On 08/22/2010 04:37 AM, Avi Kivity wrote:
On 08/19/2010 11:49 PM, Anthony Liguori wrote:
The bus does not need to have any connection to existence or
non-existence of real buses. In SoCs or ASICs, all devices and buses
may reside inside a chip.
Well, I think this is part of the trouble with the current qdev
object model. There are really two distinct types of devices. There
are chips that have pins whereas the meaning of those pins are
defined by the chip itself. For instance, a UART16650A is a chip
that has a well defined pin layout.
Then there are buses which typically multiplex signals for many
devices over a single set of wires. Usually you need some type of
logic that decodes the bus signals to the actual chips that sit on
the card.
So really, I think this suggests that some devices shouldn't have any
requirement to sit on a bus. A UART16650A does not sit on bus. It
sits on a card and is wired to the ISA bus or is sometimes wired
directly to pins on a CPU on a SoC.
I don't think we want to model individual resistors on a serial card
as separate qdev objects. We want the serial card itself to be a qdev
(as it is a hotpluggable entity) and the individual serial interfaces
on that card (as they are duplicates of each other and of interest to
the user).
You're missing the fundamental problem which arises because we've
introduced an object model without thinking through how devices ought to
be modelled.
All devices should have a DeviceState associated with them. Otherwise,
there's really no point in having qdev at all.
We have lots of devices today that don't have DeviceState's associated
with them because the have a separate qdev representation with a
reference to the non-DeviceState object.
We have non-DeviceState objects because otherwise we end up with an
inheritance diamond. We have this problem because we want to have
relationships like: DeviceState <- SystemDeviceState <- ISADevice <-
ISASerialDevice.
But ISASerialDevice is not the only type of serial device. You can also
have a SystemSerialDevice that's directly attached to the System bus.
That means you'd have to have:
SerialDevice -> ISASerialDevice -> SystemDeviceState -> DeviceState
-> SystemSerialDevice -> SystemDeviceState ->
DeviceState
Which is a classic MI diamond. The only way to resolve this modelling
problem is to split out the common code and rely on a has-a relationship
instead of an is-a. That gives you:
ISASerialDevice->SystemDeviceState->DeviceState
SystemSerialDevice->SystemDeviceState->DeviceState
ISASerialDevice has-a SerialDevice
SystemSerialDevice has-a SerialDevice
And since we want SerialDevice inherit from a DeviceState (recall, all
devices should have DeviceStates):
SerialDevice->DeviceState
No more MI diamond and all devices have DeviceStates. Coincidentally,
it matches more closely how hardware works..
Generally speaking, any time we have one device that needs to sit on
multiple busses, we're going to have to model it in this fashion.
Regards,
Anthony Liguori
