On Wed, 4 Mar 2026, Peter Maydell wrote:
On Wed, 4 Mar 2026 at 12:25, Markus Armbruster <[email protected]> wrote:
We might be at a crossroads here.
Paul Brook designed qdev around "a device plugs into some bus, and may
provide buses". To make this work, he added a main system bus to serve
as root of a single tree.
The parent of a device in that tree is the bus it plugs into, and the
parent of a bus is the device that provides it, except for the main
system bus, which has no parent. "info qtree" shows this tree.
Note a machine had just this one system bus back then.
The system bus isn't something hardware people would call a bus.
The qtree is a qdev thing. It is *not* the QOM composition tree ("info
qom-tree /").
We actually abandoned "a device plugs into some bus" a long time ago.
Moe on that below.
Qdev was later rebased onto QOM, as follows.
A bus QOM type is a subtype of "bus".
A device QOM type is a subtype of "device".
If a device type has a @bus_type, then it plugs into a bus of that QOM
type. Such a concrete device is commonly[*] not a direct subtype of
"device". Instead, it's a subtype of the abstract device that plugs
into that bus type. Example: "isa-serial" is a direct subtype of
"isa-device", and it inherits its bus type "ISA" from there.
If a device has no @bus_type, it doesn't plug into any bus. It doesn't
show up in "info qtree" (it does in "info qom-tree", of course).
Example: "serial" is a direct subtype of "device".
This lets us build devices from components without having to make up
buses to connect them. Example: "isa-serial" has a direct QOM child of
type "serial". Good!
Except infrastructure useful to such bus-less devices lives in sysbus,
where bus-less devices can't access it. This tempts us to make devices
sysbus devices just to be able to use the infrastructure, and to create
additional sysbuses.
There is only one sysbus -- it is impossible to create more than one,
and there wouldn't be much point in creating a second one anyway.
We can elect to go down this path further: make more sysbus devices.
Create more sysbuses. If this is the right path, then bus-less devices
were a mistake, and we should consider eliminating them.
Or we can pick the other branch: put the infrastructure where bus-less
devices can use it. If this is the right path, then sysbus is
obsolescent.
I do think that at some point it would be good to move all that
stuff into the base "device" class, and drop sysbus. The real
problem, though, is reset. Currently reset propagates along the
qtree (the tree of buses). So something (like TYPE_SERIAL, and
also like CPU objects) that is a direct child of TYPE_DEVICE does
not get automatically reset. The major overriding reason why
(as it currently stands) I favour making things sysbus devices
and not plain devices is that devices that don't get reset are
either broken or need all their users to take extra steps that
most don't take, or do in a weird way. We can't just push the
reset handling into TYPE_DEVICE because by definition it is tied
to "being a thing on a bus, i.e. the sysbus".
Disentangling reset is extremely painful -- I don't even know
what it ought to be doing. Maybe propagating along the QOM tree?
So for now, we muddle along.
This suggests maybe we need a way to represent qtree in QOM. So while QOM
devices are organised by parent-child relationship in the qom-tree they
could have standardised properties that tell which bus they are connected
to (or buses have properties that list devices connected to them,
whichever is easier to walk) then the reset or info qtree could select
devices based on these properties. Could that replace the qtree of sysbus
devices and thus allow merging SysBusDevice into Device? There are link
properties in QOM to connect objects so it should be possible to represent
object relationships with such properties not just by their parent-child
(which I think are also represented by properties but I did not check it).
Regards,
BALATON Zoltan
