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Date: Thu, 15 Sep 2011 11:33:00 -0500
From: Anthony Liguori <aliguori@us.ibm.com>
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Subject: Re: [Qemu-devel] [RFC] Plan for moving forward with QOM
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To: Gleb Natapov <gleb@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Jan Kiszka <jan.kiszka@siemens.com>, qemu-devel <qemu-devel@nongnu.org>, Markus Armbruster <armbru@redhat.com>, Gerd Hoffmann <kraxel@redhat.com>, "Edgar E. Iglesias" <edgar.iglesias@gmail.com>, Paolo Bonzini <pbonzini@redhat.com>

On 09/15/2011 10:38 AM, Gleb Natapov wrote:
> On Thu, Sep 15, 2011 at 10:28:52AM -0500, Anthony Liguori wrote:
>> On 09/15/2011 09:25 AM, Gleb Natapov wrote:
>>
>> There is no canonical parent link.  A device may have multiple (more
>> or less equivalent) parents.
>>
>> What should be treated as the "canonical" link depends on what
>> you're trying to do.  In the case of OF, you want to treat the bus
>> as a parent.  If a device happens to sit on multiple buses, I'm not
>> really sure what you do.
>>
> Yes, "canonical" is a link to a bus. Can you give an example of a device
> that sits on multiple buses?

Not all devices buses that they sit on.

A good example is our favorite one to debate--the PIIX3.  Devices like the UART 
don't sit on a bus.  They don't have any links at all.

Instead, the PIIX3 itself bridges the public interface of the UART via its PC 
interface.  So it looks something like this:

class Serial : public Device
{
    uint8_t read(uint8_t addr);
};

class PIIX3 : public PciDevice, implements PciBus
{
    Serial *tty[4];
};

uint32_t PIIX3::pio_access(uint16_t addr, int size)
{
    if (addr == 0x3f8 && this->tty[0]) {
        return this->tty[0]->read(addr - 0x3f8);
    } else {
        ...
    }
}

There is no backlink in the Serial device so there's no way of walking up the 
graph from the Serial device itself.  You have to transverse to it to build a path.

Regards,

Anthony Liguori

>
> --
> 			Gleb.