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  • Bjorn Helgaas's avatar
    PCI: Sleep rather than busy-wait for VPD access completion · c521b014
    Bjorn Helgaas authored
    Use usleep_range() instead of udelay() while waiting for a VPD access to
    complete.  This is not a performance path, so no need to hog the CPU.
    Rationale for usleep_range() parameters:
      We clear PCI_VPD_ADDR_F for a read (or set it for a write), then wait for
      the device to change it.  For a device that updates PCI_VPD_ADDR between
      our config write and subsequent config read, we won't sleep at all and
      can get the device's maximum rate.
      Sleeping a minimum of 10 usec per 4-byte access limits throughput to
      about 400Kbytes/second.  VPD is small (32K bytes at most), and most
      devices use only a fraction of that.
      We back off exponentially up to 1024 usec per iteration.  If we reach
      1024, we've already waited up to 1008 usec (16 + 32 + ... + 512), so if
      we miss an update and wait an extra 1024 usec, we can still get about
      1/2 of the device's maximum rate.
    Tested-by: default avatarShane Seymour <>
    Tested-by: default avatarBabu Moger <>
    Signed-off-by: default avatarBjorn Helgaas <>
    Reviewed-by: default avatarHannes Reinecke <>
access.c 19.4 KB