Re: [Qemu-devel] [RFC] Plan for moving forward with QOM

[Anthony Liguori]

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.