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.
Paolo
