On 4/24/21 1:06 AM, Philippe Mathieu-Daudé wrote: > On 4/22/21 4:20 PM, Philippe Mathieu-Daudé wrote: >> On 4/22/21 3:21 PM, Markus Armbruster wrote: >>> Peter Maydell <peter.mayd...@linaro.org> writes: > >>> Most qdevs plug into a qbus, but some don't. >>> >>> DeviceClass member @bus_type names the kind of bus the device plugs >>> into. It's a QOM type name. Example: for a PCI device, it's >>> TYPE_PCI_BUS, and the device must be plugged into an instance of a >>> (subtype of) TYPE_PCI_BUS. >>> >>> If @bus_type is null, the device does not plug into any qbus. >>> >>> The qbus a device is plugged into is also called the parent bus. Not to >>> be confused with the QOM parent. >>> >>>>> But even without parent they end in the /unattached >>>>> container below /machine, so if the reset were there, the >>>>> machine could still iterate over the /unattached children. >>>> >>>> ...yes, /unattached is what I was thinking about. >>>> >>>> My current half-thought-through view is that where we ought >>>> to try to end up is something like: >>>> >>>> * "real" buses should continue to propagate reset >>>> (A "real" bus is like PCI, SCSI, and other buses where the real >>>> hardware has a concept of a "bus reset" or where the power to the >>>> plugged device comes from the bus so that powercycling the >>>> controller naturally powercycles the devices. Sysbus is not a >>>> "real" bus; I haven't checked the others to see if we have any >>>> other non-real buses) >>>> * reset should follow the QOM tree for objects not on a "real" bus >>>> (that is, the qdev "reset this device" function should do >>>> "iterate through my QOM children and reset those which are not >>>> on a real bus" as well as its current "reset myself" and "reset >>>> every qbus I have") >>>> * instead of reset starting with the sysbus and working along the >>>> qbus hierarchy, we start by resetting the machine. That should >>>> include resetting all the QOM children of the machine. Any >>>> device which has a qbus should reset the qbus as part of its >>>> reset, but only "real" buses reset their children when reset. >>> >>> Sounds like an approximation of reset wire modelling :) >>> >>> In a real machine, the reset signal travels along "wires" (in quotes, >>> because it need not be a dedicated wire, although it commonly is) >>> >>> We're not modelling these wires explicitly so far. Instead, we make >>> assumptions such as "reset flows along the qdev tree", which are close >>> enough except when they aren't. >>> >>> What you propose is likely closer to reality than what we have now. >> >> Then maybe reality is easier to model =) >> >>> Do I make sense? >> >> I guess so. Now I wonder if Peter's approach is doable while still >> having "incompletely QOMified devices". >> >> But if we can propagate reset tree via QOM, it is a good excuse >> to finish QOM'ifying devices and enforce the API to prohibit non-QOM >> ones. >> >> And remove the crutch in device_set_realized(). >> >>>> That means that, for instance, if you reset an SoC container object >>>> it will reset all the sub-devices within the SoC and the miscellaneous >>>> bits of glue logic like OR gates it might also own[*]. It also means that >>>> CPU objects should no longer need weird special casing, because they >>>> are part of the QOM hierarchy and get reset that way. >>>> >>>> [*] Fun fact: TYPE_OR_IRQ inherits directly from TYPE_DEVICE which >>>> means that pretty much no instances of it ever get reset. >>>> >>>> There is of course a massive unsolved problem with this idea, which >>>> is the usual "how do we get there from here" one. >>>> >>>> (Eventually I think we might be able to collapse TYPE_SYS_BUS_DEVICE >>>> down into TYPE_DEVICE: there is no particular reason why a TYPE_DEVICE >>>> can have GPIO inputs and outputs but only a TYPE_SYS_BUS_DEVICE can >>>> claim to have MMIO regions and IRQs. "Only sysbus devices get reset" >>>> is a big part of why a lot of devices today are sysbus.) > > Looking at qemu_register_reset() uses I found this commit: > > commit 0c7322cfd3fd382c0096c2a9f00775818a878e13 > Date: Mon Jun 29 08:21:10 2015 +0200 > > watchdog/diag288: correctly register for system reset requests > > The diag288 watchdog is no sysbus device, therefore it doesn't get > triggered on resets automatically using dc->reset. > > Let's register the reset handler manually, so we get correctly notified > again when a system reset was requested. Also reset the watchdog on > subsystem resets that don't trigger a full system reset. > > Why is the reset() handler in DeviceClass and not in SysbusDeviceClass > if "Only sysbus devices get reset"? ...
Ah, probably because the problem is generic to all busses (ISA, ...) and not just sysbus. >>> Sysbus may habe been a design mistake. It goes back the qdev design >>> assumption "every device plugs into exactly one bus, every bus is part >>> of exactly one device, and the main system bus is the root of this >>> tree". The assumption ceased to hold long ago, but we still have >>> sysbus. >
