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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
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.