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

[Michał Żygowski (Code Review)]

Attention is currently required from: Angel Pons, Arthur Heymans, Felix Held, Krystian Hebel, Nico Huber, Patrick Rudolph.

Michał Żygowski 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
......................................................................

Patch Set 10:

(1 comment)

File src/device/pciexp_device.c:

https://review.coreboot.org/c/coreboot/+/77338/comment/9d48df88_7786efb1 :
PS10, Line 709: pciexp_configure_max_payload
> > dev == dev->bus->dev is how we identify the root device. Don't ask […]

This is what I did:

```C
static bool pciexp_device_is_root_port(struct device *dev)
{
    u16 flags;
    u16 cap = pci_find_capability(dev, PCI_CAP_ID_PCIE);
    if (!cap)
    	return false;

flags = pci_read_config16(dev, cap + PCI_EXP_FLAGS) & PCI_EXP_FLAGS_TYPE;
    return ((flags >> 4) == PCI_EXP_TYPE_ROOT_PORT);
}

static void pciexp_configure_max_payload_size(struct device *dev)
{
    unsigned int root_cap, cap;
    struct device *pdev = dev;

/* Walk up the chain and adjust MaxPayload to the minimum supported in the chain */
    while (pdev->bus && pdev->bus->dev && !pciexp_device_is_root_port(pdev)) {
    	cap = pci_find_capability(pdev, PCI_CAP_ID_PCIE);
    	root_cap = pci_find_capability(pdev->bus->dev, PCI_CAP_ID_PCIE);
    	printk(BIOS_DEBUG, "Setting Max Paylaod Size for %s device and %s parent\n",
    			   dev_path(pdev), dev_path(pdev->bus->dev));
    	if (!cap || !root_cap)
    		return;

if (pdev->bus->dev == pdev || pdev->bus->dev->path.type != DEVICE_PATH_PCI)
    		break;

/* Adjust Max_Payload_Size */
    	pciexp_set_max_payload_size(pdev->bus->dev, root_cap, pdev, cap);
    	pdev = pdev->bus->dev;
    };
}

static void pciexp_tune_dev(struct device *dev)
{
...
    pciexp_configure_max_payload_size(dev);

pciexp_configure_ltr(root, root_cap, dev, cap);
}

```

What I get in cbmem log as a result:

https://paste.dasharo.com/?d1b97978265f7ede#ABLvEctaviQN4acGhVKMgURvSWaxH43W...

As one can see, the pciexp_set_max_payload_size does not get called for the root port as it should (because pdev->bus->dev never points to a parent bridge/root port).