drm/i915: Pad ringbuffer with NOOPs before wrapping

According to the docs, the ringbuffer is not allowed to wrap in the middle
of an instruction.

G45 PRM, Vol 1b, p101:
  While the “free space” wrap may allow commands to be wrapped around the
  end of the Ring Buffer, the wrap should only occur between commands.
  Padding (with NOP) may be required to follow this restriction.

Do as commanded.

[Having seen bug reports where there is evidence of split commands, but
apparently the GPU has continued on merrily before a bizarre and untimely
death, this may or may not fix a few random hangs.]

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
CC: Eric Anholt <eric@anholt.net>
Signed-off-by: Eric Anholt <eric@anholt.net>

1 files changed, 28 insertions, 1 deletions

diff --git a/drivers/gpu/drm/i915/i915_dma.c b/drivers/gpu/drm/i915/i915_dma.c
index 50d1f782768c..f135bdcc6d5b 100644
--- a/drivers/gpu/drm/i915/i915_dma.c
+++ b/drivers/gpu/drm/i915/i915_dma.c
@@ -80,6 +80,34 @@ int i915_wait_ring(struct drm_device * dev, int n, const char *caller)
 	return -EBUSY;
 }
 
+/* As a ringbuffer is only allowed to wrap between instructions, fill
+ * the tail with NOOPs.
+ */
+int i915_wrap_ring(struct drm_device *dev)
+{
+	drm_i915_private_t *dev_priv = dev->dev_private;
+	volatile unsigned int *virt;
+	int rem;
+
+	rem = dev_priv->ring.Size - dev_priv->ring.tail;
+	if (dev_priv->ring.space < rem) {
+		int ret = i915_wait_ring(dev, rem, __func__);
+		if (ret)
+			return ret;
+	}
+	dev_priv->ring.space -= rem;
+
+	virt = (unsigned int *)
+		(dev_priv->ring.virtual_start + dev_priv->ring.tail);
+	rem /= 4;
+	while (rem--)
+		*virt++ = MI_NOOP;
+
+	dev_priv->ring.tail = 0;
+
+	return 0;
+}
+
 /**
  * Sets up the hardware status page for devices that need a physical address
  * in the register.
@@ -200,7 +228,6 @@ static int i915_initialize(struct drm_device * dev, drm_i915_init_t * init)
 		}
 
 		dev_priv->ring.Size = init->ring_size;
-		dev_priv->ring.tail_mask = dev_priv->ring.Size - 1;
 
 		dev_priv->ring.map.offset = init->ring_start;
 		dev_priv->ring.map.size = init->ring_size;