Peter Maydell <peter.mayd...@linaro.org> writes: > On 24 March 2017 at 16:58, Markus Armbruster <arm...@redhat.com> 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*? Devices plugging into a bus have to expose their MMIO regions, IRQ lines, etc. in a certain way dictated by the bus. In return, you don't have to wire up each resource manually, you simply plug into the bus and are done. That's what makes a bus a bus for me. "Sysbus" does nothing of the sort. >> 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". Of course. > You could > argue for moving all that into Device I suppose. Yup. >> We got a "platform bus", which is really not the same as "sysbus", but >> we shoehorned it into "sysbus" anyway. > > I agree 'platform bus' is a mess, and I'd rather it didn't exist. > Unfortunately people really really want to be able to do device > pass through of random memory-mapped devices :-( The "platform" bus adds certain constraints over "sysbus", precisely to make these uses possible. And that's precisely why it should be its own thing instead of complicating "sysbus".