On Denverton's integration of the Intel(R) Trace Hub (for a reference and
overview see Documentation/trace/intel_th.rst) the reported size of one of
its resources (RTIT_BAR) doesn't match its actual size, which leads to
overlaps with other devices' resources.

In practice, it overlaps with XHCI MMIO space, which results in the xhci
driver bailing out after seeing its registers as 0xffffffff, and perceived
disappearance of all USB devices:

  intel_th_pci 0000:00:1f.7: enabling device (0004 -> 0006)
  xhci_hcd 0000:00:15.0: xHCI host controller not responding, assume dead
  xhci_hcd 0000:00:15.0: xHC not responding in xhci_irq, assume controller is dead
  xhci_hcd 0000:00:15.0: HC died; cleaning up
  usb 1-1: USB disconnect, device number 2

For this reason, we need to resize the RTIT_BAR on Denverton to its actual
size, which in this case is 4MB.  The corresponding erratum is DNV36 at the
link below:

  DNV36.       Processor Host Root Complex May Incorrectly Route Memory
               Accesses to Intel® Trace Hub

  Problem:     The Intel® Trace Hub RTIT_BAR (B0:D31:F7 offset 20h) is
	       reported as a 2KB memory range.  Due to this erratum, the
	       processor Host Root Complex will forward addresses from
	       RTIT_BAR to RTIT_BAR + 4MB -1 to Intel® Trace Hub.

  Implication: Devices assigned within the RTIT_BAR to RTIT_BAR + 4MB -1
               space may not function correctly.

  Workaround:  A BIOS code change has been identified and may be
               implemented as a workaround for this erratum.

  Status:      No Fix.

Note that 5118ccd3 ("intel_th: pci: Add Denverton SOC support") updates
the Trace Hub driver so it claims the Denverton device, but the resource
overlap exists regardless of whether that driver is loaded or that commit
is included.

Link: https://www.intel.com/content/dam/www/public/us/en/documents/specification-updates/atom-c3000-family-spec-update.pdfSigned-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
[bhelgaas: include erratum text, clarify relationship with 5118ccd3]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org