Re: [Qemu-devel] [PATCH v3 1/6] hw/misc/dyn_sysbus_binding: helpers for sysbus device dynamic binding

[Alexander Graf]

On 10.09.14 12:05, Paolo Bonzini wrote:
> Il 10/09/2014 11:56, Alexander Graf ha scritto:
>>
>>
>> On 10.09.14 11:43, Paolo Bonzini wrote:
>>> Il 10/09/2014 11:31, Alexander Graf ha scritto:
>>>>>> Yeah, please do the registration in sysbus.c, not in virt.c.  There is
>>>>>> no reason to make the platform_bus_init_notify+DynSysbusNotifier
>>>>>> interface public.  The code in sysbus.c can fill in the fields.
>>>> Sysbus != Platform bus. Sysbus is an in-QEMU representation of a
>>>> pseudo-bus that we put all devices onto that we consider unsorted.
>>>>
>>>> Platform bus is a machine representation of an actual bus that devices
>>>> are attached to. These devices usually are sysbus devices.
>>>
>>> Is there any difference between the two?
>>>
>>> Take a machine that has two chips, a SoC that does everything except
>>> USB, and a USB controller chip.
>>>
>>> Strictly speaking the USB controller chip would be on a "platform bus",
>>> but we would likely put it on sysbus.
>>>
>>> Why should it matter whether the devices are static or dynamic, for the
>>> sake of calling something the "system" or the "platform" bus?  I would
>>> say that QEMU calls "sysbus" the platform bus.
>>>
>>> Some devices (e.g. the local APIC in x86, or the in-core timers and GIC
>>> in ARM) should probably not be in sysbus at all, and should attach
>>> directly to the CPU address space.  But that is a quirk in the modeling
>>> of those devices, it shouldn't mean that sysbus is not a "platform" bus.
>>
>> On e500 for example, we have a predefined CCSR region. That is a machine
>> defined "platform bus". The offsets inside that region are strictly
>> defined by the spec.
>>
>> Now take the serial ports. We have space for 2 serial ports inside of
>> that CCSR region. We can spawn these 2 ports in the machine file based
>> on -serial, but if you want to spawn them with -device, how do you tell
>> the machine whether they should go into the "big bucket platform bus" or
>> the "CCSR platform bus"?
> 
> Two possibilities:
> 
> 1) you would use two instances of sysbus (one default, one created by
> the board) and specify ",bus=ccsr" on the command line when you want to
> add the device to the CCSR region.
> 
> The two would work exactly the same way, only with different algorithms
> for resource allocation.
> 
> 2) similar to ISA, you would create a new ccsr-bus device and a new
> ccsr-serial device, and use -device ccsr-serial,index=[0|1],chardev=foo
> to specify which of the two serial ports this is for.  Most of the fdt
> magic could be shared by the sysbus and CCSR cases.
> 
> I think I prefer (2)...

Fair enough.

As far as moving "platform bus" logic into sysbus, I'd really like to
hold back and see what this whole thing ends up getting used for first.

So for now, I'd definitely prefer to keep "platform bus" logic and
"sysbus" logic separate. If we realize that every user only ever uses
the dynamic sysbus creation in conjunction with our "platform bus"
implementation, we can merge them.


Alex