIe r5.0, sec 7.8.5.2, fixed bus numbers of a bridge must be zero
when no function that uses EA is located behind it.  Hence, if EA supplies
bus numbers of zero, assign bus numbers normally.  A secondary bus can
never have a bus number of zero, so setting a bridge's Secondary Bus Number
to zero makes downstream devices unreachable.

[bhelgaas: retain bool return value so "zero is invalid" logic is local]
Fixes: 2dbce59 ("PCI: Assign bus numbers present in EA capability for bridges")
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Subbaraya Sundeep <[email protected]>
Signed-off-by: Bjorn Helgaas <[email protected]>
Cc: [email protected]	# v5.2+

Author: Subbaraya Sundeep

drivers/pci/probe.c

@@ -1090,14 +1090,15 @@ static unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
  * @sec: updated with secondary bus number from EA
  * @sub: updated with subordinate bus number from EA
  *
- * If @dev is a bridge with EA capability, update @sec and @sub with
- * fixed bus numbers from the capability and return true.  Otherwise,
- * return false.
+ * If @dev is a bridge with EA capability that specifies valid secondary
+ * and subordinate bus numbers, return true with the bus numbers in @sec
+ * and @sub.  Otherwise return false.
  */
 static bool pci_ea_fixed_busnrs(struct pci_dev *dev, u8 *sec, u8 *sub)
 {
 	int ea, offset;
 	u32 dw;
+	u8 ea_sec, ea_sub;
 
 	if (dev->hdr_type != PCI_HEADER_TYPE_BRIDGE)
 		return false;
@@ -1109,8 +1110,13 @@ static bool pci_ea_fixed_busnrs(struct pci_dev *dev, u8 *sec, u8 *sub)
 
 	offset = ea + PCI_EA_FIRST_ENT;
 	pci_read_config_dword(dev, offset, &dw);
-	*sec =  dw & PCI_EA_SEC_BUS_MASK;
-	*sub = (dw & PCI_EA_SUB_BUS_MASK) >> PCI_EA_SUB_BUS_SHIFT;
+	ea_sec =  dw & PCI_EA_SEC_BUS_MASK;
+	ea_sub = (dw & PCI_EA_SUB_BUS_MASK) >> PCI_EA_SUB_BUS_SHIFT;
+	if (ea_sec  == 0 || ea_sub < ea_sec)
+		return false;
+
+	*sec = ea_sec;
+	*sub = ea_sub;
 	return true;
 }