[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, 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 10:

(2 comments)

File src/device/pciexp_device.c:

https://review.coreboot.org/c/coreboot/+/77338/comment/8a085bf7_08d9d686 :
PS10, Line 645: 	pciexp_limit_max_payload_ctl(dev->bus->dev, pciexp_configured_max_payload(dev), cap);
> pciexp_scan_bus discovers devices from the root towards end devices. This means that iterating over child->sibling will always execute pciexp_configure_max_payload on device(s) closer to root before those further down.

No, it shouldn't. It recurses into pci_scan_bus() first, before the loop, which
will call pciexp_scan_bus() again unless there are legacy PCI bridges. So the
loop runs for the lowest level first.

https://review.coreboot.org/c/coreboot/+/77338/comment/807e1385_62571472 :
PS10, Line 709: pciexp_configure_max_payload
> Should work.

How? pciexp_set_max_payload_size() only reads capabilities, not what was
programmed. Without additional input, it can't propagate anything, AFAICS.
You could always call it with the same `dev` pointer but changing parent,
but that could be quite irritating to the reader because it would also
program the same `dev` over and over again.