Re: [PATCH 0/9] hw/sysbus/platform-bus: Introduce TYPE_DYNAMIC_SYS_BUS_DEVICE

[Philippe Mathieu-Daudé]

On 28/1/25 13:57, BALATON Zoltan wrote:
> On Tue, 28 Jan 2025, Peter Maydell wrote:
> > On Tue, 28 Jan 2025 at 10:42, Gerd Hoffmann <kraxel@redhat.com> wrote:
> > > On Sat, Jan 25, 2025 at 07:13:34PM +0100, Philippe Mathieu-Daudé wrote:
> > > > Some SysBus devices can optionally be dynamically plugged onto
> > > > the sysbus-platform-bus (then virtual guests are aware of
> > > > mmio mapping and IRQs via device tree / ACPI rules).
> > >
> > > Do we have some sane way to have user-pluggable sysbus devices on arm?
> >
> > The answer in a general sense is "no, because user pluggable
> > sysbus is a weird idea". "sysbus" means "it's wired into a
> > specific bit of the memory map and to specific IRQs, and whoever
> > does that needs to know what IRQs and bits of memory are usable,
> > and the guest OS needs to know it's there". "user-pluggable" means
> > "it's all automatic and the guest can just do some kind of
> > probing for what is or isn't present". All the platform bus stuff
> > is a nasty mess that's working around the things people want
> > to plug in not being clean devices on probeable buses :-(
> > And the platform bus is only supported on the "virt" board,
> > because that's the only one where QEMU is generating its
> > own dtb or ACPI tables where it can tell the guest "hey,
> > there's some device here".
>
> There are some SoCs that have memory mapped devices but different 
> versions in the same family have different devices. Either older ones 
> missing some devices or have less USB or network ports while newer SoCs 
> have more of those or they have PCIe instead of PCI. Modelling these 
> could use pluggable sysbus devices so one could add the devices needed 
> for a SoC version without having to write or modify a board code. I 
> think Bernhard's attempt to try creating e500 SoCs from a device tree 
> goes in that direction too. We could also model this by having a SoC 
> that can instantiate devices based on some properties but maybe 
> pluggable devices could be more generic for this. The issue seems to be 
> how to tell the board or SoC where to map it and what IRQ to connect it 
> as this is done by the board and not the device so properties on the 
> device to set these does not really help unless the board can somehow 
> query it and instantiate the devices based on that. Otherwise whatever 
> handles the -device option to create the device would need knowledge 
> about the board. (E.g. the e500 devices are mapped in the CCSR memory 
> region so one can't just use system address space for them.)

IIRC Bernard's series takes a DTB as input and create the machine
matching this DTB.

As Peter explained, sysbus-platform-bus fits TYPE_DYNAMIC_SYS_BUS_DEVICE
in free slots, then generates the corresponding ACPI/DTB.

What you describe seems closer to the QEMU Dynamic Machine project,
following Damien's idea:
https://lore.kernel.org/qemu-devel/20220223090706.4888-1-damien.hedde@greensocs.com/
We are not quite there yet...