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Diffstat (limited to 'drivers/gpu/drm/i915/intel_display.c')
-rw-r--r--drivers/gpu/drm/i915/intel_display.c9
1 files changed, 9 insertions, 0 deletions
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 3cabe52a4e3b..569717a12723 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -12203,6 +12203,15 @@ static void update_scanline_offset(struct intel_crtc *crtc)
* type. For DP ports it behaves like most other platforms, but on HDMI
* there's an extra 1 line difference. So we need to add two instead of
* one to the value.
+ *
+ * On VLV/CHV DSI the scanline counter would appear to increment
+ * approx. 1/3 of a scanline before start of vblank. Unfortunately
+ * that means we can't tell whether we're in vblank or not while
+ * we're on that particular line. We must still set scanline_offset
+ * to 1 so that the vblank timestamps come out correct when we query
+ * the scanline counter from within the vblank interrupt handler.
+ * However if queried just before the start of vblank we'll get an
+ * answer that's slightly in the future.
*/
if (IS_GEN2(dev_priv)) {
const struct drm_display_mode *adjusted_mode = &crtc->config->base.adjusted_mode;