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>
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