[Qemu-devel] [RFC] hw/core/bus.c: Only the main system bus can have no parent

[Peter Maydell]

In commit 80376c3fc2c38fdd453 in 2010 we added a workaround for
some qbus buses not being connected to qdev devices -- if the
bus has no parent object then we register a reset function which
resets the bus on system reset.

Nearly a decade later, we have now no buses in the tree which
are created with non-NULL parents, so we can remove the
workaround and instead just assert that if the bus has a NULL
parent then it is the main system bus.

(The absence of other parentless buses was confirmed by
code inspection of all the callsites of qbus_create() and
qbus_create_inplace() and cross-checked by 'make check'.)

Signed-off-by: Peter Maydell <address@hidden>
---
While I was reviewing Damian's reset patchset I noticed this
code which meant that we theoretically had multiple 'roots' to
the set of things being reset, so I wondered what was actually
using it. It turns out nothing was :-)

Commit 80376c3fc2c38fdd453 also added a TODO in vl.c suggesting
that there is the wrong place to register the reset function
which effectively resets the whole system starting at the
root which is the main system bus:
   qemu_register_reset(qbus_reset_all_fn, sysbus_get_default());
I don't understand why vl.c is a bad place to put that, and I'd
rather not move it to qdev.c (where in qdev.c?) because that
would reshuffle reset ordering which seems liable to cause
regressions. So maybe we should just delete that TODO comment?

---
 hw/core/bus.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/hw/core/bus.c b/hw/core/bus.c
index e09843f6abe..e50287c2b35 100644
--- a/hw/core/bus.c
+++ b/hw/core/bus.c
@@ -96,10 +96,9 @@ static void qbus_realize(BusState *bus, DeviceState *parent, 
const char *name)
         bus->parent->num_child_bus++;
         object_property_add_child(OBJECT(bus->parent), bus->name, OBJECT(bus), 
NULL);
         object_unref(OBJECT(bus));
-    } else if (bus != sysbus_get_default()) {
-        /* TODO: once all bus devices are qdevified,
-           only reset handler for main_system_bus should be registered here. */
-        qemu_register_reset(qbus_reset_all_fn, bus);
+    } else {
+        /* The only bus without a parent is the main system bus */
+        assert(bus == sysbus_get_default());
     }
 }
 
-- 
2.20.1