[Bug 91579] [HSW] kms_flip atomic warnings

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=91579

--- Comment #13 from Jesse Barnes <jbarnes at virtuousgeek.org> ---
Update from yesterday's experiments.  I confirmed that the frame counter is
incrementing correctly and at the right rate, but the scanline register read
returns 0 when read around the start of vblank.  A sequence of reads might
return:
  0
  0
  0
  <delay 1us>
  1600

This happens right around the time the frame counter increments, so when this
happens we trigger the warning in the atomic update end function.  I posted a
patch to workaround this by ignoring scanline read results for the first couple
of lines in the display; that should be safe everywhere and lets us schedule
things correctly when we hit this case.

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