Re: [PATCH v2 06/13] mac_newworld: Simplify creation of Uninorth devices

[Markus Armbruster <armbru@redhat.com>]

Philippe Mathieu-Daudé <f4bug@amsat.org> writes:

> Cc'ing CLI refactor team.
>
> On 29/9/22 09:39, Mark Cave-Ayland wrote:
>> On 25/09/2022 13:38, BALATON Zoltan wrote:
>> 
>>> Avoid open coding sysbus_mmio_map() and map regions in ascending
>>> otder. Reorganise code a bit to avoid some casts.
>>>
>>> Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
>>> ---
>>>   hw/ppc/mac_newworld.c | 42 +++++++++++++++++-------------------------
>>>   1 file changed, 17 insertions(+), 25 deletions(-)
>
>> Same comment here re: sysbus. Also the patch seems correct here, but it is worth noting that the PCI bus initialisation is order sensitive: the 
>> last bus created is the one that becomes the default PCI bus for -device, so changing this would break quite a few command lines...
>
> Eh, I was not aware of this API fragility. So when using -device without
> expliciting the 'bus' key, the default is the latest bus created... OK.

Yes, our external interface is in part defined implicitly by the order
in board code execution.  It goes deeper than just CLI, I'm afraid.

Omitting bus= is a convenience feature for users.  It's clearly useful.
But what's the default?  We walk the qdev tree rooted at the main system
bus looking for a bus of suitable bus type that is not full, or else of
suitable type that is full.  See qdev_find_recursive().  The tree walk
visits children in creation order.  Therefore, bus creation order is
ABI.

Related: bus names.

For user-created buses, the bus name depends on the owning device's qdev
ID (specified with id=ID).  If it has none, the system picks a bus name;
more on that below.  Else, it is ID.N, where N counts from zero.  When a
device creates multiple buses of the same type, its bus creation order
is ABI.

For onboard devices, the bus name can be specified by board code.  If
board code elects not to, the system picks a name.

System-picked names are TYPE.N, where TYPE is the bus type name such as
"pci" or "isa", and N counts from zero separately for each type.  Again,
bus creation order is ABI.

This is qbus_init_internal().

Letting users omit qdev IDs is a convenience feature.

Letting developers delegate bus name picking to the system is also a
convenience feature.  Without it, we'd need bus name logic in board code
and not just qbus_init_internal().

I dislike implicit ABI definitions.

Rarely met a QEMU convenience feature that didn't bite us in the
posterior later (the sensitivity of my posterior may well cloud my
memory, though).