Re: [Qemu-devel] [PATCH v2 15/18] rtc: add a dynamic property for retrieving the date

[Kevin Wolf <kwolf@redhat.com>]

Am 09.12.2011 14:08, schrieb Anthony Liguori:
> On 12/09/2011 05:26 AM, Kevin Wolf wrote:
>> Am 02.12.2011 21:20, schrieb Anthony Liguori:
>>> This really shows the power of dynamic object properties compared to qdev
>>> static properties.
>>>
>>> This property represents a complex structure who's format is preserved over the
>>> wire.  This is enabled by visitors.
>>>
>>> It also shows an entirely synthetic property that is not tied to device state.
>>>
>>> Signed-off-by: Anthony Liguori<aliguori@us.ibm.com>
>>
>> There's one thing that I was hoping to find answered when I would have
>> reviewed the whole series, but it hasn't happened: There is no doubt
>> that dynamic properties (in the sense of being able to modify them after
>> constructions) are a useful thing. But you also claim that class-based
>> properties are not enough for QOM and that we need object-based ones,
>> which is a requirement not immediately obvious to me.
>>
>> Can you provide some examples where we would explicitly need
>> object-based properties?
> 
> Sure.  Any property that's dynamic needs to be object based.  A good example 
> would be PCI slots.
> 
> Today, we unconditionally advertise 32 slots in our ACPI tables.  It could be 
> desirable to eventually make this configurable.  So you can imagine where you 
> would have an 'slot-count' property and if that was set to 16, it would result 
> in 'slot[0]..slot[15]' being created.
> 
> There are other good examples too.

So is it mostly about variably sized arrays, which just happen to be
considered independent properties in your approach? Or are there cases
where a logically separate property may be there or missing depending on
some condition, or possibly even that a new property is created during
runtime?

Kevin