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Re: [Qemu-devel] q35 and sysbus devices

From: Cornelia Huck
Subject: Re: [Qemu-devel] q35 and sysbus devices
Date: Mon, 27 Mar 2017 10:44:40 +0200

On Fri, 24 Mar 2017 16:23:18 -0300
Eduardo Habkost <address@hidden> wrote:

> On Fri, Mar 24, 2017 at 05:08:56PM +0000, Peter Maydell wrote:
> > 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.
> > 
> > > We fixed the design to permit bus-less devices, but we didn't get rid of
> > > "sysbus".
> > 
> > Call it what you want, but we should have some common code support
> > for "I want to have MMIOs and IRQs and GPIO lines". You could
> > argue for moving all that into Device I suppose.
> Even if we don't move all that into Device, this sounds like an
> argument for the existence of struct SysBusDevice. But I still
> don't understand the reason TYPE_SYSTEM_BUS still exists.

ISTR some surprises that reset (or some other) callbacks were not
called as expected if there wasn't a sysbus device among the ancestors.
Don't know if that's still true.

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