On Wed, Jun 26, 2019 at 12:14:15PM +0200, Paolo Bonzini wrote:
> On 26/06/19 06:46, Markus Armbruster wrote:
> >> I'm not sure how to wire it together without the bus abstraction? So
> >> I'll stick with the bus for now. It *is* extremely convenient!
> >
> > As far as I can tell offhand, a common use of bus-less connections
> > between devices is wiring together composite devices. Example:
> >
> > static void designware_pcie_host_init(Object *obj)
> > {
> > DesignwarePCIEHost *s = DESIGNWARE_PCIE_HOST(obj);
> > DesignwarePCIERoot *root = &s->root;
> >
> > object_initialize_child(obj, "root", root, sizeof(*root),
> > TYPE_DESIGNWARE_PCIE_ROOT, &error_abort,
> > NULL);
> > qdev_prop_set_int32(DEVICE(root), "addr", PCI_DEVFN(0, 0));
> > qdev_prop_set_bit(DEVICE(root), "multifunction", false);
> > }
> >
> > This creates a TYPE_DESIGNWARE_PCIE_ROOT device "within" the
> > TYPE_DESIGNWARE_PCIE_HOST device.
> >
> > Bus-less connections between separate devices (i.e. neither device is a
> > part of the other) are also possible. But I'm failing at grep right
> > now. Here's an example for connecting a device to a machine:
> >
> > static void mch_realize(PCIDevice *d, Error **errp)
> > {
> > int i;
> > MCHPCIState *mch = MCH_PCI_DEVICE(d);
> >
> > [...]
> > object_property_add_const_link(qdev_get_machine(), "smram",
> > OBJECT(&mch->smram), &error_abort);
> > [...]
> > }
>
> This is a link to a memory region. A connection to a separate device
> can be found in hw/dma/xilinx_axidma.c and hw/net/xilinx_axienet.c,
> where you have
>
> data stream <------------> data stream
> / \
> dma enet
> \ /
> control stream <------> control stream
>
> where the horizontal links in the middle are set up by board code, while
> the diagonal lines on the side are set up by device code.
>
> > Paolo, can you provide guidance on when to use a bus, and when not to?
>
> I would definitely use a bus if 1) it is common for the user (and not
> for machine code) to set up the connection 2) the relationship is
> parent-child. Link properties are basically unused on the command line,
> and it only makes sense to make something different if the connection is
> some kind of graph so bus-child does not cut it.
>
Definitely looks like the bus is the way to go. The controller/namespace
relationship is strictly parent-child.
Thanks both of you for the advice!
Klaus
