• Bjorn Helgaas's avatar
    PCI/ACPI: Request LTR control from platform before using it · af8bb9f8
    Bjorn Helgaas authored
    Per the PCI Firmware spec r3.2, sec 4.5, an ACPI-based OS should use _OSC
    to request control of Latency Tolerance Reporting (LTR) before using it.
    
    Request control of LTR, and if the platform does not grant control, don't
    use it.
    
    N.B. If the hardware supports LTR and the ASPM L1.2 substate but the BIOS
    doesn't support LTR in _OSC, we previously would enable ASPM L1.2.  This
    patch will prevent us from enabling ASPM L1.2 in that case.  It does not
    prevent us from enabling PCI-PM L1.2, since that doesn't depend on LTR.
    See PCIe r40, sec 5.5.1, for the L1 PM substate entry conditions.
    Signed-off-by: default avatarBjorn Helgaas <bhelgaas@google.com>
    Reviewed-by: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
    af8bb9f8
pci_root.c 25.1 KB