drm/core: Add drm_accurate_vblank_count, v5.

This function is useful for gen2 intel devices which have no frame
counter, but need a way to determine the current vblank count without
racing with the vblank interrupt handler.

intel_pipe_update_start checks if no vblank interrupt will occur
during vblank evasion, but cannot check whether the vblank handler has
run to completion. This function uses the timestamps to determine
when the last vblank has happened, and interpolates from there.

Changes since v1:
- Take vblank_time_lock and don't use drm_vblank_count_and_time.
Changes since v2:
- Don't return time of last vblank.
Changes since v3:
- Change pipe to unsigned int. (Ville)
- Remove unused documentation for tv_ret. (kbuild)
Changes since v4:
- Add warning to docs when the function is useful.
- Add a WARN_ON when get_vblank_timestamp is unavailable.
- Use drm_vblank_count.

Cc: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> #v4
Acked-by: David Airlie <airlied@linux.ie> #irc, v4
Link: http://patchwork.freedesktop.org/patch/msgid/1463490484-19540-2-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Mario Kleiner <mario.kleiner.de@gmail.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/drm/drmP.h b/include/drm/drmP.h
index 360b2a74e1ef..ed890384b938 100644
--- a/include/drm/drmP.h
+++ b/include/drm/drmP.h
@@ -1002,6 +1002,7 @@ extern void drm_crtc_vblank_off(struct drm_crtc *crtc);
 extern void drm_crtc_vblank_reset(struct drm_crtc *crtc);
 extern void drm_crtc_vblank_on(struct drm_crtc *crtc);
 extern void drm_vblank_cleanup(struct drm_device *dev);
+extern u32 drm_accurate_vblank_count(struct drm_crtc *crtc);
 extern u32 drm_vblank_no_hw_counter(struct drm_device *dev, unsigned int pipe);
 
 extern int drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev,