1. 19 Apr, 2021 4 commits
  2. 15 Apr, 2021 4 commits
  3. 14 Apr, 2021 4 commits
  4. 13 Apr, 2021 3 commits
  5. 12 Apr, 2021 5 commits
  6. 11 Apr, 2021 4 commits
  7. 09 Apr, 2021 2 commits
  8. 08 Apr, 2021 13 commits
  9. 01 Apr, 2021 1 commit
    • Ville Syrjälä's avatar
      drm/vblank: Do not store a new vblank timestamp in drm_vblank_restore() · 167b4002
      Ville Syrjälä authored
      drm_vblank_restore() exists because certain power saving states
      can clobber the hardware frame counter. The way it does this is
      by guesstimating how many frames were missed purely based on
      the difference between the last stored timestamp vs. a newly
      sampled timestamp.
      
      If we should call this function before a full frame has
      elapsed since we sampled the last timestamp we would end up
      with a possibly slightly different timestamp value for the
      same frame. Currently we will happily overwrite the already
      stored timestamp for the frame with the new value. This
      could cause userspace to observe two different timestamps
      for the same frame (and the timestamp could even go
      backwards depending on how much error we introduce when
      correcting the timestamp based on the scanout position).
      
      To avoid that let's not update the stored timestamp at all,
      and instead we just fix up the last recorded hw vblank counter
      value such that the already stored timestamp/seq number will
      match. Thus the next time a vblank irq happens it will calculate
      the correct diff between the current and stored hw vblank counter
      values.
      
      Sidenote: Another possible idea that came to mind would be to
      do this correction only if the power really was removed since
      the last time we sampled the hw frame counter. But to do that
      we would need a robust way to detect when it has occurred. Some
      possibilities could involve some kind of hardare power well
      transition counter, or potentially we could store a magic value
      in a scratch register that lives in the same power well. But
      I'm not sure either of those exist, so would need an actual
      investigation to find out. All of that is very hardware specific
      of course, so would have to be done in the driver code.
      
      Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
      Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
      Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
      Signed-off-by: default avatarVille Syrjälä <ville.syrjala@linux.intel.com>
      Link: https://patchwork.freedesktop.org/patch/msgid/20210218160305.16711-1-ville.syrjala@linux.intel.comReviewed-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
      167b4002