linux-imx

PCI: Sleep rather than busy-wait for VPD access completion

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: Shane Seymour <shane.seymour@hpe.com>
Tested-by: Babu Moger <babu.moger@oracle.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>