Re: Resetting non-qdev children in a 3-phase reset device

[Philippe Mathieu-Daudé]

On 4/22/21 4:20 PM, Philippe Mathieu-Daudé wrote:
> On 4/22/21 3:21 PM, Markus Armbruster wrote:
>> Peter Maydell <peter.maydell@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.