[Intel-gfx] [PATCH v2 8/5] drm/i915: Fix gen2 scanline counter

[ville.syrjala at linux.intel.com]

From: Ville Syrjälä <ville.syrjala at 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.syrjala at 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