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: 2dbce5901179 ("PCI: Assign bus numbers present in EA capability for bridges")
Link: https://lore.kernel.org/r/1572850664-9861-1-git-send-email-sundeep.lkml@gmail.com
Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org	# v5.2+

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index bdbc849..d303387 100644 (file)
--- a/drivers/pci/probe.c
+++ b/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;
 }