Set the ISA Bus link property on the PIIX IDE object from the
two unique users, the PC machine and the PIIX4 function.

Add a check in pci_piix_ide_realize() to be sure this property
is set.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 hw/i386/pc_piix.c |  2 ++
 hw/ide/piix.c     |  5 +++++
 hw/isa/piix4.c    | 10 +++++++---
 3 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index fb606c14768..d799c8004df 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -246,6 +246,8 @@ static void pc_init1(MachineState *machine,
 
         dev = pci_new(piix3_devfn + 1,
                       xen_enabled() ? "piix3-ide-xen" : "piix3-ide");
+        object_property_set_link(OBJECT(dev), "isa-bus",
+                                 OBJECT(isa_bus), &error_abort);
         pci_realize_and_unref(dev, pci_bus, &error_abort);
         pci_ide_create_devs(dev);
         idebus[0] = qdev_get_child_bus(&dev->qdev, "ide.0");
diff --git a/hw/ide/piix.c b/hw/ide/piix.c
index 48da68da37f..3aef9b1e21c 100644
--- a/hw/ide/piix.c
+++ b/hw/ide/piix.c
@@ -152,6 +152,11 @@ static void pci_piix_ide_realize(PCIDevice *dev, Error **errp)
     PCIIDEState *d = PCI_IDE(dev);
     uint8_t *pci_conf = dev->config;
 
+    if (!d->isa_bus) {
+        error_setg(errp, "piix-ide: 'isa-bus' link not set");
+        return;
+    }
+
     pci_conf[PCI_CLASS_PROG] = 0x80; // legacy ATA mode
 
     bmdma_setup_bar(d);
diff --git a/hw/isa/piix4.c b/hw/isa/piix4.c
index d60f161ecf4..ef11c2d1f8d 100644
--- a/hw/isa/piix4.c
+++ b/hw/isa/piix4.c
@@ -248,20 +248,24 @@ static void piix4_register_types(void)
 
 type_init(piix4_register_types)
 
-DeviceState *piix4_create(PCIBus *pci_bus, ISABus **isa_bus, I2CBus **smbus)
+DeviceState *piix4_create(PCIBus *pci_bus, ISABus **isa_busp, I2CBus **smbus)
 {
     PCIDevice *pci;
     DeviceState *dev;
+    ISABus *isa_bus;
     int devfn = PCI_DEVFN(10, 0);
 
     pci = pci_create_simple_multifunction(pci_bus, devfn,  true,
                                           TYPE_PIIX4_PCI_DEVICE);
     dev = DEVICE(pci);
-    if (isa_bus) {
-        *isa_bus = ISA_BUS(qdev_get_child_bus(dev, "isa.0"));
+    isa_bus = ISA_BUS(qdev_get_child_bus(dev, "isa.0"));
+    if (isa_busp) {
+        *isa_busp = isa_bus;
     }
 
     pci = pci_new(devfn + 1, "piix4-ide");
+    object_property_set_link(OBJECT(pci), "isa-bus",
+                             OBJECT(isa_bus), &error_abort);
     pci_realize_and_unref(pci, pci_bus, &error_abort);
     pci_ide_create_devs(pci);
 
-- 
2.26.3