hw/pci-host/designware: Prevent device attachment on internal PCIe root bus

On the real device, the PCIe root bus is only connected to a PCIe bridge and
does not allow for direct attachment of devices. Doing so in QEMU results in no
PCI devices being detected by Linux. Instead, PCI devices should plug into the
secondary PCIe bus spawned by the internal PCIe bridge.

Unfortunately, QEMU defaults to plugging devices into the PCIe root bus. To work
around this, every PCI device created on the command line needs an extra
`bus=dw-pcie` option which is error prone. Fix that by marking the PCIe root bus
as full which makes QEMU decend into the child PCIe bus.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Message-id: 20250223114708.1780-3-shentey@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/pci-host/designware.c

@@ -55,6 +55,17 @@
 #define DESIGNWARE_PCIE_ATU_DEVFN(x)               (((x) >> 16) & 0xff)
 #define DESIGNWARE_PCIE_ATU_UPPER_TARGET           0x91C
 
+static void designware_pcie_root_bus_class_init(ObjectClass *klass, void *data)
+{
+    BusClass *k = BUS_CLASS(klass);
+
+    /*
+     * Designware has only a single root complex. Enforce the limit on the
+     * parent bus
+     */
+    k->max_dev = 1;
+}
+
 static DesignwarePCIEHost *
 designware_pcie_root_to_host(DesignwarePCIERoot *root)
 {
@@ -699,7 +710,7 @@ static void designware_pcie_host_realize(DeviceState *dev, Error **errp)
                                      &s->pci.memory,
                                      &s->pci.io,
                                      0, 4,
-                                     TYPE_PCIE_BUS);
+                                     TYPE_DESIGNWARE_PCIE_ROOT_BUS);
     pci->bus->flags |= PCI_BUS_EXTENDED_CONFIG_SPACE;
 
     memory_region_init(&s->pci.address_space_root,
@@ -754,6 +765,11 @@ static void designware_pcie_host_init(Object *obj)
 
 static const TypeInfo designware_pcie_types[] = {
     {
+        .name           = TYPE_DESIGNWARE_PCIE_ROOT_BUS,
+        .parent         = TYPE_PCIE_BUS,
+        .instance_size  = sizeof(DesignwarePCIERootBus),
+        .class_init     = designware_pcie_root_bus_class_init,
+    }, {
         .name           = TYPE_DESIGNWARE_PCIE_HOST,
         .parent         = TYPE_PCI_HOST_BRIDGE,
         .instance_size  = sizeof(DesignwarePCIEHost),

include/hw/pci-host/designware.h

@@ -25,12 +25,19 @@
 #include "hw/pci/pci_bridge.h"
 #include "qom/object.h"
 
+#define TYPE_DESIGNWARE_PCIE_ROOT_BUS "designware-pcie-root-BUS"
+OBJECT_DECLARE_SIMPLE_TYPE(DesignwarePCIERootBus, DESIGNWARE_PCIE_ROOT_BUS)
+
 #define TYPE_DESIGNWARE_PCIE_HOST "designware-pcie-host"
 OBJECT_DECLARE_SIMPLE_TYPE(DesignwarePCIEHost, DESIGNWARE_PCIE_HOST)
 
 #define TYPE_DESIGNWARE_PCIE_ROOT "designware-pcie-root"
 OBJECT_DECLARE_SIMPLE_TYPE(DesignwarePCIERoot, DESIGNWARE_PCIE_ROOT)
 
+struct DesignwarePCIERootBus {
+    PCIBus parent;
+};
+
 typedef struct DesignwarePCIEViewport {
     DesignwarePCIERoot *root;