From: Ville Syrjälä <ville.syrj...@linux.intel.com>

On gen2 the scanline counter behaves a bit differently from the
later generations. Instead of adding one to the raw scanline
counter value, we must subtract one.

v2: Remove the gen3 FIXME. Gen3 behaves like gen4

Signed-off-by: Ville Syrjälä <ville.syrj...@linux.intel.com>
---
 drivers/gpu/drm/i915/i915_irq.c | 34 +++++++++++++++++++++-------------
 1 file changed, 21 insertions(+), 13 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index fc49fb6..40adce0 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -724,26 +724,34 @@ static int __intel_get_crtc_scanline(struct intel_crtc 
*crtc)
        struct drm_i915_private *dev_priv = dev->dev_private;
        const struct drm_display_mode *mode = &crtc->config.adjusted_mode;
        enum pipe pipe = crtc->pipe;
-       int vtotal = mode->crtc_vtotal;
-       int position;
+       int position, vtotal;
 
+       vtotal = mode->crtc_vtotal;
        if (mode->flags & DRM_MODE_FLAG_INTERLACE)
                vtotal /= 2;
 
+       /*
+        * The scanline counter increments at the leading edge of hsync.
+        *
+        * On most platforms it starts counting from vtotal-1 on the
+        * first active line. That means the scanline counter value is
+        * always one less than what we would expect. Ie. just after
+        * start of vblank, which also occurs at start of hsync (on the
+        * last active line), the scanline counter will read vblank_start-1.
+        *
+        * Gen2 is the exception as the scanline counter starts counting
+        * from 1 instead of vtotal-1, so we have to subtract one (or
+        * rather add vtotal-1 to keep the value positive), instead of
+        * adding one.
+        */
        if (IS_GEN2(dev))
-               position = __raw_i915_read32(dev_priv, PIPEDSL(pipe)) & 
DSL_LINEMASK_GEN2;
+               position = (__raw_i915_read32(dev_priv, PIPEDSL(pipe)) &
+                           DSL_LINEMASK_GEN2) + vtotal - 1;
        else
-               position = __raw_i915_read32(dev_priv, PIPEDSL(pipe)) & 
DSL_LINEMASK_GEN3;
+               position = (__raw_i915_read32(dev_priv, PIPEDSL(pipe)) &
+                           DSL_LINEMASK_GEN3) + 1;
 
-       /*
-        * Scanline counter increments at leading edge of hsync, and
-        * it starts counting from vtotal-1 on the first active line.
-        * That means the scanline counter value is always one less
-        * than what we would expect. Ie. just after start of vblank,
-        * which also occurs at start of hsync (on the last active line),
-        * the scanline counter will read vblank_start-1.
-        */
-       return (position + 1) % vtotal;
+       return position % vtotal;
 }
 
 static int i915_get_crtc_scanoutpos(struct drm_device *dev, int pipe,
-- 
1.8.3.2

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