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Date: Mon, 07 Sep 2009 13:02:42 +0200
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: Re: [Qemu-devel] Defaults for qdev properties inherited from bus
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To: Markus Armbruster <armbru@redhat.com>
Cc: qemu-devel@nongnu.org

>> The second group has instance-dependent defaults (see ${device}_io
>> arrays in pc.c and elsewhere), i.e. first ne2k_isa at 0x300, second at
>> 0x320, etc.  Property defaults can cover instance #1 only.
>
> The instance-dependent defaults are occasionally convenient, but I'm not
> sure the functionality worth replicating for -device.

For -serial (and maybe -parallel) you probably want to have that because 
there are standard i/o port for serial lines 1 -> 4.  More than one 
serial line isn't that uncommon too, and even in a pci machine these are 
isa devices.

For sb16, ne2k_isa, ... we can probably simply go with default values 
(so -device $name just works for a single card) and expect the user to 
override iobase in case he wants to plug multiple cards into his isapc.

>> We can move the io properties from the bus to the devices which
>> actually need it.  Then this would probably become something like
>> this:
>>
>>        if (ne2k_isa->iobase == -1)
>>            ne2c_isa->iobase = default_base[instance];
>
> Not sure I get what you mean.  If your proposal has the above code in
> the init() callback, then it's not much of an improvement.

Only needed for devices which want to have different default values per 
instance (i.e. serial, see above).  Otherwise property defaults (or no 
iobase property at all) will do just fine.

> The cleanest way to implement defaults is to get the job done in
> qdev_create().  No second-guessing in init() whether the default has
> been overridden by the user.

Sure.  Might be overkill for just one or two use cases though.

cheers,
   Gerd