PCI: Store number of supported End-End TLP Prefixes

PCI: Store number of supported End-End TLP Prefixes

eetlp_prefix_path in the struct pci_dev tells if End-End TLP Prefixes
are supported by the path or not, and the value is only calculated if
CONFIG_PCI_PASID is set.

The Max End-End TLP Prefixes field in the Device Capabilities Register 2
also tells how many (1-4) End-End TLP Prefixes are supported (PCIe r6.2 sec
7.5.3.15). The number of supported End-End Prefixes is useful for reading
correct number of DWORDs from TLP Prefix Log register in AER capability
(PCIe r6.2 sec 7.8.4.12).

Replace eetlp_prefix_path with eetlp_prefix_max and determine the number of
supported End-End Prefixes regardless of CONFIG_PCI_PASID so that an
upcoming commit generalizing TLP Prefix Log register reading does not have
to read extra DWORDs for End-End Prefixes that never will be there.

The value stored into eetlp_prefix_max is directly derived from device's
Max End-End TLP Prefixes and does not consider limitations imposed by
bridges or the Root Port beyond supported/not supported flags. This is
intentional for two reasons:

  1) PCIe r6.2 spec sections 2.2.10.4 & 6.2.4.4 indicate that a TLP is
     malformed only if the number of prefixes exceed the number of Max
     End-End TLP Prefixes, which seems to be the case even if the device
     could never receive that many prefixes due to smaller maximum imposed
     by a bridge or the Root Port. If TLP parsing is later added, this
     distinction is significant in interpreting what is logged by the TLP
     Prefix Log registers and the value matching to the Malformed TLP
     threshold is going to be more useful.

  2) TLP Prefix handling happens autonomously on a low layer and the value
     in eetlp_prefix_max is not programmed anywhere by the kernel (i.e.,
     there is no limiter OS can control to prevent sending more than N TLP
     Prefixes).

Link: https://lore.kernel.org/r/20250114170840.1633-7-ilpo.jarvinen@linux.intel.com
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Yazen Ghannam <yazen.ghannam@amd.com>