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From: Knut Omang <knut.omang@oracle.com>
Date: Wed, 20 Aug 2014 15:08:51 +0200
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Subject: Re: [Qemu-devel] [PATCH 2/3] ioh3420: Provide a unique bus name and
 an interrupt mapping function
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Juan Quintela <quintela@redhat.com>, Alexey Kardashevskiy <aik@ozlabs.ru>, Marcel Apfelbaum <marcel.a@redhat.com>, qemu-devel@nongnu.org, Markus Armbruster <armbru@redhat.com>, Gonglei <arei.gonglei@huawei.com>, Igor Mammedov <imammedo@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>

On Wed, 2014-08-20 at 13:36 +0200, Michael S. Tsirkin wrote:
> On Wed, Aug 20, 2014 at 11:30:55AM +0200, Knut Omang wrote:
> > On Wed, 2014-08-20 at 10:52 +0200, Paolo Bonzini wrote:
> > > Il 20/08/2014 08:53, Knut Omang ha scritto:
> > > > A unique bus name is necessary to be able to refer to each instance
> > > > from the command line and monitors.
> > > 
> > > Is it needed?  Can't you just add id= to the -device option?
> > 
> > Yes, as far as I understand the problem is that the id= would work on
> > the ioh3420 device itself, while what is needed here is to name the
> > secondary bus of the ioh3420, which I haven't found a way to name from
> > the command line.
> 
> Did you try using the device name?

I believe I did, I tried a lot back and forth back then...

> For pci bridges, unless you set bus_name, bus name will
> match device itself. See this code:
> 
>      * If we don't specify the name, the bus will be addressed as
>      * <id>.0, where id is the device id.
>      * Since PCI Bridge devices have a single bus each, we don't need
>      * the index:
>      * let users address the bus using the device name.
>      */
>     if (!br->bus_name && dev->qdev.id && *dev->qdev.id) {
>             br->bus_name = dev->qdev.id;
>     }

but my testing of this may well have been before your patch with this
logic - the cost of being so slow with my patches - it won't happen
again...
 
Both with the somewhat counterintuitive second re-provision of the bus
name in

-    pci_bridge_map_irq(br, g_strdup(tmp), pci_swizzle_map_irq_fn);
+    pci_bridge_map_irq(br, br->bus_name, pci_swizzle_map_irq_fn);

and actually with removing it due to Alex's commit 659fefee which was
also not in when this was conceived, I am now able to instantiate two
devices on two ports by means of:

-device ioh3420,slot=0,id=pcie_port.0
-device ioh3420,slot=1,id=pcie_port.1
-device <ari_capable_device1>,bus=pcie_port.0
-device <ari_capable_device2>,bus=pcie_port.1

In light of Markus' enlightening description of the mess here, unless
there are more issues, I'll repost the rest of the changes without this
commit,

Thanks,

Knut

> > Maybe an even better solution would be to have default names for
> > everything, if not specified, from a user friendliness perspective? 
> >
> > I suppose this is a more general issue of sensible default values
> > though, but the fact that it is easy to create devices which cannot be
> > referred has caused me some confusion from time to time.
> > 
> > > Paolo
> > 
> > Thanks,
> > 
> > Knut