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"? ... >> >> 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.