hw/dma/i82374: Avoid double creation of the 82374 controller

QEMU fails when used with the following command line:

    ./ppc64-softmmu/qemu-system-ppc64 -S -machine 40p -device i82374
    qemu-system-ppc64: hw/isa/isa-bus.c:110: isa_bus_dma: Assertion `!bus->dma[0] && !bus->dma[1]' failed.

The 40p machine type already creates the device i82374. If specified in the
command line, it will try to create it again, hence generating the error. The
function isa_bus_dma() isn't supposed to be called twice for the same bus.
Check the bus doesn't already have a DMA controller registered before creating
the device.

Fixes: https://bugs.launchpad.net/qemu/+bug/1721224Signed-off-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180326153441.32641-2-f4bug@amsat.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/dma/i82374.c

@@ -23,6 +23,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "qapi/error.h"
 #include "hw/isa/isa.h"
 #include "hw/dma/i8257.h"
 
@@ -118,13 +119,19 @@ static const MemoryRegionPortio i82374_portio_list[] = {
 static void i82374_realize(DeviceState *dev, Error **errp)
 {
     I82374State *s = I82374(dev);
+    ISABus *isa_bus = isa_bus_from_device(ISA_DEVICE(dev));
+
+    if (isa_get_dma(isa_bus, 0)) {
+        error_setg(errp, "DMA already initialized on ISA bus");
+        return;
+    }
+    i8257_dma_init(isa_bus, true);
 
     portio_list_init(&s->port_list, OBJECT(s), i82374_portio_list, s,
                      "i82374");
     portio_list_add(&s->port_list, isa_address_space_io(&s->parent_obj),
                     s->iobase);
 
-    i8257_dma_init(isa_bus_from_device(ISA_DEVICE(dev)), true);
     memset(s->commands, 0, sizeof(s->commands));
 }