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

[Krystian Hebel (Code Review)]

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

Krystian Hebel 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:

(4 comments)

File src/device/pciexp_device.c:

https://review.coreboot.org/c/coreboot/+/77338/comment/34f0ccc4_7b202c1b :
PS11, Line 594: root_devcap
The name no longer describes what this variable holds.

https://review.coreboot.org/c/coreboot/+/77338/comment/a4dee813_1d3a4cd3 :
PS11, Line 598: awlays
always

https://review.coreboot.org/c/coreboot/+/77338/comment/059ccb24_7eeb1a46 :
PS11, Line 608:  & PCI_EXP_DEVCAP_PAYLOAD
No need to mask it here, it was already masked right after reading.

https://review.coreboot.org/c/coreboot/+/77338/comment/6e5a16dd_9e217dc7 :
PS11, Line 630: 	root->max_payload_set = 1;
I think this still doesn't catch case where two endpoints have different max payload sizes, for example:
- root has max size = 5, there are two endpoints directly below root, A and B (they may be whole chains, but let's keep it simple), scanned in that order
- endpoint A has max size = 4, the code sees it and limits root's payload size to 4
- endpoint B has max size = 3, the code sees it and limits root's payload size to 3

At this point endpoint A and root have different max payload sizes configured. I think this would be fixed by restarting the loop in `pciexp_scan_bus()` (in fact, `pciexp_configure_max_payload_size()` would be enough) `if ((root->max_payload_set == 1) && (max_payload < root_max_payload))`. Best not to do this recurrently though, with many devices a lot of stack space could be wasted.