[M] Change in coreboot[main]: device/pciexp_device.c: Fix setting Max Payload Size

[Nico Huber (Code Review)]

Attention is currently required from: Angel Pons, Arthur Heymans, Felix Held, Krystian Hebel, Kyösti Mälkki, Michał Żygowski, Patrick Rudolph.

Nico Huber has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/77338?usp=email )

Change subject: device/pciexp_device.c: Fix setting Max Payload Size
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Patch Set 11:

(1 comment)

File src/device/pciexp_device.c:

https://review.coreboot.org/c/coreboot/+/77338/comment/11d90c44_04e59b8f :
PS11, Line 630: 	root->max_payload_set = 1;
> This would also work, and is much easier than what I suggested. […]

That's a good catch, Krystian! This means we couldn't avoid walking the tree
a second time. TBH, to fix this, I would start from scratch, as all the designs
we had so far tried to avoid exactly this. When walking the tree a second time,
it becomes much simpler, I believe:

1. On the way down, set every device to its own maximum. On the way up, propagate the minimum from the endpoints up to the root port (making sure that we use the minimum of what a bridge was configured and the value from the child, to avoid problems with multiple children and different MPS). i.e. bridge mps = MIN(bridge mps, child mps)
2. Walk a second time, this time we have the minimum configured at the root port and have to propagate that down to all bridges and the endpoints. i.e. simply, child mps = parent mps.