• Sarah Sharp's avatar
    xhci: Avoid "dead ports", add roothub port polling. · c52804a4
    Sarah Sharp authored
    The USB core hub thread (khubd) is designed with external USB hubs in
    mind.  It expects that if a port status change bit is set, the hub will
    continue to send a notification through the hub status data transfer.
    Basically, it expects hub notifications to be level-triggered.
    
    The xHCI host controller is designed to be edge-triggered on the logical
    'OR' of all the port status change bits.  When all port status change
    bits are clear, and a new change bit is set, the xHC will generate a
    Port Status Change Event.  If another change bit is set in the same port
    status register before the first bit is cleared, it will not send
    another event.
    
    This means that the hub code may lose port status changes because of
    race conditions between clearing change bits.  The user sees this as a
    "dead port" that doesn't react to device connects.
    
    The fix is to turn on port polling whenever a new change bit is set.
    Once the USB core issues a hub status request that shows that no change
    bits are set in any USB ports, turn off port polling.
    
    We can't allow the USB core to poll the roothub for port events during
    host suspend because if the PCI host is in D3cold, the port registers
    will be all f's.  Instead, stop the port polling timer, and
    unconditionally restart it when the host resumes.  If there are no port
    change bits set after the resume, the first call to hub_status_data will
    disable polling.
    
    This patch should be backported to stable kernels with the first xHCI
    support, 2.6.31 and newer, that include the commit
    0f2a7930 "USB: xhci: Root hub support."
    There will be merge conflicts because the check for HC_STATE_SUSPENDED
    was moved into xhci_suspend in 3.8.
    Signed-off-by: default avatarSarah Sharp <sarah.a.sharp@linux.intel.com>
    Acked-by: default avatarAlan Stern <stern@rowland.harvard.edu>
    Cc: stable@vger.kernel.org
    c52804a4
xhci-ring.c 122 KB