Re: [Qemu-devel] q35 and sysbus devices

[Thomas Huth]

On 24.03.2017 18:59, Markus Armbruster wrote:
> Peter Maydell <address@hidden> writes:
> 
>> On 24 March 2017 at 16:58, Markus Armbruster <address@hidden> wrote:
>>> "Sysbus" isn't a bus.  In qdev's original design, every device had to
>>> plug into a bus, period.  The ones that really didn't were made to plug
>>> into "sysbus".
>>>
>>> Pretty much the only thing "sysbus" devices had in common was that they
>>> couldn't be used with device_add and device_del.
>>
>> This isn't really true. Sysbus devices support having MMIO regions
>> and IRQ lines and GPIO lines. If you need those you're a
>> sysbus device; otherwise you can probably just be a plain old Device.
> 
> Well, "device has MMIO regions, IRQ lines and GPIO lines" is about as
> "device contains virtual silicon".  What would a device without any of
> these *do*?

On ppc64, we've got for a example pseudo-devices that takes care of
certain hypercalls, e.g. the spapr-rng or the spapr-rtc devices (well,
the latter is still a TYPE_SYS_BUS_DEVICE ... we should convert it to
TYPE_DEVICE instead one day...). These devices do not have any MMIO, IRQ
or GPIO lines at all, so I think the differentiation between a very
generic TYPE_DEVICE class and a "SYS_BUS_DEVICE" (with MMIO/IRQ/GPIO)
class still makes sense.

 Thomas