drm/i915/ringbuffer: Ignore failure to setup the ring on Sandybridge

The ring buffer registers return 0 whilst idle (for some values of idle)
on early Sandybridge hw. Persevere even when all appears hopeless...
Fortunately the head auto-reporting prevents most hangs.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=31370
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

1 files changed, 24 insertions, 8 deletions

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 9033697783b9..f5d6151c953f 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -180,14 +180,30 @@ static int init_ring_common(struct intel_ring_buffer *ring)
         if ((I915_READ_CTL(ring) & RING_VALID) == 0 ||
             I915_READ_START(ring) != obj_priv->gtt_offset ||
             (I915_READ_HEAD(ring) & HEAD_ADDR) != 0) {
-                DRM_ERROR("%s initialization failed "
-                                "ctl %08x head %08x tail %08x start %08x\n",
-                                ring->name,
-                                I915_READ_CTL(ring),
-                                I915_READ_HEAD(ring),
-                                I915_READ_TAIL(ring),
-                                I915_READ_START(ring));
-                return -EIO;
+                if (IS_GEN6(ring->dev) && ring->dev->pdev->revision <= 8) {
+                        /* Early revisions of Sandybridge do not like
+                         * revealing the contents of the ring buffer
+                         * registers whilst idle. Fortunately, the
+                         * auto-reporting mechanism prevents most hangs,
+                         * but this will bite us eventually...
+                         */
+                        DRM_DEBUG("%s initialization failed "
+                                  "ctl %08x head %08x tail %08x start %08x. Ignoring, hope for the best!\n",
+                                  ring->name,
+                                  I915_READ_CTL(ring),
+                                  I915_READ_HEAD(ring),
+                                  I915_READ_TAIL(ring),
+                                  I915_READ_START(ring));
+                } else {
+                        DRM_ERROR("%s initialization failed "
+                                  "ctl %08x head %08x tail %08x start %08x\n",
+                                  ring->name,
+                                  I915_READ_CTL(ring),
+                                  I915_READ_HEAD(ring),
+                                  I915_READ_TAIL(ring),
+                                  I915_READ_START(ring));
+                        return -EIO;
+                }
         }
 
         if (!drm_core_check_feature(ring->dev, DRIVER_MODESET))