• Tobias Waldekranz's avatar
    net: dsa: mv88e6xxx: Improve performance of busy bit polling · 35da1dfd
    Tobias Waldekranz authored
    Avoid a long delay when a busy bit is still set and has to be polled
    again.
    
    Measurements on a system with 2 Opals (6097F) and one Agate (6352)
    show that even with this much tighter loop, we have about a 50% chance
    of the bit being cleared on the first poll, all other accesses see the
    bit being cleared on the second poll.
    
    On a standard MDIO bus running MDC at 2.5MHz, a single access with 32
    bits of preamble plus 32 bits of data takes 64*(1/2.5MHz) = 25.6us.
    
    This means that mv88e6xxx_smi_direct_wait took 26us + CPU overhead in
    the fast scenario, but 26us + 1500us + 26us + CPU overhead in the slow
    case - bringing the average close to 1ms.
    
    With this change in place, the slow case is closer to 2*26us + CPU
    overhead, with the average well below 100us - a 10x improvement.
    
    This translates to real-world winnings. On a 3-chip 20-port system,
    the modprobe time drops by 88%:
    
    Before:
    
    root@coronet:~# time modprobe mv88e6xxx
    real    0m 15.99s
    user    0m 0.00s
    sys     0m 1.52s
    
    After:
    
    root@coronet:~# time modprobe mv88e6xxx
    real    0m 2.21s
    user    0m 0.00s
    sys     0m 1.54s
    Signed-off-by: default avatarTobias Waldekranz <tobias@waldekranz.com>
    Reviewed-by: default avatarAndrew Lunn <andrew@lunn.ch>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    35da1dfd
chip.c 188 KB