tree 1f9da7034ffee2c96ef95b9a5dd1cf045c1da412
parent a07fd74d5ea9c45a5c6e41f7cb4b997cf40d50f3
author Denys Vlasenko <dvlasenk@redhat.com> 1461167156 +0200
committer Jeff Kirsher <jeffrey.t.kirsher@intel.com> 1463176595 -0700

e1000e: e1000e_cyclecounter_read(): do overflow check only if needed

SYSTIMH:SYSTIML registers are incremented by 24-bit value TIMINCA[23..0]

er32(SYSTIML) are probably moderately expensive (they are pci bus reads).
Can we avoid one of them? Yes, we can.

If the SYSTIML value we see is smaller than 0xff000000, the overflow
into SYSTIMH would require at least two increments.

We do two reads, er32(SYSTIML) and er32(SYSTIMH), in this order.

Even if one increment happens between them, the overflow into SYSTIMH
is impossible, and we can avoid doing another er32(SYSTIML) read
and overflow check.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>