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Re: [Qemu-devel] q35 and sysbus devices

From: Thomas Huth
Subject: Re: [Qemu-devel] q35 and sysbus devices
Date: Mon, 27 Mar 2017 10:00:27 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0

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.


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