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

[Anthony Liguori]

On 09/15/2011 03:29 PM, Gleb Natapov wrote:
> On Thu, Sep 15, 2011 at 12:51:23PM -0500, Anthony Liguori wrote:
> > On 09/15/2011 11:59 AM, Gleb Natapov wrote:
> > > On Thu, Sep 15, 2011 at 11:33:00AM -0500, Anthony Liguori wrote:
> > > > 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.
> > > Missing "have"? If device has no bus how do you talk to it? Who carries
> > > the signal from a cpu to a device?
> > >
> > > > A good example is our favorite one to debate--the PIIX3.  Devices
> > > PIIX3 is a collection of devices, not a device.
> > >
> > > > like the UART don't sit on a bus.  They don't have any links at all.
> > > In PC UART sits on isa bus. How device can have no links at all? It just
> > > glued to a motherboard not touching any wires?
> >
> > A bus implies a bidirectional relationship.  IOW, the device has to
> > know that it sits on a ISA bus to be an ISA device.
> And ISA device with UART on it definitely knows that.
>
> > The UART has no knowledge of the fact that is mapped behind ISA.
> > The UART exposes a public interface (through it's pins) that's
> > orthogonal to any buses.
> The UART itself has no knowledge, yes. But UART does not exists in
> vacuum. It is always a part of other device that provides bus logic.
> Original PC provided 2 or 4 ISA devices with UART on them. That is how
> we need to model them on a PC. You can (or could) easily buy PCI card
> with many more additional UARTs. You wouldn't claim that those UARTs are
> not on the PCI bus, would you?

Let's consider the following.

Let's say that we emulated a simpler micro controller that exposes a GPIO 
interface.  Something like an Amtel or maybe even a simple ARM chip.

IRL, you would wire the UART pins directly to the GPIO pins and call it a day. 
There is no bus and there is no intermediate layer.

In an everything has a bus world, how does something like this get modelled?

> > How do you "walk up the device graph" from a 16650A?  What signals
> > are you going to send out of the pins to do that?
> 16650A is not a device. ISA card it resides on is a device.
>
> > If a device can always do self->parent->parent->parent->send_io(foo)
> > then the design is fundamentally broken and you will end up with
> > devices that do things that they shouldn't do.
> Why?

Because a serial device has no business calling functions in the i440fx device. 
 It's a layering violation.

Regards,

Anthony Liguori

> --
>                         Gleb.