The CM4 has an actual PCIe slot, so the device filtering
need to be a little less restrictive WRT busses with more
than 1 device given that switches can now appear in the
topology. Since it is possible to start numbering the
busses with a non-zero value, the bus restriction should
be based on the secondary side of the root port. This
isn't likely but its better than hard-coding the limit.

Suggested-by: René Treffer <treffer+groups.io@measite.de>
Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
---
 .../Library/Bcm2711PciSegmentLib/PciSegmentLib.c    | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/Silicon/Broadcom/Bcm27xx/Library/Bcm2711PciSegmentLib/PciSegmentLib.c b/Silicon/Broadcom/Bcm27xx/Library/Bcm2711PciSegmentLib/PciSegmentLib.c
index 44ce3b4b99..6d15e82fa2 100644
--- a/Silicon/Broadcom/Bcm27xx/Library/Bcm2711PciSegmentLib/PciSegmentLib.c
+++ b/Silicon/Broadcom/Bcm27xx/Library/Bcm2711PciSegmentLib/PciSegmentLib.c
@@ -19,6 +19,7 @@
 #include <Library/PciSegmentLib.h>
 #include <Library/UefiLib.h>
 #include <IndustryStandard/Bcm2711.h>
+#include <IndustryStandard/Pci30.h>
 
 typedef enum {
   PciCfgWidthUint8 = 0,
@@ -78,6 +79,9 @@ PciSegmentLibGetConfigBase (
   UINT64        Base;
   UINT64        Offset;
   UINT32        Dev;
+  UINT32        Bus;
+  UINT32        Data;
+  UINT32        HostPortSec;
 
   Base = PCIE_REG_BASE;
   Offset = Address & 0xFFF;         /* Pick off the 4k register offset */
@@ -89,17 +93,20 @@ PciSegmentLibGetConfigBase (
     Base += PCIE_EXT_CFG_DATA;
     if (mPciSegmentLastAccess != Address) {
       Dev = EFI_PCI_ADDR_DEV (Address);
+      Bus = EFI_PCI_ADDR_BUS (Address);
+      HostPortSec = MmioRead8 (PCIE_REG_BASE +
+                               PCI_BRIDGE_SECONDARY_BUS_REGISTER_OFFSET);
+
       /*
-       * Scan things out directly rather than translating the "bus" to a device, etc..
-       * only we need to limit each bus to a single device.
+       * There can only be a single device on bus 1 (downstream of root).
+       * Subsequent busses (behind a PCIe switch) can have more.
        */
-      if (Dev < 1) {
-          MmioWrite32 (PCIE_REG_BASE + PCIE_EXT_CFG_INDEX, Address);
-          mPciSegmentLastAccess = Address;
-      } else {
-          mPciSegmentLastAccess = 0;
+      if (Dev > 0 && (Bus <= HostPortSec)) {
           return 0xFFFFFFFF;
       }
+
+      MmioWrite32 (PCIE_REG_BASE + PCIE_EXT_CFG_INDEX, Address);
+      mPciSegmentLastAccess = Address;
     }
   }
   return Base + Offset;
--
2.13.7