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    ixgbe: Hardware Timestamping + PTP Hardware Clock (PHC) · 3a6a4eda
    Jacob Keller authored
    
    
    This patch enables hardware timestamping for use with PTP software by
    extracting a ns counter from an arbitrary fixed point cycles counter.
    The hardware generates SYSTIME registers using the DMA tick which
    changes based on the current link speed. These SYSTIME registers are
    converted to ns using the cyclecounter and timecounter structures
    provided by the kernel. Using the SO_TIMESTAMPING api, software can
    enable and access timestamps for PTP packets.
    
    The SO_TIMESTAMPING API has space for 3 different kinds of timestamps,
    SYS, RAW, and SOF. SYS hardware timestamps are hardware ns values that
    are then scaled to the software clock. RAW hardware timestamps are the
    direct raw value of the ns counter. SOF software timestamps are the
    software timestamp calculated as close as possible to the software
    transmit, but are not offloaded to the hardware. This patch only
    supports the RAW hardware timestamps due to inefficiency of the SYS
    design.
    
    This patch also enables the PHC subsystem features for atomically
    adjusting the cycle register, and adjusting the clock frequency in
    parts per billion. This frequency adjustment works by slightly
    adjusting the value added to the cycle registers each DMA tick. This
    causes the hardware registers to overflow rapidly (approximately once
    every 34 seconds, when at 10gig link). To solve this, the timecounter
    structure is used, along with a timer set for every 25 seconds. This
    allows for detecting register overflow and converting the cycle
    counter registers into ns values needed for providing useful
    timestamps to the network stack.
    
    Only the basic required clock functions are supported at this time,
    although the hardware supports some ancillary features and these could
    easily be enabled in the future.
    
    Note that use of this hardware timestamping requires modifying daemon
    software to use the SO_TIMESTAMPING API for timestamps, and the
    ptp_clock PHC framework for accessing the clock. The timestamps have
    no relation to the system time at all, so software must use the posix
    clock generated by the PHC framework instead.
    
    Signed-off-by: default avatarJacob E Keller <jacob.e.keller@intel.com>
    Tested-by: default avatarStephen Ko <stephen.s.ko@intel.com>
    Signed-off-by: default avatarJeff Kirsher <jeffrey.t.kirsher@intel.com>
    3a6a4eda