Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path()

[Markus Armbruster <armbru@redhat.com>]

Alex Williamson <alex.williamson@redhat.com> writes:

> On Wed, 2010-06-16 at 10:23 +0200, Markus Armbruster wrote:
>> Alex Williamson <alex.williamson@redhat.com> writes:
>> 
>> > On Tue, 2010-06-15 at 12:28 +0100, Paul Brook wrote:
>> >> > > Alex proposed to disambiguate by adding "identified properties of the
>> >> > > immediate parent bus and device" to the path component.  For PCI, these
>> >> > > are dev.fn.  Likewise for any other bus where devices have unambigous
>> >> > > bus address.  The driver name carries no information!
>> >> > 
>> >> > From user POV, driver names are very handly to address a device
>> >> > intuitively - except for the case you have tones of devices on the same
>> >> > bus that are handled by the same driver. For that case we need to
>> >> > augment the device name with a useful per-bus ID, derived from the bus
>> >> > address where available, otherwise based on instance numbers.
>> >> 
>> >> This is where I think you're missing a trick. We don't need to augment the 
>> >> name, we just need to allow the bus id to be used instead.
>> >
>> > For the case of a hot remove, I agree.  If the user specifies "pci_del
>> > pci.0/03.0", that's completely sufficient because we don't care what's
>> > in that slot, just remove it.  However, I still see some use cases for
>> > device names in the path.  Take for example:
>> >
>> > (A): /i440FX-pcihost/pci.0/e1000.05.0
>> >
>> > vs
>> >
>> > (B): /pci.0/05.0
>> >
>> > (removing both the root bridge driver name and the device driver name)
>> 
>> / is the main system bus.  System bus defines no bus address at the
>> moment.  Therefore, you have to use the driver name i440FX-pcihost.
>
> So is the general rule "If a device's parent bus does not provide an
> address, print device name"?

I think the general rule for constructing a *canonical* qdev path should
be:

* If it's the main system bus, the path is /.

* If it's another bus, the path is P/B, where P is the canonical path of
  the device providing the bus, and B is the bus name.  Unambiguous,
  since no device ever defines two buses with the same name.

* If it's a device, the path is P/D, where P is the canonical path of
  the bus.  If the bus defines bus addresses, then D is @A, where A is
  the device's bus address.

  We haven't made up our minds whether the else case exists, or what to
  do if it does.  The simple "else D is the device model driver's name"
  works only if the bus can't take multiple device models with the same
  driver.

The canonical path is not the only path.  For instance, a qdev ID is a
valid path, but it's not canonical.  /i440FX-pcihost/pci.0/e1000 is
another valid, non-canonical path.

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