drm/i915: Add a small adjustment to the pixel counter on interlaced modes

In interlaced modes, the pixel counter counts all pixels,
so one field will have htotal more pixels. In order to avoid
the reported position from jumping backwards when the pixel
counter is beyond the length of the shorter field, just
clamp the position the length of the shorter field. This
matches how the scanline counter based position works since
the scanline counter doesn't count the two half lines.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: "Akash Goel <akash.goels@gmail.com>"
Reviewed-by: "Sourab Gupta <sourabgupta@gmail.com>"
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 20432762d30fab2b67e1c12275e5c6f16eba40d9..cfa4d369c98c46e4c7fff5f5d159751a413a13b4 100644 (file)
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -914,6 +914,18 @@ static int i915_get_crtc_scanoutpos(struct drm_device *dev, int pipe,
                vbl_end *= htotal;
                vtotal *= htotal;
 
+               /*
+                * In interlaced modes, the pixel counter counts all pixels,
+                * so one field will have htotal more pixels. In order to avoid
+                * the reported position from jumping backwards when the pixel
+                * counter is beyond the length of the shorter field, just
+                * clamp the position the length of the shorter field. This
+                * matches how the scanline counter based position works since
+                * the scanline counter doesn't count the two half lines.
+                */
+               if (position >= vtotal)
+                       position = vtotal - 1;
+
                /*
                 * Start of vblank interrupt is triggered at start of hsync,
                 * just prior to the first active line of vblank. However we