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Re: [Qemu-devel] [PATCH] Name the default PCI bus "pci.0" on all archite
From: |
Paul Brook |
Subject: |
Re: [Qemu-devel] [PATCH] Name the default PCI bus "pci.0" on all architectures |
Date: |
Wed, 2 Jun 2010 16:13:56 +0100 |
User-agent: |
KMail/1.13.3 (Linux/2.6.33-2-amd64; KDE/4.4.3; x86_64; ; ) |
> Paul Brook <address@hidden> writes:
> >> The system emulators for each arch are using inconsistent
> >> naming for the default PCI bus "pci" vs "pci.0". Since it
> >> is conceivable we'll have multiple PCI buses in the future
> >> standardize on "pci.0" for all architectures. This ensures
> >> mgmt apps can rely on a name when assigning PCI devices an
> >> address on the bus using eg '-device e1000,bus=pci.0,addr=3'
> >
> > No. Bus names are local to the parent device. None of the host bridges
> > support multiple bridges, so the ".0" suffix makes no sense. The parent
> > device has no idea whether it owns the "default" pci bus or not.
> > If you have multiple PCI busses then you can identify them by the device
> > path.
>
> From qbus_create_inplace():
>
> if (name) {
> /* use supplied name */
> bus->name = qemu_strdup(name);
> } else if (parent && parent->id) {
> /* parent device has id -> use it for bus name */
> len = strlen(parent->id) + 16;
> buf = qemu_malloc(len);
> snprintf(buf, len, "%s.%d", parent->id, parent->num_child_bus);
> bus->name = buf;
> } else {
> /* no id -> use lowercase bus type for bus name */
> len = strlen(info->name) + 16;
> buf = qemu_malloc(len);
> len = snprintf(buf, len, "%s.%d", info->name,
> parent ? parent->num_child_bus : 0);
> for (i = 0; i < len; i++)
> buf[i] = qemu_tolower(buf[i]);
> bus->name = buf;
> }
>
> If appending ".0" really makes no sense when the device has just one
> bus, then we shouldn't append it in cases 2 & 3.
IMO the code you quote is completely bogus. All devices should specify names
for their child busses, failure to do so is a bug.
These bus names are local to the parent device. Trying to use them as global
identifiers is just plain wrong. A given device [type] should always have the
same set of properties/child busses regardless of where it occurs in the
device tree.
Using a single counter for all busses is also wrong. The order of bus creation
is a device implementation detail, under no circumstances should it be part of
the user visible API. Consider a device that has both a PCI and I2C bus. It
makes no sense to call there pci.0 and i2c.1.
Paul