How about this option? Instead of one (coreboot) PCIe domain per (Xeon-SP) PCIe stack, we do one (coreboot) PCIe domain per root bus assignment. Regarding resource windows, we could adjust the remaining windows after assignment for a PCIe domain is completed.

Jonathan

From: Arthur Heymans <arthur@aheymans.xyz>
Date: Thursday, March 17, 2022 at 11:04 AM
To: coreboot <coreboot@coreboot.org>
Subject: [coreboot] Multi domain PCI resource allocation: How to deal with multiple root busses on one domain

Hi

I've recently tried to improve the soc/intel/xeon_sp codebase.

I want to make it use more native coreboot structures and codeflows instead of parsing the FSP HOB again and again to do things. Ideally the HOB is parsed only once in ramstage, parsed into adequate native coreboot structures (struct device, struct bus, chip_info, ...) and used later on.

The lowest hanging fruit in that effort is resource allocation.

Currently the coreboot allocator is sort of hijacked by the soc code and done over again.

The reason for this is that xeon_sp platforms operate a bit differently than most AMD and Intel Client hardware: there are multiple root busses. This means that there are PCI busses that are in use, but are not downstream from PCI bus 0. In hardware terminology those are the IIO and other type of Stacks.

Each Stack has its own range of usable PCI Bus numbers and decoded IO and MEM spaces below and above 4G. I tried to map these hardware concepts to the existing coreboot 'domain' structure. Each domain has resource windows that are used to allocate children devices on, which would be the PCI devices on the stacks.
The allocator needs some tweaks to allow for multiple resources of a type (MEM or IO), but nothing major. See https://review.coreboot.org/c/coreboot/+/62353/ and https://review.coreboot.org/c/coreboot/+/62865 (allocator rewrite/improvement based on Nico's excellent unmerged v4.5 work) This seems to work really well and arguably even better than how it is now with more elegant handling of above and below 4G resources.

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 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.

Do you have any suggestions to move forward?

Kind regards

Arthur Heymans