Hi Arthur,

On 17.03.22 19:03, Arthur Heymans wrote:
> Now my question is the following:
> On some Stacks there are multiple root busses, but the resources need to be
> allocated on the same window. My initial idea was to add those root busses
> as separate struct bus in the domain->link_list. However currently the
> allocator assumes only one bus on domains (and bridges).
> In the code you'll see a lot of things like
>
> for (child = domain->link_list->children; child; child = child->sibling)
>       ....

this is correct, we often (if not always by now) ignore that `link_list`
is a list itself and only walk the children of the first entry.

>
> This is fine if there is only one bus on the domain.
> Looping over link_list->next, struct bus'ses is certainly an option here,
> but I was told that having only one bus here was a design decision on the
> allocator v4 rewrite. I'm not sure how common that assumption is in the
> tree, so things could be broken in awkward ways.

I wouldn't say it was a design choice, probably rather a convenience
choice. The old concepts around multiple buses directly downstream of
a single device seemed inconsistent, AFAICT. And at the time the allo-
cator v4 was written it seemed unnecessary to keep compatibility around.

That doesn't mean we can't bring it back, of course. There is at least
one alternative, though.

The currently common case looks like this:


          PCI bus 0
             |
             v

  domain 0 --.
             |-- PCI 00:00.0
             |
             |-- PCI 00:02.0
             |
             :


Now we could have multiple PCI buses directly below the domain. But
instead of modelling this with the `link_list`, we could also model
it with an abstract "host" bus below the domain device and another
layer of "host bridge" devices in between:

          host bus
             |
             v

  domain 0 --.
             |-- PCI host bridge 0 --.
             |                       |-- PCI 00:00.0
             |                       |
             |                       `-- PCI 00:02.0
             |
             |
             |-- PCI host bridge 1 --.
             |                       |-- PCI 16:00.0
             |                       |
             |                       :
             :


I guess this would reduce complexity in generic code at the expense of
more data structures (devices) to manage. OTOH, if we'd make a final
decision for such a model, we could also get rid of the `link_list`.
Basically, setting in stone that we only allow one bus downstream of
any device node.

I'm not fully familiar with the hierarchy on Xeon-SP systems. Would
this be an adequate solution? Also, does the term `stack` map to our
`domain` 1:1 or are there differences?

Nico
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