David Gibson <da...@gibson.dropbear.id.au> writes:
> On Fri, Dec 04, 2015 at 09:05:51AM +0100, Markus Armbruster wrote:
>> David Gibson <da...@gibson.dropbear.id.au> writes:
>>
>> > On Thu, Dec 03, 2015 at 04:30:31PM +0100, Markus Armbruster wrote:
>> >> 1. Before commit 94649d4 "spapr: Don't use QOM [*] syntax for DR
>> >> connectors", the indexes were small integers:
>> >>
>> >> (qemu) info qom-tree
>> >> /machine (pseries-2.4-machine)
>> >> /unattached (container)
>> >> [...]
>> >> /device[5] (spapr-pci-host-bridge)
>> >> /p...@800000020000000.mmio[0] (qemu:memory-region)
>> >> /p...@800000020000000.mmio-alias[0] (qemu:memory-region)
>> >> /p...@800000020000000.io[0] (qemu:memory-region)
>> >> /p...@800000020000000.io-alias[0] (qemu:memory-region)
>> >> /pci.0 (PCI)
>> >> /pci@800000020000000.iommu-root[0] (qemu:memory-region)
>> >> /dr-connector[0] (spapr-dr-connector)
>> >> /dr-connector[1] (spapr-dr-connector)
>> >> /dr-connector[2] (spapr-dr-connector)
>> >> [...]
>> >>
>> >> Since then, they're big ones:
>> >>
>> >> /dr-connector[1073741824] (spapr-dr-connector)
>> >> /dr-connector[1073741825] (spapr-dr-connector)
>> >> /dr-connector[1073741826] (spapr-dr-connector)
>> >>
>> >> The commit message doesn't quite spell out this change, and I'm
>> >> therefore double-checkint it's intentional. Is it?
>> >
>> > Yes, it's intentional. The small integers were arbitrarily allocated
>> > by the QOM magic [*] code, whereas the big integers are actually
>> > meaningful values (essentially the DRC's global ID for the dynamic
>> > reconfiguration hypervisor interfaces).
>>
>> Good.
>>
>> >> 2. Before commit 6c2f9a1 "qapi: Make output visitor return qnull()
>> >> instead of NULL", qom-get returned {}:
>> >>
>> >> Since then, it returns null:
>> >>
>> >> QMP> { "execute": "qom-get", "arguments": { "path":
>> >> "/machine/unattached/device[5]/dr-connector[1073741950]", "property":
>> >> "fdt" } }
>> >> {"return": null}
>> >>
>> >> Does anyone care?
>> >
>> > Hm, I'm guessing this is a case where fdt is NULL internally. Which I
>>
>> Yes.
>>
>> > think will happen before a device gets hotplugged into the DRC. In
>> > that case null seems more correct to me than {}, since {} would also
>> > be what's shown for a present-but-empty device tree.
>>
>> It was {} in 2.4. Changing it to null so we can distingish "nothing"
>> from "empty" is an incompatible change. May make sense anyway, but I
>> can't judge it.
>
> Strictly speaking it's an incompatible change, yes. But I find it
> hard to imagine anything would be relying on the {} behaviour. This
> property is essentially a debugging interface to start with, and the
> missing / empty case is examining it in a state that's unlikely to be
> interesting.
I'm not against changing it, I just want it changed intentionally rather
than by accidental side effect :)
If you tell me you want null here going forward, I'll make sure it gets
changed to null in the next development cycle, with a nice commit
message.
If you want it to be null in 2.5, NAK "[PATCH for-2.5 2/3] spapr_drc:
Change value of property "fdt" from null back to {}". It'll remain an
implicit change then, not documented in commit messages. I expect we'll
eventually get a patch similar to the NAKed one regardless, because
we'll tighten up the visitor contracts, and returning without visiting
anything should become a programming error then.
