[PATCH] drm/i915: Clear scanline waits before disabling the pipe.

[Chris Wilson <chris@chris-wilson.co.uk>]

If we disable the pipe and the GPU is currently waiting on a scanline
WAIT_FOR_EVENT, the GPU will hang. Fortunately, there is a magic bit
which we can write on i915+ to break this wait before disabling the
pipe.

References:

  Bug 29252 - [Arrandale] Hung WAIT_FOR_EVENT when running rss-glx-skyrocket
  https://bugs.freedesktop.org/show_bug.cgi?id=29252

  Bug 28964 - [i965gm] GPU infinite MI_WAIT_FOR_EVENT while watching video in Totem
  https://bugs.freedesktop.org/show_bug.cgi?id=28964

and many others.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
---
 drivers/gpu/drm/i915/i915_reg.h      |    2 ++
 drivers/gpu/drm/i915/intel_display.c |   29 +++++++++++++++++++++++++++--
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 97a35a4..2953b0d 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -288,6 +288,8 @@
 #define   RING_VALID_MASK	0x00000001
 #define   RING_VALID		0x00000001
 #define   RING_INVALID		0x00000000
+#define   RING_WAIT_I8XX	(1<<0) /* gen2, PRBx_HEAD */
+#define   RING_WAIT		(1<<11) /* gen3+, PRBx_CTL */
 #define PRB1_TAIL	0x02040 /* 915+ only */
 #define PRB1_HEAD	0x02044 /* 915+ only */
 #define PRB1_START	0x02048 /* 915+ only */
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 2bb3196..696767c 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -2410,6 +2410,28 @@ static int i9xx_crtc_dpms(struct drm_crtc *crtc, int mode)
 	return 0;
 }
 
+/*
+ * When we disable a pipe, we need to clear any pending scanline wait events
+ * to avoid hanging the ring, which we assume we are waiting on.
+ */
+static void intel_clear_scanline_wait(struct drm_device *dev)
+{
+	struct drm_i915_private *dev_priv = dev->dev_private;
+
+	if (IS_GEN2(dev)) {
+		if (I915_READ(PRB0_HEAD) & RING_WAIT_I8XX) {
+			DRM_INFO("Forcing GPU idle to flush scanline wait");
+			i915_gpu_idle(dev);
+		}
+	} else {
+		u32 tmp = I915_READ(PRB0_CTL);
+		if (tmp & RING_WAIT) {
+			I915_WRITE(PRB0_CTL, tmp);
+			POSTING_READ(PRB0_CTL);
+		}
+	}
+}
+
 /**
  * Sets the power management mode of the pipe and plane.
  */
@@ -2429,12 +2451,15 @@ static int intel_crtc_dpms(struct drm_crtc *crtc, int mode)
 	 * with multiple pipes prior to enabling to new pipe.
 	 *
 	 * When switching off the display, make sure the cursor is
-	 * properly hidden prior to disabling the pipe.
+	 * properly hidden and there are no pending waits prior to
+	 * disabling the pipe.
 	 */
 	if (mode == DRM_MODE_DPMS_ON)
 		intel_update_watermarks(dev);
-	else
+	else {
 		intel_crtc_update_cursor(crtc);
+		intel_clear_scanline_wait(dev);
+	}
 
 	dev_priv->display.dpms(crtc, mode);
 
-- 
1.7.1