• Matthew Garrett's avatar
    PCI: Rework ASPM disable code · 3c076351
    Matthew Garrett authored
    Right now we forcibly clear ASPM state on all devices if the BIOS indicates
    that the feature isn't supported. Based on the Microsoft presentation
    "PCI Express In Depth for Windows Vista and Beyond", I'm starting to think
    that this may be an error. The implication is that unless the platform
    grants full control via _OSC, Windows will not touch any PCIe features -
    including ASPM. In that case clearing ASPM state would be an error unless
    the platform has granted us that control.
    
    This patch reworks the ASPM disabling code such that the actual clearing
    of state is triggered by a successful handoff of PCIe control to the OS.
    The general ASPM code undergoes some changes in order to ensure that the
    ability to clear the bits isn't overridden by ASPM having already been
    disabled. Further, this theoretically now allows for situations where
    only a subset of PCIe roots hand over control, leaving the others in the
    BIOS state.
    
    It's difficult to know for sure that this is the right thing to do -
    there's zero public documentation on the interaction between all of these
    components. But enough vendors enable ASPM on platforms and then set this
    bit that it seems likely that they're expecting the OS to leave them alone.
    
    Measured to save around 5W on an idle Thinkpad X220.
    Signed-off-by: default avatarMatthew Garrett <mjg@redhat.com>
    Signed-off-by: default avatarJesse Barnes <jbarnes@virtuousgeek.org>
    3c076351
pci_root.c 17.2 KB