PCI: Remove old_size limit from bridge window sizing

PCI: Remove old_size limit from bridge window sizing

calculate_memsize() applies lower bound to the resource size before
aligning the resource size making it impossible to shrink bridge window
resources. I've not found any justification for this lower bound and
nothing indicated it was to work around some HW issue.

Prior to the commit 3baeae36039a ("PCI: Use pci_release_resource() instead
of release_resource()"), releasing a bridge window during BAR resize
resulted in clearing start and end address of the resource.  Clearing
addresses destroys the resource size as a side-effect, therefore nullifying
the effect of the old size lower bound.

After the commit 3baeae36039a ("PCI: Use pci_release_resource() instead of
release_resource()"), BAR resize uses the aligned old size, which results
in exceeding what fits into the parent window in some cases:

  xe 0030:03:00.0: [drm] Attempting to resize bar from 256MiB -> 16384MiB
  xe 0030:03:00.0: BAR 0 [mem 0x620c000000000-0x620c000ffffff 64bit]: releasing
  xe 0030:03:00.0: BAR 2 [mem 0x6200000000000-0x620000fffffff 64bit pref]: releasing
  pci 0030:02:01.0: bridge window [mem 0x6200000000000-0x620001fffffff 64bit pref]: releasing
  pci 0030:01:00.0: bridge window [mem 0x6200000000000-0x6203fbff0ffff 64bit pref]: releasing
  pci 0030:00:00.0: bridge window [mem 0x6200000000000-0x6203fbff0ffff 64bit pref]: was not released (still contains assigned resources)
  pci 0030:00:00.0: Assigned bridge window [mem 0x6200000000000-0x6203fbff0ffff 64bit pref] to [bus 01-04] free space at [mem 0x6200400000000-0x62007ffffffff 64bit pref]
  pci 0030:00:00.0: Assigned bridge window [mem 0x6200000000000-0x6203fbff0ffff 64bit pref] to [bus 01-04] cannot fit 0x4000000000 required for 0030:01:00.0 bridging to [bus 02-04]

The old size of 0x6200000000000-0x6203fbff0ffff resource was used as the
lower bound which results in 0x4000000000 size request due to alignment.
That exceeds what can fit into the parent window.

Since the lower bound never even was enforced fully because the resource
addresses were cleared when the bridge window is released, remove the
old_size lower bound entirely and trust the calculated bridge window size
is enough.

This same problem may occur on io window side but seems less likely to
cause issues due to general difference in alignment. Removing the lower
bound may have other unforeseen consequences in case of io window so it's
better to leave it as -next material if no problem is reported related to
io window sizing (BAR resize shouldn't touch io windows anyway).

Fixes: 3baeae36039a ("PCI: Use pci_release_resource() instead of release_resource()")
Reported-by: Simon Richter <Simon.Richter@hogyros.de>
Link: https://lore.kernel.org/r/f9a8c975-f5d3-4dd2-988e-4371a1433a60@hogyros.de/
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20251219174036.16738-6-ilpo.jarvinen@linux.intel.com