PCI: Preserve bridge window resource type flags

When a bridge window is found unused or fails to assign, the flags of the
associated resource are cleared. Clearing flags is problematic as it also
removes the type information of the resource which is needed later.

Thus, always preserve the bridge window type flags and use IORESOURCE_UNSET
and IORESOURCE_DISABLED to indicate the status of the bridge window. Also,
when initializing resources, make sure all valid bridge windows do get
their type flags set.

Change various places that relied on resource flags being cleared to check
for IORESOURCE_UNSET and IORESOURCE_DISABLED to allow bridge window
resource to retain their type flags. Add pdev_resource_assignable() and
pdev_resource_should_fit() helpers to filter out disabled bridge windows
during resource fitting; the latter combines more common checks into the
helper.

When reading the bridge windows from the registers, instead of leaving the
resource flags cleared for bridge windows that are not enabled, always
set up the flags and set IORESOURCE_UNSET | IORESOURCE_DISABLED as needed.

When resource fitting or assignment fails for a bridge window resource, or
the bridge window is not needed, mark the resource with IORESOURCE_UNSET or
IORESOURCE_DISABLED, respectively.

Use dummy zero resource in resource_show() for backwards compatibility as
lspci will otherwise misrepresent disabled bridge windows.

This change fixes an issue which highlights the importance of keeping the
resource type flags intact:

  At the end of __assign_resources_sorted(), reset_resource() is called,
  previously clearing the flags. Later, pci_prepare_next_assign_round()
  attempted to release bridge resources using
  pci_bus_release_bridge_resources() that calls into
  pci_bridge_release_resources() that assumes type flags are still present.
  As type flags were cleared, IORESOURCE_MEM_64 was not set leading to
  resources under an incorrect bridge window to be released (idx = 1
  instead of idx = 2). While the assignments performed later covered this
  problem so that the wrongly released resources got assigned in the end,
  it was still causing extra release+assign pairs.

There are other reasons why the resource flags should be retained in
upcoming changes too.

Removing the flag reset for non-bridge window resource is left as future
work, in part because it has a much higher regression potential due to
pci_enable_resources() that will start to work also for those resources
then and due to what endpoint drivers might assume about resources.

Despite the Fixes tag, backporting this (at least any time soon) is highly
discouraged. The issue fixed is borderline cosmetic as the later
assignments normally cover the problem entirely. Also there might be
non-obvious dependencies.

Fixes: 5b28541552ef ("PCI: Restrict 64-bit prefetchable bridge windows to 64-bit resources")
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20250829131113.36754-11-ilpo.jarvinen@linux.intel.com

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